Status Report Report on Commissioning of the First RF

TIARA-REP-WP7-2014-002

Test Infrastructure and Accelerator Research Area

Status Report

Report on commissioning of the ﬁrst RF ampliﬁer system in the ICTF Hall

Moss, A. (STFC) et al

31 January 2014

The research leading to these results has received funding from the European Commission under the FP7-INFRASTRUCTURES-2010-1/INFRA-2010-2.2.11 project TIARA (CNI-PP). Grant agreement no 261905. This work is part of TIARA Work Package 7: ICTF R&D Infrastructure. The electronic version of this TIARA Publication is available via the TIARA web site at http://www.eu-tiara.eu/database or on the CERN Document Server at the following URL: http://cds.cern.ch/search?p=TIARA-REP-WP7-2014-002

TIARA WP 7.2 Report: Demonstration of the RF Power System for the ICTF

A. Moss, C. White, A. Grant, STFC Daresbury Laboratory

T. Stanley STFC Rutherford Appleton Laboratory

S. Alsari, K. Long Department of Physics, Imperial College

C.G. Whyte & K. Ronald SUPA and Department of Physics, The University of Strathclyde

Table of Contents 1 Introduction ...... 4 1.1 MICE ...... 4 1.2 The ICTF ...... 5 1.3 ICTF prototype amplifier ...... 5 2 Amplifier Systems ...... 6 2.1 RF Design and Build of Prototype Triode and Tetrode amplifiers ...... 6 2.2 Status of Build of RF systems for amplifiers 2 through 4 ...... 7 2.3 MICE RF - High Voltage Power Supplies ...... 9 2.3.1 Power Supplies - general ...... 9 2.3.2 20 kV Power Supply (4616) ...... 10 2.3.3 Auxiliary Rack (4616) ...... 12 2.3.4 40 kV Power Supply (TH116) ...... 14 2.3.5 Auxiliary Rack (TH116) ...... 16 2.3.6 TH116 Amplifier Enclosure ...... 17 3 Tests of the Prototype Amplifier ...... 18 3.1 Demonstration of required RF performance ...... 18 3.2 Installation and operation in ICTF ...... 20 4 Summary ...... 23 5 Acknowledgements ...... 23

List of Figures

Figure 1: MICE in STEP VI configuration, highlighting the two separate accelerator units and three absorber cells. The solenoids at each end surround scintillating particle trackers that form the momentum spectrometers. Not shown are the time of flight, Cherenkov and EMR detectors, momentum selection and emittance control is performed by upstream dipoles and a diffuser...... 4 Figure 2: The diagram on the LHS illustrates the layout of the test system at Daresbury, the middle and right hand images show the intermediate and final stage prototype amplifiers ...... 7 Figure 3: Image showing the second and third tetrode amplifier systems, assembly completed at Daresbury laboratory ...... 8 Figure 4: LHS shows the fourth tetrode amplifier under test at 250kW, whilst the RHS shows the third and fourth triode amplifiers, ready for operational testing. These amplifiers differ from slightly from nos 1 and 2 in that they have two 6” output lines, the flanges for which are visible on either side of the upper blue cylindrical cavity section, in place of the single 9” line ...... 8 Figure 5: Control panels for the 4616 tetrode intermediate amplifier systems 20kV power supply ...... 9 Figure 6: 4616 Amplifier, Power Supply System: Overview of key components ...... 10 Figure 7: TH116 Amplifier System Power Supplies: Overview of key subsystem ...... 13 Figure 8: HT power supply rack for the final stage Thales TH116 Triode valve amplifier ...... 14 Figure 9: Illustration of key components in the TH116 valve power supply, on the left is a prototype e2v thyratron rated for high charge switching installed as the crowbar switch, and on the right is the cathode modulator...... 15 Figure 10: LHS showing the electrical cabinet and RHS showing the cooling and electrical power circuits installed in the base of the triode amplifier assembly ...... 17 Figure 11: Graphs showing the variation of the intermediate amplifiers output power and zeroth order Fourier component of the anode current as a function of drive power, anode voltage 19kV, control grid bias 170V and screen grid at 1.74kV ...... 19 Figure 12: Illustrating the variation of triode output power with bias voltage ...... 19 Figure 13: The power meter reading on the output from the final stage amplifier demonstrating the required peak output power...... 20 Figure 14: Installation for TIARA tests in the MICE hall. Shown are 4 of the 5 racks (one rack not visible held the SSPA, Oscillator and timing pulse generator), consisting of the auxiliary and main PSU racks for the tetrode and triode amplifiers. The tetrode amplifier is visible on the mezzanine whilst the triode amplifier is the tall purple structure on the ground floor with the valve cap protruding through the mezzanine...... 20 Figure 15: Images of the installation of the final stage amplifier and showing the completed installation with the water distribution system and the MRI funded loads and RF distribution components ...... 21 Figure 16: Installation of the power supply, auxiliary and control systems, also showing the tetrode amplifier and the top cap of the triode amplifier...... 21 Figure 17: Left hand shows the power meters gated measurement of the output power from the triode amplifier whilst on the right we see the input drive from the tetrode and the reflected power back from the triode ...... 22 Figure 18: MICE Step VI distribution network, all components procured through University of Mississippi MRI grant, installation to commence from January 2014 ...... 22

