! ESS AD Technical Note ! ESS/AD/0033 ! ! ! ! ! ! !!!!!!!!!! ! !!!Accelerator Division ! ! ! !
! ! ! ! ! ! Comparative Overview of
Inductive Output Tubes
Rihua Zeng, Anders J. Johansson, Karin Rathsman and Stephen Molloy
Influence of the Droop and Ripple of Modulator onRebecca Klystron SeviourOutput
June 2011
23 February 2012
I. Introduction
An IOT is a beam driven vacuum electronic RF amplifier. This document represents a comparative overview of the Inductive Output Tube (IOT). Starting with an overview of the IOT, we progress to a comparative discussion of the IOT relative to other RF amplifiers, discussing the advantages and limitations within the frame work of the RF amplifier requirements for the ESS. A discussion on the current state of the art in IOTs is presented along with the status of research programmes to develop 352MHz and 704MHz IOT’s.
II. Background
The Inductive Output Tube (IOT) RF amplifier was first proposed by Haeff in 1938, but not really developed into a working technology until the 1980s. Although primarily developed for the television transmitters, IOTs have been, and currently are, used on a number of international high- powered particle accelerators, such as; Diamond, LANSCE, and CERN. This has created a precedence and expertise in their use for accelerator applications.
IOTs are a modified form of conventional coaxial gridded tubes, similar to the tetrode, although modified towards a linear beam structure device, similar to a Klystron. This hybrid construct is sometimes described as a cross between a klystron and a triode, hence Eimacs trade name for IOTs, the Klystrode. A schematic of an IOT, taken from [1], is shown in Figure 1. Similar to a tetrode the initial part of an IOT consists of a cathode with a control grid in front, the drive RF causes the beam passing through the grid to undergo modulation, operating the grid in this fashion can enable the IOT to act as a Class C amplifier. The beam is then accelerated by passing through a high voltage DC region, where magnetic lens focus the modulated high-energy electron beam through a small drift tube, similar to a klystron. This drift tube prevents the backflow of electromagnetic radiation. The bunched electron beam passes through a resonant cavity, equivalent to the output cavity of a klystron. The electron bunches excite the cavity, and the electromagnetic energy of the beam is extracted by a coaxial transmission line [1].
The highest frequency of operation achievable in an IOT is limited by the grid-to-cathode spacing. The electrons must be accelerated off the cathode and pass the grid before the RF electric field reverses direction, the limit in voltage stand-off to frequency dependence limits the upper frequency attainable of IOT’s to approximately 1.3 GHz.
Figure 1 schematic of an IOT
One key issue, common to all gridded tubes, is the effect of heat from the cathode evaporating cathode material which then condenses on the grid, as material accretes on the grid the gap between grid and cathode narrows eventually electrically shorting the cathode and grid. In addition, the emissive cathode material on the grid causes a negative grid current, although using modern coatings/materials on the cathode/grid can mitigate these effects.
IOT’s by virtue of their design can operate as Class C amplifiers, with achievable efficiencies between 70-80% obtained in operation. IOTs also offer a monotonic dependence of Pin to Pout, as shown in Figure 3, where the maximal efficiency is obtainable over the power range of operation. Due to the simplicity of design the IOT represents a very compact (electrically short) RF amplifier with gains usually between 20-23 dB. Modern IOT designs are able to achieve an average lifetime of approximately 40 K hrs, with the most common failure arising from damage to the cathode.
III. ESS Requirements
The ESS requires a number of RF sources and amplifiers at 352MHz and 704 MHz, operating over a range of RF powers. Current thoughts are to use 1MW 704MHz Klystrons to supply the medium beta and high beta cavities. For the spoke cavities the current proposal is to use 28 tetrodes at 352 MHz with a forward power of 360KW. These tetrodes will be driven, by 20KW solid-state RF amplifiers/sources. For the DTL tanks and the RFQ 1MW 352 MHz Klystrons will be used. These requirements also have to be balanced against the other key deliverables of the ESS, namely achieving at least 95% availability and as a green “carbon neutral” facility. This necessitates that all technologies used in the ESS must be highly efficient, with long life times and high reliability. This last requirement is critical for the RF sources, where the majority of failures and energy losses occur in current accelerator facilities.
Currently no manufacture has a working 704 MHz klystron, Thales is working on a 704MHZ prototype for CERN. Thales does market a 1MW 352MHz klystron suitable for application in the ESS, and this model has already been tested with parameters similar to those required for operation in the ESS. Existing tetrodes have been identified (TH 781) that can supply the 360KW power at 352 MHz, although the current identified tube is required to operate outside the manufactures specified operational range.
