High Power IOTs

Morten Jensen

www.europeanspallationsource.se FCC, Washington, 24 March 2015 The Inductive Output Tube

Invented in 1938 by Andrew V. Haeff as a source for radar

 To overcome limitation of output power by grid interception  Pass beam trough a resonant cavity  Achieved: 100 W at 450 MHz, 35% efficiency

Used first in 1939 to transmit television images from the Empire State Building to the New York World Fair

IOTs then lay dormant  Intense competition with velocity modulated tubes ( had just been invented by the Varian brothers)  Difficult to manufacture

The IOT is often described as a cross between a klystron and a hence Eimac’s trade name ‘Klystrode’

Why use IOTs

 High efficiency at point of operation  Small  Power is pulsed by RF instead of HV  Good Linearity X Lower Gain

Good for machines which require the to operate at different power levels – varying power loads – Non uniform power profiles – Margins for overhead for regulation – One-to-one relationship with amplifier to accelerating structure

Proton Linacs in particular can benefit Circulating machines with high regulation requirements

Often what prohibits technology is the lack of demand 3

The Main Principles

Klystron (Density modulated) RF input RF output Cathode (DC Beam) Collector

IOT (Density modulated)

RF input

Biased RF output IOT and Klystron Comparison

Klystrons: IOTs: • Velocity modulated DC beam • Density modulated beam – Grid controls current • Maximum theoretical efficiency is 100%, • Maximum theoretical efficiency is also same as for Class C amplifier 100% – Beam to be infinitely short – Needs zero length bunches which – Cross the gap in zero time means zero current! – reduces efficiency – Cross the gap in zero time – Efficiencies of > 80% possible but – Class B operation more common, 65% typical at saturation efficiency 70-75% typical • Klystron saturates at maximum power • IOT does not saturate as a klystron • Efficiency drops rapidly for operation at • Efficiency drops slowly at reduced output reduced output (DC input is constant) power • Collector has to handle full DC beam • Low pushing factor; helps power supply (e.g. at Eff. 50% Pcol = twice RF power) design • Collector only ever handles spent RF beam (e.g. at Eff. 50% Pcoll= RF power) 5

Efficiency comparison of and IOTs

IOT measurements courtesy of M. Boyle, L3 IOT • Based on broadcast IOT L-4444 • System setup limited Klystron by drive power and beam voltage • IOT setup for maximum gain (not efficiency) without breakdown • No optimisation of coupling, grid • Klystron assumed to have same saturated voltages etc. for efficiency as the IOT different power levels • No optimisation of coupling, voltages, perveance for different power levels 6

Typical Broadcast IOT

Courtesy of e2v 3rd Generation Light Sources

Three 500 MHz 300 kW amplifier for 6 RF plants of 160 kW Storage Ring: 4 x 80 kW IOT combined 500 MHz One 80 kW for the Booster 2 IOTs combined per cavity IOTs from E2V IOTs from Thales Electron Devices and L3

8 Selection of other Facilities using IOTs

NSLS II CERN L3 IOT, 500 MHz 800 MHz 80 kW CW 60 kW

Metrology Light Source (Willy Wien Laboratory) CPI 90 kW IOT (K5H90W1)

Elettra 500 MHz 150 kW IOT based amplifier for Combination of 2x80 kW

9 700 MHz HOM IOT Experience

RF Input Design Parameters value units Power Output 1000 kW (min) Beam Voltage 45 kV (max) Beam Current 31 A (max) Frequency 700 MHz Gun 1dB Bandwidth ± 0.7 MHz (min) Gain 23 dB (min) Efficiency @ 31kV 71 % (min) Diameter 30/76 in/cm Height 51/130 in/cm Solenoid, Weight 1000/450 lbs./kg Collector Coolant Flow 220 gpm O/P Cavity Body Coolant Flow 10 gpm O/P Window Cooling (Air) 35 cfm RF Output

Collector

Test Results (pulsed)

VHP-8330A IOT 10 Other IOT technology

IOTs designed for various applications Series production has been < 100 kW

267 MHz Depressed collector IOTs 300 kW CW

L3 wide band IOT 11 The ESS Requirement

Opportunity to develop Super Power IOT Accelerating Structure Freq. Quantity Max Beam (MHz) Power (kW) RFQ, DTL 352 6 2200** Spoke 352 26 330** Elliptical Medium Beta 704 36 860**

Elliptical High Beta 704 84 1100**

** Plus overhead for control The ESS Superconducting Power Profile > 150 cavities/couplers

1 RFQ and 5 DTL tanks 352 MHz 2.8 MW Klystrons Total Power: 13 MW 84 High Beta 26 Cavities 704 MHz (5 cell) 352 MHz 1.2 MW IOT 2*200 kW MB Klystron as backup Total: 90 MW Total: 8 MW 36 Medium Beta 704 MHz (6 cell) 1.5 MW Klystrons Total: 22 MW