1 Introduction

1.1 MICE The Muon Ionisation Cooling Experiment (MICE) aims to demonstrate that a 10% reduction in the emittance of a synthesised muon beam (representative in its phase space distribution to that expected for the muon source for a neutrino factory or muon collider) can be achieved by ionisation cooling as the beam transits through a single cell of the baseline design ‘cooling cell’ for the neutrino factory [1,2]. The baseline design consists of passing Muons of an appropriate energy through

‘absorbers’ consisting of low Z materials (particularly Liquid H2 and LiH) to reduce all components of the momentum with subsequent re-acceleration in RF cavities. Periodic re-acceleration between absorbers is vital since ionisation cooling dominates over scattering (and hence emittance growth) only over a limited energy range. MICE will test the cooling process for Muons in the range 140- 240MeV/c. The single cell to be tested in MICE (in its STEP VI configuration) consists of three absorber elements bracketing and dividing two RF accelerator sections, Figure 1. This is the minimum unit required to allow the experiment to maintain momentum after transit of the cooling cell. It is anticipated that any neutrino factory will exploit a chain of some 50 such cells to reduce the emittance rapidly to within the acceptance of a suitable accelerator system which can in turn boost the muon energy to relativistic levels before excessive decay can take place. The MICE project will hence demonstrate that energy neutral deceleration of the muons can be achieved, essential for sustained cooling over 50 cells of the cooling lattice. For a muon collider, even more sophisticated 6D cooling must be achieved requiring the implementation of techniques for emittance exchange. The MICE experiment will also be able to demonstrate the principles of this approach.

1.2 The ICTF To realise the MICE experiment requires the scientific community to create a unique experimental infrastructure, the Ionisation Cooling Test Facility (ICTF). In addition to demonstrating the principles of ionisation cooling, the facility will provide a resource for precision muon experiments for a range of applications. The experiment requires several technically challenging elements of apparatus, including unusually large superconducting magnets (and their operation and interactions in a range of magnetic configurations within a small space), detectors with high sensitivity to particle phase space or transit time, the operation of high gradient RF accelerators in strong magnetic fields, the development of handling systems for the absorbers (Liq. H2 being particularly complex though now demonstrated) and high power RF driving systems. Again the density of the cryogenic (He and H2) systems, magnets and HPRF systems poses complex problems, and understanding and resolving these issues will result in impact beyond the scope of the MICE measurements themselves. From the perspective of the RF accelerators the problems of operating a high field accelerator in a strong magnetic field poses a particular problem due to the known effect whereby the attainable gradients are significantly impacted, whilst the development of a suitably compact RF driver system to accommodate the constraints imposed by the density of complex cryogenic and magnetic systems is also a key challenge. It is addressing this challenge that is the key focus of TIARA WP 7.2.

The creation of the ICTF is being undertaken by a large scale international collaboration with contributions from UK, USA, Japan, EU and China. The key requirement for such a system is the availability of a high power, energetic proton drive beam, hence the apparatus is being built at the Rutherford Appleton Laboratory where it can take advantage of colocation with the ISIS facility, and particularly its proton synchrotron [3].

The principle of the experiment is that a target (titanium in the case of the ICTF [4]) is inserted into the fringe of the proton beam, resulting in the production of pions. These pions are selected by passage through bending dipoles and transported by solenoidal and quadrupole magnets into a decay area where a significant fraction decay into muons. To be able to measure a 10% reduction with confidence, it is vital that the emittance is precisely measured. For this reason the ICTF is currently configured to operate at a relatively low muon fluence, this allows each individual muon to be measured at the beginning and end of the cooling process and the results of a large number of individual particles, forming a synthesised beam, compared to the predictions of the models of the ionisation cooling process. To determine the momentum the experiment uses both precision timing detectors (e.g. scintillation instruments known as Time of Flight or ToF detectors) [5] and a pair of scintillating fibre trackers [6] which follow the trajectory of a muon through a solenoidal magnetic field to plot its initial and final locations in velocity space. An additional detector is being developed which will determine the RF phase during the transit of each muon since only particles at the correct RF phase (in a 5 degree range) will be used to synthesise the cooling process for a realistic beam (in this regard the selection represents the performance of the prebuncher which will be used in the front end of a practical accelerator.