IV. Comparison with other RF amplifiers
At this point it is salient to compare IOTs with other RF amplifiers, such as the; Klystron, Tetrode, Triode, Traveling Wave Tubes, and solid-state. Solid-state devices can act both as oscillators and amplifiers, with efficiencies of over 85%, although this technology does have a number of issues, including current technologies readiness state, and will be discussed in greater detail in a separate document. Other vacuum electronic amplifiers such as the Gyrotron and Bakertron only operate at frequencies well beyond the range of interest for the ESS and hence are not discussed here. Magnetrons do offer high power, high efficiency, at frequencies ideal for the ESS, and are cheap to manufacture, but the innate phase drift inherent from the principle of operation means multiply sources need to be phased locked [2]. Complex schemes for locking the phase of magnetrons do exist [2], but the associated cost, physical footprint, and technology readiness state renders magnetrons currently unsuitable for deployment in the ESS. The Traveling Wave Tube (TWT) is a beam driven, vacuum electronics, amplifier, the principle of operation is the synchronization between wave dispersion curve and electron beam, at periodic points to initiate energy transfer [3]. The principle is to effectively slow the wave down to match the velocities of wave and beam, this is achieved in a variety of ways (folded wave guide, Helix, dielectrically loaded), but the result is the same, a slow wave structure. Although simple in concept TWTs present the most challenging vacuum electronic amplifier to manufacture, resulting in the largest cost per W to produce. TWTs operate with varying degrees of efficiency, ranging from Class A-F, over frequency ranges from 300MHz to 100 GHz. The maximum power output that has been achieved to date from a TWT is 15KW[3], with very low efficiency. The low power and high cost make TWT unsuitable for accelerator applications compared to alternative amplifiers.
Tetrode/Triode Both Tetrodes and Triodes belong to the same class of vacuum electronics as the IOT, gridded vacuum tubes. One key advantage of the IOT over both the tetrode and triode is gain, tetrodes and triodes offer a gain of between 13-15dB [3], with efficiencies around 60% (c.f. TH391 [4]). The gain of a tetrode can be increased but at the expense of both efficiency and the maximum sustainable drive power. In comparison the IOT offers a gain of between 20-23 dB, with an efficiency of >70% [3]. Although the most important discriminator is average life-time, tetrodes and triodes have expected life- times <20K hrs [5], compared to the IOT with lifetimes >40K hrs. For example in 2010 CERN had to replace 25% of their Tetrodes on the LHC [6]. The cost of an individual Tetrode is the cheapest HPA (compared to IOT/Klystron), the cost due to replacements due to failure (2-3 Tetrodes replacements per IOT/klystron replacement) balances over all cost.
The anode of a tetrode is part of the output circuit and therefore restricted in size, and operated at high-voltage and hence more suited for forced air-cooling than for water-cooling. This limits the tetrodes power capability and shortens its lifetime expectancy. The collector of an IOT is entirely separated electrically and grounded, enabling water cooling, and hence can be designed for almost any power capability, thus contributing to high lifetime figures [8].
Other options considered for the ESS include using the TH781 tube to provide RF power for the spoke cavities at 352 MHz with 360KW power. Experience in operation of the TH781 has been gain from LANSCE [7]. Figure 2 shows the operation of the TH781 tetrode at 200MHz, not that for 360KW power the gain is 13.5 dB and the efficiency is 50%. Rolling the tube outside of its operational range to meet an ESS requirement of 352MHz is possible, but the efficiency will be reduced to approximately 40%. This difference in gain between tetrode and IOT has a marked effect on the cost of the drive RF source and substantially reduces the cost difference between the IOT and tetrode.
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Amplitude sensitivity to beam supply variation is also different between the1080 klystron and IOT. The fundamental- frequency value of cathode current is directly determined by the RF input, and is relatively independent of beam voltage. For a given value of RF output there will be an optimum beam voltage. Less than optimum voltage saturates the output, leading to excessive body current (an effect also seen in klystrons). Excessive beam voltage produces higher collector dissipation. Small voltage variations around the optimum value have much less effect on RF power output than comparable variations in a klystron.
The efficiency of a Klystron is highly dependent upon drive power, with maximum efficiency only available at saturation. Although for effective control over the phase/frequency the Klystron must be operated off saturation, i.e. below maximal efficiency. This is not true for the IOT where the monotonic dependence of Pin vs Pout means an IOT operates at maximal efficiency at points below maximal power, as shown below in Figure 3.
Figure 3 Klystron and IOT Pin vs. Pout. Taken from presentation made to ESS by CPI.
V. State of the art
As the IOT was primarily developed in the 1980’s as a klystron replacement for TV transmitters, availability of current off-the-shelf IOTs is limited in frequency to 470-800 MHz, with power ranges up to 500 KW CW, and 1 MW pulsed. To the best information available, currently, no manufacturer provides IOTs which would meet the ESS specifications, of 352MHz at 360KW/1MW or 704MHz at 1MW. Although recently interest in the use of IOT’s for particle accelerator, security and high-energy directed weaponry is stimulating interest in the development of IOTs suitable for high- power, high/low-frequency applications, which would also meet the ESS requirements. The current status of all know (to the author) companies, research projects to establish high-power IOT’s are listed below; a) Calabazas Creek Research, Calabazas Creek Research Incorporated (CCR) is a research organization with extensive expertise in R&D of vacuum electronics. CCR has recently completed fabrication and assembly of a 350 MHz, 200 kW CW, Multiple Beam IOT (MBIOT) prototype. The target efficiency is 70% with a predicted gain exceeding 22 dB. All parts have been received and fabrication is in progress with an expected completion date of September 2012, with testing scheduled for fall 2012 [9]. The specifications of this tube would be ideal for application within the ESS. b) CPI, CPI are one of the largest manufactures of vacuum electronics, with extensive expertise in IOT research and manufacture. CPI does not currently have an IOT that would meet the ESS requirements the company does have tubes similar in specification, such as, the 2KDW250PA (250 kW CW at 267 MHz), and the SDI IOT (500 kW peak, 50 kW avg. at 425 MHz..