Total High Power RF: 133 MW peak (4% duty) plus overhead An IOT for ESS

Parameter Comment Frequency 704.42 MHz Bandwidth > +/- 0.5 MHz Maximum 1.2 MW Average power during the pulse Power RF Pulse length Up to 3.5 ms Beam pulse 2.86 ms Duty factor Up to 5% Pulse rep. frequency fixed to 14 Hz Efficiency Target > 65% High Voltage Low Expected < 50 kV Design Lifetime > 50,000 hrs Work is being carried out in collaboration with CERN  ESS to procure prototypes 3.3 MW power  CERN to make space and utilities available for testing reduction by using IOTs Target: Approval for ESS series production in 2017/18 for High Beta Two IOTs to be delivered in 2016

• Two Multi-Beam IOTs being designed – Thales/CPI Consortium – L3 • Contracts signed in September 2014 • Project duration: 24 months • Long term testing at CERN • Approval for series tender 2017/18

15 Multi-Beam IOTs for ESS

10 Beam Multi-Beam IOT 1.2 MW 704 MHz Output Cavity and DC Beam Studies Courtesy of L3 Communications

• Ten beams on a single bolt circle • Output cavity supports a large number of modes • HFSS used to map modes near harmonics of the drive frequency

Fundamental Near Second Near Third Harmonic Harmonic

TM at 704 MHz TM1,24,0 at 2124 MHz 1,0,0 TM1,16,0 at 1417 MHz

Beam transport simulations

17 Beam transport and RF Interactions Courtesy of L3 Communications

Beam profile Propagated in the presence of the imported here magnetic field imported from Maxwell Output Drift tunnel Gap Drift tunnel taper

Particles shown at 4 phases of an RF cycle

18 RF Output Circuit and Output Window Courtesy of L3 Communications

• Air cooled SLAC design Coaxial window from B-factory klystron – 1.2 MW CW operation, 476 MHz – TiN coated and has modest peak electric field

SLAC Proprietary • T-bar Coax-to-Waveguide transition in air • Design was rescaled Concept provided by SLAC for 704 MHz using HFSS Return Loss = -30 dB at 704 MHz 2 1.8 1.6 1.4 1.2 1 36kV Operational Optimisations 0.8 40kV 0.6 45kV

Output Power [MW] Power Output 0.4 0.2 50kV Courtesy of L3 Communications 0 0 2 4 6 8 10 12 14 16 Input Power [kW] 2 1.4 1.8 36kV 1.2 1.3 MW 1.6 40kV 1.4 1 45kV 70% eff 1.2 0.8 1 50kV 36kV 0.60.8 40kV 0.6 0.4 45kV

Output Power [MW] Power Output 0.4

0.20.2 50kV Body Current Per Beam [A] Beam Per Current Body Power and Efficiency 0 0 0 0 2 2 4 4 6 6 8 8 1010 1212 1414 1616 Impact of HV Input Power [kW] Plot shows maximum Input Power [kW] 1.4 0.80 36kV achievable efficiency for 0.751.2 40kV 0.70 1 0.65 45kV

various operating points 0.600.8 50kV 0.55 0.6 36kV

0.50 Efficiency 0.450.4 40kV 0.400.2 45kV 0.35[A] Beam Per Current Body 50kV Increased beam voltage provides 0 0.30 0 2 4 6 8 10 12 14 16 78 0 2 4 6 8 10 12 14 16 for better performance Input Power [kW] 76 Input Power [kW] 0.80 • Increases gain 0.7574 0.70

0.6572

• Increases efficiency 0.60 70 Efficiency[%] 0.55 36kV 0.50 Efficiency 68 • Decreases body current 0.45 40kV 0.4066 45kV Simulations are for 10 beams 0.3535 40 45 50 55 50kV60 0.30 Voltage [kV] 0 2 4 6 8 10 12 14 16 Input Power [kW] MAGIC Prediction of MB-IOT Performance Courtesy of Thales and CPI

Power Transfer Curve Efficiency & Gain vs Output Power 1.2 MW Efficiency

Gain

Vk = 48 kV Class-C

MAGIC-3D simulation of one beam with MB-IOT off- axis B-field • At 1.2 MW, h = 72% with Vk = 48 kV • At 600 kW

• h = 59% with Vk = 48 kV • h = 68% with Vk = 34 kV

Future Prospects

Combined amplifier and ESS requires 1.2 MW plus overhead High current accelerating cavity • Short development time available emission structures • Preference for low voltage cathodes • ‘Proven’ technology • Factor of 10 up in power Optical emission cathodes for ultra high Future machines require: repetition rate • More power another factor of 10? • Better efficiency • Better reliability Energy recovery / • Smaller footprint, etc Grid controlled emission Depressed collectors with bunch forming High voltage, 1 MW, cavities single cathode Double grid devices and current 10 MW MBIOT by manipulation for bunch shortening combination of 1 MW • Efficiency, higher frequency tubes 22 Short Bunch Length for High Efficiency Courtesy of L3 Communications

80

• L-3 developed a novel 70 technique for very short 60 pulse generation 50 40

• Bunch is formed, then [A] Current 30 accelerated to high voltage 20 10

• Electrons arrive at the 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 output cavity over a very Time [ns] small portion of the RF • Simulation performed with cycle, with very small MICHELLE-TD of a IOT-type gun energy spread designed to produce short bunches • FWHM of electron bunch is 3.6 degrees of RF cycle

23 Acknowledgements

Special thanks to

Thales, CPI and L3 for agreeing to publish some of the design details, calculations and predictions