1.3 ICTF prototype amplifier The TIARA project has, under work package 7.2 supported the development of an RF amplifier system which is suitable for installation in the tight confines of the ICTF experimental area (sometimes referred to as the MICE Hall) to provide the drive to the RF cavities. The key deliverable were to demonstrate that an RF amplifier system capable of delivering the required 2MW, 1ms

5 pulses of 201.25MHz radiation at a repetition rate of 1Hz can be realised in the confined space of the ICTF hall. This has encompassed preparation of the physical structures and the tuning controls for the amplifier cavities (section 2.1) and the power supplies (section 0) required to provide the bias on the RF valves used in the two high power amplifiers, and integration of these components together with the solid state power amplifier and source which provides their drive signal. The prototype was to be proven at 2MW output power (section 3.1) and demonstrated operating in the ICTF (section 3.1). In addition preparations for the other three amplifier systems required for the MICE project were to be taken forward. In this last regard a change in the priorities of the MICE project has influenced the rebuilding programme. MICE originally intended to use its RF system in two phases (known as STEP V and STEP VI). The first of these used two RF amplifiers to drive a single accelerator module with four cavities (each cavity receiving 1MW of RF power), whilst step VI was to exploit four amplifier chains driving eight cavities in 2 RF accelerator modules. Revised plans for magnetic screening of the experiment have changed the emphasis to requiring all the RF amplifiers at the same time and therefore rather than prioritise the second amplifier as originally planned all three amplifiers have been mechanically completed in parallel.

2 Amplifier Systems The amplifier systems will consist of two valve stages fed by an SSPA driven in turn by a solid state synthesised oscillator (which will in due course be replaced by an LLRF system). The SSPA boosts the oscillator signal from ~mW level to ~kW level, with the intermediate amplifier (a tetrode valve system) providing up to 250kW and the final stage (based on a triode valve) delivering up to 2MW.

2.1 RF Design and Build of Prototype Triode and Tetrode amplifiers The amplifier systems follow designs of both CERN [8] and RAL in their layout, using many similar components and design philosophies to produce a robust reliable RF system in the ICTF for the MICE experiment.

A 4kW Dressler solid state power amplifier (SSPA) is used as the first stage of amplification. This amplifier was produced to CERN requirements for their LINAC sections and includes interlocking of critical parameters and a remote control interface. The input to this SSPA will be an RF signal from the low level RF system (LLRF) of up to 10mW and will allow control of the amplifier chain, cavity phase angle and voltage using feedback. However for the purposes of the initial tests of the amplifier chain, the seed signal was provided by a synthesised RF oscillator.

The intermediate stage is a 250kW tetrode valve amplifier system. These amplifier systems and valves were produced by Burle (now Photonis) [8] and have been used in many previous accelerator projects with proven reliability. The valve is a 4616 power tetrode, which is fitted into an amplifier cavity that provides the ability to operate the tube filament, provide water and air cooling, adjust input frequency and match, also output frequency and coupling factors using movable controls.

The SSPA output is routed into the input cavity of the 4616 amplifier cavity to provide the necessary excitation at 201MHz. The main and associated power supplies connected to the 4616 amplifier cavity allow control of various parameters within the tube so that the system can be optimised to provide high gain and good electrical to RF conversion efficiency, necessary to prolong tube life and provide reliable operation, see Section 2.3.2. The output of the Tetrode system is a 3 inch rigid coax

6 with a directional coupler for monitoring forward and reflected power. This connects to the final amplifier stage.

Figure 2: The diagram on the LHS illustrates the layout of the test system at Daresbury, the middle and right hand images show the intermediate and final stage prototype amplifiers

The final stage of the RF amplifier chain is a triode valve amplifier consists of a Thales TH116 valve [9] and a coaxial amplifier cavity system that supports the valve within the RF circuit and allows the cooling and HT bias voltage supplies to be connected. The amplifier cavity supports the RF input and output operation by movable concentric tuner sections. Since these may need to be adjusted in routine operation, these tuner stubs have been installed with motor controls to allow remote tuning of the system.

The components of the system were routinely tested in the Daresbury Laboratory test facility as elements of the amplifier systems and power supplies were built and assessed for their performance and suitability. A number of technical issues were identified during these test operations and appropriate solutions were implemented. The layout of the test system at Daresbury is illustrated on the LHS in Figure 2 whilst the figure also shows photographs of the final and intermediate stage amplifiers installed at Daresbury.

The RF distribution system for the ICTF was designed to transmit the power from the amplifier’s to the acceleration sections within the tight space confines of the MICE hall. This has made the design very challenging in respect of ensuring the phase length of each coax transmission system is preserved so that the RF power adds inside the accelerating structure. RF components at this frequency are physically large so a great deal of effort has gone into the design of the distribution system. The system has been designed on CAD to provide fully integrated plans of the system fitted into the MICE hall with all the equipment as it will finally be. This allows the build-up of the system inside the hall to be programmed into the optimum time slot so that the natural order of the project is maintained. The installation of the system in the ICTF validated this design effort.

2.2 Status of Build of RF systems for amplifiers 2 through 4 As previously noted, the plan for the second through fourth amplifier systems was modified during the course of the project (though with no impact on deliverables), due to changes in the schedule and the priorities in the MICE project, which will now require all four amplifier chains at broadly the same time. This was driven by a change to the magnetic shielding configuration planned for the ICTF. This changed the emphasis from bringing the second set of amplifiers to completion (which would have been the priority for MICE Step V) to proceeding in parallel with all four amplifier chains

7 required for MICE Step VI, the full implementation of the ICTF. The amplifiers for the next sections of the project are at various advanced stages of build. Two of the 250kW tetrode amplifiers are mechanically refurbished and complete. These systems require electrical wire up and then testing within the test facility at DL.

Figure 3: Image showing the second and third tetrode amplifier systems, assembly completed at Daresbury laboratory

A new tetrode amplifier was purchased from Photonis through a US MRI programme led by the University of Mississippi, this being the fourth amplifier needed for the project. This has been operated at 250kW and will be integrated into a housing rack so that ancillary supplies can be connected.

Figure 4: LHS shows the fourth tetrode amplifier under test at 250kW, whilst the RHS shows the third and fourth triode amplifiers, ready for operational testing. These amplifiers differ from slightly from nos 1 and 2 in that they have two 6” output lines, the flanges for which are visible on either side of the upper blue cylindrical cavity section, in place of the single 9” line

The second final triode amplifier system has been fully mechanically prepared by Daresbury laboratory. It requires electrical wire up and testing with the DL test facility. Final amplifiers three and four have been prepared at CERN and are mechanically and electrically complete. These systems require assembly at DL and testing with the test facility.

2.3 MICE RF - High Voltage Power Supplies

2.3.1 Power Supplies - general Each of the valve amplifier system has a dedicated high voltage power supply developed for this specific application; one rated at 20 kV for the 4616 amplifier and the other at 40 kV for the TH116 amplifier. In addition auxiliary systems are required for amplifier operation, interlocking and safety. These systems are housed in adjacent racks. All racks are Schroff type, steel-framed, with 19” rack fittings. Side panels, rear door and blanking panels used to cover any gaps at the front where 19” cases are not fitted ensure the system is fully closed. Electrical switches are fitted at the rear and sides for interlocking with the control system to prevent operation without all panels being in position. An example of one of these racks is shown in Figure 5 (the supply rack for the intermediate amplifier). The following sections detail the specification of the amplifier system.

Figure 5: Control panels for the 4616 tetrode intermediate amplifier systems 20kV power supply

Figure 6: 4616 Amplifier, Power Supply System: Overview of key components

The overall 4616 system is shown in Figure 6, whilst the overall TH116 system is shown in Figure 7.

2.3.2 20 kV Power Supply (4616) The 20 kV power supply rack is shown in Figure 5. It accommodates the HT pulsed power systems, chargers and certain supporting subsystems for the intermediate amplifier.

2.3.2.1 Lamp Display Unit This is a 2U plate with two lamp units mounted on it, to provide a clear indication of the status of the system. One is illuminated red when the HV power supply is turned on; the other is illuminated green when the system is earthed.

2.3.2.2 Mains Power Module The mains power module contains the protective circuit breakers, and distributes the mains power via several rear-mounted connectors. It also contain a contactor which controls the output power to the charging power supply; this contactor is activated via the mechanical Castell key switch and a digital output from the control unit.

2.3.2.3 Capacitor Charging Power Supply The capacitor charging power supply is a shoebox type design; a Lambda 500A rated at 20 kV, 500 J/s. It is controlled by the Charger Interface Unit via a 15-way D type connector.

2.3.2.4 Charger Interface Unit The charger interface unit comprises a 2U 19” crate and connects to the control connector on the charging power supply via a 15 way D-type connector. It has LEDs for status indication and switches for Enable/Reset and Inhibit functions and provides manual control of the charging (output) voltage via a potentiometer.

2.3.2.5 Control Unit The control and interlocking of the HV power supply is undertaken by the Control Unit, a 4U crate. Its front panel has LED indications to illustrate the status of the system:

Remote interlocks Not used prior to cavity installation. Guardline 1 Not used prior to cavity installation. Guardline 2 Not used prior to cavity installation. Doors Monitors the rear door switch and side panel switches. Connectors in Prevents operation unless the connectors fitted to the rear of the control unit are in place. PLC OK Signal from the main PLC that the interlocks it monitors are all OK for the HV PSU to be turned on. These include the panels on the amplifier enclosure, cooling water and compressed air. Crowbar heater Monitors the crowbar heater current via a current monitoring relay and times its operation to ensure the crowbar is at full working temperature before the HV PSU can be turned on. Earthed Indicates that the earth switch is in the earth position. Crowbar ready Indicates that the crowbar circuit is operational. After a crowbar trip it must be manually reset before the HV PSU can be turned on. Energy dumped Indicates that the dump relay is closed and hence that the energy in the capacitor has been discharged.

2.3.2.6 HV Capacitor The energy storage for the power supply is accomplished by an AVX capacitor rated at 29.2 uF and 23.8 kV. It has a single HV bushing with the return connection via a stud welded to the steel case. It is bolted to support plates fixed to the enclosure lower frame.

2.3.2.7 Potential Divider The potential divider consists of a series of high voltage resistors mounted in series on an insulated plate. The output voltage is taken from the last resistor in the series giving a 10000:1 ratio. The output voltage is routed via a unity gain buffer amplifier allowing it to drive the meters in the control unit whilst drawing minimal current from the system. This circuit is provided to allows the voltage on the system to be monitored.

2.3.2.8 Dump Relay The relay is a Ross high-voltage relay rated at 25 kV. It has a double break contact system and is spring-loaded in the closed position. It is operated by a 230 V ac coil. To minimise heating in the coil during operation the voltage is switched to a lower level once the relay has pulled in. An auxiliary switch indicates the relay state to the control system.

2.3.2.9 Earth Switch The earth switch is a manually operated and provides a permanent earth connection to the capacitor bank. It is interlocked via mechanical (Castell) keys to ensure the rear doors are closed before the earth switch can be opened. Once the earth is removed another key allows the capacitor charging HV power supply to be energised.

2.3.2.10 Crowbar It is essential that the valuable tetrode valve be protected from an overcurrent fault, this protection is provided by the crowbar circuit whose function is to rapidly reduce the charge stored in the capacitors and hence reduce the voltage over, and the current through the fault. The crowbar switch is a National Electronics NL508A ignitron, a mercury switch. The anode is heated to prevent mercury condensation during use; this is achieved by local heaters powered from a 230V 30 kV isolation transformer. The ignitron is fired by a current pulse from a North Star firing unit which is triggered via a pulse from a fibre optic cable. Detection of an overcurrent is from a Stangenes current transducer (10 A/V) and a solid-state comparator circuit which compares the actual current during the pulse with a pre-set trip level. If this is exceeded for more than a few micro-seconds the circuit activates its optical output and the crowbar fires, discharging the capacitor and protecting the 4616 tube from arc damage. The crowbar heater is monitored via a current monitoring relay; a timer prevents the high voltage power supply being switched on until the crowbar is at working temperature.

2.3.3 Auxiliary Rack (4616) In addition to the HT modulator, the 4616 valve amplifier requires power supplies and modulator circuits for regulation of its control grid and screen grid voltages, and also power for its heater system. These functions are provided by the auxiliary rack which also contains the front end of the RF chain (the oscillator and the SSPA), as well as timing control circuits. The subsystems are detailed below:

2.3.3.1 RF Signal Generator This unit is a tuneable synthesised RF source and it generates the stable 201 MHz signal for the RF system. In addition to defining the frequency for the system, it also controls the power level into the amplifier chain. Once cavities are installed in the ICTF its role will be taken by the LLRF system. The output of the synthesiser is connected to the input of the Solid-state RF Amplifier.

2.3.3.2 Programmable Logic Controller (PLC) The PLC is built into a 2U case and uses Siemens S7-200 series hardware; it comprises a central processing unit (CPU) with digital input and relay output modules plus some analogue inputs.

2.3.3.3 Filament Heater Power Supply The 4616 tube requires its cathode to be heated at a current of 500A 0.95V. A step-down transformer adjacent to the tube provides this voltage from a 110V ac input. The power supply uses a Eurotherm TE10P 16A constant power thyristor module to control the heater power. It is linked to a Eurotherm 3504 Controller which provides functions such as current ramp-up and ramp-down (necessary to prevent thermal stressing of the heater and to minimise bulb current effects) plus an interface to the manual control switches and the feedback control of the heater power. The TE10P operates at 230 V ac so a 230:110V step-down transformer is incorporated to match the 110V transformer in the amplifier assembly.

2.3.3.4 Grid DC Power Supply The grid power supply is a Xantrex XFR 300-4 DC power supply rated at 300V 4A. It is controlled manually via its front panel to set the Grid 1 voltage on the 4616 amplifier tube. A Transient Voltage Suppressor is fitted at its output terminals to protect against any transient voltages which may be generated from the 4616 tube. It is typically set in the range between 150V to 200V.

2.3.3.5 Screen Pulser Power Supply The screen pulser power supply is a pulse power unit designed to provide the trigger pulse to the 4616 screen (grid 2) to turn the valve on. The pulse rating is up to 2000 V for 1 ms at a repetition rate of 1 Hz. The unit takes the DC from its associated dc power supply and charges up a capacitor to typically 330 V dc. This is then discharged via the screen by an IGBT (Insulated-gate bipolar transistor) switch via a 6:1 pulse transformer to give a pulse up to 2000 V amplitude. Control of the pulse amplitude is achieved by adjusting the charging voltage of the capacitor, i.e. the output of the DC Power Supply. The pulse timing and duration is controlled via an input trigger connector which receives the timing signal from the main system pulse generator.

2.3.3.6 Screen DC Power Supply The screen dc power supply provides the bulk dc power to the screen pulser. It is a Xantrex XFR 600- 2 DC power supply rated at 600V 2A. It is controlled manually via its front panel to set the capacitor charging voltage for the Screen Pulser Power Supply.

2.3.3.7 Pulse Generator The timing for the various pulses required for system operation is provided via a 4 channel Quantum Composers pulse generator unit mounted in the auxiliary rack. For the purposes of the tests undertaken in the TIARA project it operates as a stand-alone unit generating pulses for:

 4616 Screen Pulser  RF Solid-state Amplifier (Dressler)  TH116 Cathode modulator

2.3.3.8 Solid-state RF Amplifier (Dressler) The RF signal from the synthesised oscillator is first amplified by a solid-state amplifier rated to 4 kW, this provides the power level required to drive the tetrode valve. Its output is pulsed at a rate set by a trigger from the Pulse Generator.

Figure 7: TH116 Amplifier System Power Supplies: Overview of key subsystem

2.3.4 40 kV Power Supply (TH116) The 40 kV power supply, illustrated in Figure 7, contains the HT modulator and certain supporting subsystems for the final stage amplifier system, the subsystems are detailed in the following sections.

2.3.4.1 Lamp Display Unit This is a 2U plate with two lamp units mounted on it, to provide a clear indication of the status of the system. One is illuminated red when the HV power supply is turned on; the other is illuminated green when the system is earthed.

2.3.4.2 Mains Power Module The mains power module contain the protective circuit breakers, distributes the mains power (3- phase input) via several rear-mounted connectors. It also contain a contactor which controls the output power to the charging power supply; this contactor is activated via the mechanical Castell key switch and a digital output from the control unit

Figure 8: HT power supply rack for the final stage Thales TH116 Triode valve amplifier

Figure 9: Illustration of key components in the TH116 valve power supply, on the left is a prototype e2v thyratron rated for high charge switching installed as the crowbar switch, and on the right is the cathode modulator.

2.3.4.3 Capacitor Charging Power Supply The capacitor charging power supply is a rack mounting air-cooled unit; a Lambda 802L rated at 40 kV, 8000 J/s. It is controlled via its front panel controls. It can be remotely controlled but this facility is not used for the tests undertaken within the TIARA preparatory phase programme.

2.3.4.4 Control Unit The control and interlocking of the HV power supply is undertaken by the Control Unit, a 4U crate. This is virtually identical to the unit used on the 20 kV power supply. It has LED indicators to show the status of the system:

2.3.4.5 HV Capacitor The energy storage for the power supply is accomplished by two General Atomics HV capacitors each rated at 70 uF, 40 kV connected in parallel. Each has two HV bushings so the steel case is not part of the circuit. These are particularly high energy storage density units allowing the supply to remain relatively compact.

2.3.4.6 Potential Divider The potential divider consists of a series of high voltage resistors mounted in series on an insulated board and is provided to allow the voltage across the capacitors to be monitored during the discharge. The output voltage is taken from the last resistor in the series giving a 10000:1 ratio. The output voltage is routed via a unity gain buffer amplifier allowing it to drive the meters in the control unit whilst drawing minimal current from the system.

2.3.4.7 Dump Relay The relay is a Ross HV unit rated at 60 kV. It has a double break contact system and is spring-loaded in the closed position. It is operated by a 230 V ac coil. To minimise heating in the coil during operation the voltage is switched to a lower level once the relay has pulled in. An auxiliary switch indicates the relay state to the control system.

2.3.4.8 Earth Switch The earth switch is a manually operated and provides a permanent earth connection to the capacitor bank. It is interlocked via mechanical (Castell) keys to ensure the rear doors are closed before the earth switch can be opened. Once the earth is removed another key allows the capacitor charging HV power supply to be energised. An auxiliary switch indicates the relay state to the control system.

2.3.4.9 Crowbar The crowbar switch is an e2V model HX3002 thyratron, a deuterium gas switch. It has cathode and reservoir heaters which are monitored and interlocked to the control system. Grid 1 is held at a negative potential to inhibit firing. Grid 2 is pulsed at around 1000V to fire the unit; the pulse being obtained from an e2V model MA2438B firing unit with a fibre-optic input. Detection of an overcurrent is from a Stangenes current transducer (100 A/V) and a solid-state comparator circuit which compares the actual current during the pulse with a pre-set trip level. If this is exceeded for more than a few micro-seconds the trigger activates and the crowbar fires discharging the capacitor and protecting the TH116 tube from arc damage. Given the limited availability of the Thales TH116 valves, this is particularly important. See Figure 9 for an illustration of this component.

2.3.5 Auxiliary Rack (TH116) The TH116 auxiliary rack contains the following equipment to regulate the valve heater and grid power supply.

2.3.5.1 Programmable Logic Controller (PLC) The PLC uses Siemens S7-200 series hardware; it comprises a central processing unit (CPU) plus several digital input and relay output modules.

2.3.5.2 Filament Heater Power Supply The TH116 tube requires its cathode to be heated at a nominal current of 500 A 20 V. A step-down transformer adjacent to the tube provides this voltage from a 230 V ac input. The power supply uses a Eurotherm TE10P 63A constant power thyristor module to control the heater power. It is linked to

16 a Eurotherm 3504 Controller which provides functions such as current ramp-up and ramp-down (this prevents bulb inrush and thermal stressing of the heater, prolonging the life of the high value valves) plus an interface to the manual control switches and the feedback control of the heater power.

2.3.5.3 Cathode Bias Power Supply (450 V dc) A linear unregulated dc power supply provides a bias voltage to theTH116 cathode circuit. During the pulse the output current from the bias power supply is limited by an internal resistor to around 27 A. This current passes through the IGBT switch of the cathode modulator. During the rest of the cycle the current draw is very low as the cathode modulator switch is open leaving a 15k resistor in circuit.

Figure 10: LHS showing the electrical cabinet and RHS showing the cooling and electrical power circuits installed in the base of the triode amplifier assembly

2.3.6 TH116 Amplifier Enclosure The TH116 amplifier enclosure contains various items of equipment apart from the amplifier tube itself and its input and output RF circuits. See Figure 10.

2.3.6.1 Cathode Modulator The cathode modulator is built into a die-cast metal enclosure located in the base of the TH116 amplifier circuit. An IGBT switch connected across a 15 kΩ resistance operates from an external trigger pulse; when closed the series resistance between theTH116 cathode and ground is reduced from 15 kΩ to about 3 Ω which puts the TH116 into conduction. A bank of series and parallel connected zener diodes, each rated at 200V, ensure the voltage at the IGBT cannot exceed 600V. See Figure 9.

2.3.6.2 Cooling Equipment The main air cooling comes from two large fans each powered by a three-phase motor which blow air into the amplifier circuits. Two vacuum switches detect air flow from these and indicate cooling is present to the control system. Additional cooling is provided by a compressed air feed which is interlocked to the control system via a pressure switch. The tube itself is water cooled using demineralised water suitable for HV operation.

2.3.6.3 Filament Heater Transformer The main step-down transformer is located in the base of the TH116 amplifier enclosure and is rated at 10 kVA. It provides an output of 500A at 20 V maximum. A centre tap on the secondary winding provides the cathode attachment point.

2.3.6.4 Electrical Cabinet The electrical cabinet contains the electrical switch gear for the fans and a small programmable relay which sequences the fan start- ups a few seconds apart. It also houses the relays which activate the motorised tuning controls for the amplifier. These are manually driven from remote push-buttons. Linear potentiometers give a read-out of the position of each adjustable section. See Figure 10.

3 Tests of the Prototype Amplifier The tests were undertaken in two phases, the high power tests were undertaken in a specially built RF test facility at the Daresbury laboratory, shown in Figure 2, and once the required performance had been demonstrated, in an environment conducive to fine tuning the system with good space and access, the system was transported to the ICTF at the Rutherford Appleton Laboratory and operated in situ.

3.1 Demonstration of required RF performance The first tetrode amplifier has been commissioned to 240kW at 19kV anode voltage while providing up to 20dB of gain and up to 65% conversion efficiency at 1Hz 1mS. The intermediate amplifier stage proved to be very stable and forgiving, with good response over a wide range of drive conditions, with little requirement for retuning and could be controlled simply via its drive level from the SSPA. The performance is summarised in the graph shown in Figure 11.

Figure 11: Graphs showing the variation of the intermediate amplifiers output power and zeroth order Fourier component of the anode current as a function of drive power, anode voltage 19kV, control grid bias 170V and screen grid at 1.74kV

The output of the tetrode was coupled into the triode amplifier and the testing program cumulated in July 2013 with the successful operation of the whole amplifier system producing the required 2MW of RF power at 1mS 1Hz. The RF and electrical parameters of each amplifier were fully explored during this operation and the system optimised by raising the drive until the triode gain fell to around 10dB then increasing the bias voltage and continuing to raise the drive until the required performance specification was met. This process is illustrated in Figure 12 whilst Figure 13 shows the power meter displaying the pulse power level.

In the final operating condition, the drive from the synthesised oscillator was 3.7dBm yielding 2.27kW from the SSPA. The tetrode was operating at 18kV bias voltage and drawing an average current in the pulse of 15.5A, with 61% efficiency and 19dB gain yielding an output power of 170kW. The triode operated at 34kV bias voltage drawing an average anode current of 129A, operating at an efficiency of 46% and with a gain of 10.8dB developing 2.06MW of output power. The triode presented an input match with a return loss of -12.5dB (VSWR of 1.6) to its driver.

Figure 12: Illustrating the variation of triode output power with bias voltage

Figure 13: The power meter reading on the output from the final stage amplifier demonstrating the required peak output power

3.2 Installation and operation in ICTF Following the demonstration of the required performance in the test facility at Daresbury, the next step was to demonstrate that this system could be installed and operated in the MICE hall. This was required since, although the amplifier system was relatively compact for a system with this specification, both it and its essential auxiliary systems (cooling systems using air and demineralised water) are large in the context of the relatively confined space in the MICE hall.

The process commenced from late summer 2013 with the dismantling of the prototype amplifier at Daresbury and the transportation of the system to the Rutherford Laboratory where it was installed behind the shield wall. The installation is shown in Figure 14. The priority delivery, of key aspects of the distribution network from the University of Mississippi, as a small part of the overall procurement for the MICE distribution network funded by the US MRI programme, provided the parts required to install this system in these tight confines. The load, several elbows, directional couplers and lengths of 3”, 9”, 6” lines are visible in Figure 15.

Figure 14: Installation for TIARA tests in the MICE hall. Shown are 4 of the 5 racks (one rack not visible held the SSPA, Oscillator and timing pulse generator), consisting of the auxiliary and main PSU racks for the tetrode and triode amplifiers. The tetrode amplifier is visible on the mezzanine whilst the triode amplifier is the tall purple structure on the ground floor with the valve cap protruding through the mezzanine.

The installation required cooling systems to be provided in the MICE hall, including high quality de- ionised water and compressed air. A water cooling distribution panel was designed and installed to provide cooling to triode and tetrode systems and the RF Loads. This also provided monitors for the flow rate and coolant temperature, and provided interlocks to protect the amplifiers; the water panel can be seen in Figure 15. Figure 15 also shows the process of installation of the triode amplifier (which had to be substantially dismantled to facilitate transportation) and also shows the amplifier installed with the 3” input line from the tetrode in the foreground and the 6” output line for the triode amplifier also visible (A temporary load, for the 4616 amplifier, is also visible next to the left-hand wall). The inset shows the location of the triode valve in its socket seen with the ventilation top cap removed and viewed from the mezzanine. Figure 16 illustrates the configuration of the mezzanine showing the main pulsed power supply and charging power supply racks, with auxiliary racks (heaters and control systems) for each of the amplifiers.

Figure 15: Images of the installation of the final stage amplifier and showing the completed installation with the water distribution system and the MRI funded loads and RF distribution components

Figure 16: Installation of the power supply, auxiliary and control systems, also showing the tetrode amplifier and the top cap of the triode amplifier.

The tight confines of the space available under the mezzanine necessitated the use of compact loads procured primarily to act as reject loads for the final installation, this limited the immediately attainable power to about 500kW, this being the maximum peak power the manufacturer would

21 warranty the devices to achieve safely. The tetrode amplifier was commissioned on the 11th and 12th December whilst the Triode was tested on the 19th December. After a very little initial tuning, both devices performed in the same manner they had at Daresbury and soon developed very substantial levels of output power. Again the system was tuned by raising the drive until the triode gain fell to around the 10dB point, and then the triode bias was raised, followed by further incremental raising of the drive. Following this two parameter cycle allowed the triode to develop an output power of 527kW with a gain of 10.8dB at 22kV bias and with 44kW drive from the tetrode. Figure 17 illustrates the readouts on the power meters in this operating configuration. X-radiation was monitored throughout the tests using an ionisation chamber and a scintillation counter.

Figure 17: Left hand shows the power meters gated measurement of the output power from the triode amplifier whilst on the right we see the input drive from the tetrode and the reflected power back from the triode

This demonstrates the operation of the amplifier chain in the MICE hall to the limits of the loads which can be installed at this time. In realising this result the TIARA project has, in addition to direct support for the project enabled substantial gearing, since in addition to the components provided for the tests of the prototype amplifier, the US MRI funding has delivered some $1M (US) or more of equipment to realise the final RF distribution network for the ICTF, shown in Figure 18.

Figure 18: MICE Step VI distribution network, all components procured through University of Mississippi MRI grant, installation to commence from January 2014

4 Summary A prototype RF amplifier system has been designed, built and tested, meeting or exceeded the required design parameters in these initial tests. This includes the building of the RF amplifier circuits, valve cavities and tuning systems and the installation of cooling, control and load and diagnostic systems in both the test facility at Daresbury and the ICTF at Rutherford. It has also encompassed the building of the prototype power supplies. The tests showed that the intermediate amplifier can achieve over 240kW, and the triode amplifier can deliver in excess of 2MW with a drive of 170kW. The practicalities of installing the amplifiers at the ICTF have been proven and the amplifiers shown to operate in that environment. The RF cavities and circuits, for the next three amplifiers, have been completed. Now that the amplifiers and power supplies have been proven in situ, the buildup of repeat units may be undertaken with confidence.

5 Acknowledgements Vital aspects of this research project were funded by the EU through the TIARA programme and through the UK STFC, the US DoE and NSF (MRI). The MICE/ICTF RF teams are appreciative of the support of their sponsors. The authors would like to thank colleagues who made a major contribution to the project, including Don Abram, Alex Dick, Andrew Gallagher, Steve Griffiths, Philip Jeffrey, Andy Nichols, Nigel Rimmer, Nick Sabin, David Speirs, Mark Surman and Simon Windsor for their scientific, engineering and/or technical expertise and work in assembling the amplifiers and the test systems at both the Daresbury test facility and at the ICTF.'

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