CPI \ previously had an intensive research programme into developing a high-powered (1MW) coaxial IOT. CPI is considering reinitiating this development programme for the ESS, with the right motivation. To this end they have provided the following schedule for development;
352 MHz, 400KW, >70% efficiency, 23 dB gain. • Prototype – 15-18 MARO • Production – 2-4/Month at 6 After Prototype
704 MHz 1MW, >65% efficiency, 23 dB gain (5KW drive power). • Prototype – 24-30 MARO • Pre-production – 2/month 30 MARO • Production – 10/month at 42 MARO
c) E2V E2V are an established manufacture of vacuum electronics, recently their focus has shifted more to CCD manufacture, although they still offer a range of high-frequency, low-power IOTs. Also E2V in conjunction with Lancaster University had a 3-year research programme to consider a 350 MHz 1MW Radial Multiple Beam IOT. The programme ended in 2007, with no conclusive results.
Further developments are taking place in cathode technology, to produce new materials and structures to form cathodes with much higher life-times, see below in Figure 4 taken from reference [10]. As seen from reference [10] these new cathode designs offer to increase cathode life-time quite considerable, to approximately 9 years of continuous use.
M-Type Cathode Life vs Op. Temp 3 1 .10 '
100
10 kiloHours
1 2 4 8 16 32 1
0.1 800 850 900 950 1000 1050 1100 1150 1200 1250 1300 Degrees Centigrade true
Figure 4 showing developments in cathode life time [10].
VI. Discussion
As discussed above the IOT offers a number of advantages over both the Klystron and the tetrode. Table 1 shows the key parameters for each; klystron, tetrode and IOT, for the example presented, in table 1, the assumed output power from each tube is 360KW, cost does not include the cost of the power driver. Another major acquisition cost driver for any RF system based on the klystron or tetrode is the collector cooling system. As seen in table 1 the difference in cooling overhead between the IOT and other tubes is rather marked, which implies the cost differential, for a system using 30 devices each, is substantial. The size of the IOT, comparable to the tetrode, is much smaller than the klystron, this creates an easy of both maintenance are replacement, which could have a major impact on recovery time in the event of failure. The increased gain of the IOT compared to the tetrode has a dramatic impact on the cost, maintenance and footprint of the RF driver. The low life-time of the tetrode creates a major impact on the cost effectiveness of the tetrode, for example we would expect a failure rate of 3 tetrodes for each klystron/IOT that would fail. The cost figure represents the base cost of the “tube”, the number in parentheses is the cost for the RF driver (costing’s from quotes, except for the klystron).
Table 1: based on producing 360KW output RF.
Gain Efficiency Drive Power Cooling Life time Size Cost (Drive P in) overhead K euros Klystron 43dB 30% 1 8 W (40W) 840KW 50 KHrs 3m X 1m 280 (1000) Tetrode 12dB 40% 20 KW (30KW) 549KW 15 KHrs 1m x 1m 70 (97) IOT 23dB 70% 2 KW (3KW) 154KW 45 KHrs 1m x 1m 300 (10)
In summary IOTs are very useful for the following performance characteristics: ● Very high conversion efficiencies, above 70% in class C operation ● High efficiency even at reduced drive power ● Monotone increase in output power even at the point of maximal efficiency ● Linear phase characteristics ideal for phase control ● Substantial higher Gain than tetrodes (lower than a klystron) ● Much smaller and lighter than a klystron (comparable to a tetrode) ● Lifetimes comparable to klystron, much larger than for the tetrode ● Substantially lower power requirements, than a Klystron or tetrode ● Substantially smaller, cheaper, drive power than the tetrode ● Substantially lower cooling requirements, compare to Klystron and tetrode ● Reduced sensitivity to variations in supply power
Currently no IOT’s are available off the shelf that meet the requirements of the ESS, although as discussed above several companies have close to market research programmes for an IOT that will meet the specifications of the spoke cavities. These development programmes could within a 4 year period produce a multi beam IOT at 704MHz with 1MW pulsed power, ideal for the elliptical cavities of the ESS. For the 352MHz range it is conceivable to use lower power IOTs and combine then using the same technique as Diamond. Development of a suitable IOT could be achieved by adapting existing 400MHZ 200KW IOT’s, altering the electron-gun and output cavity to function at 352 MHz. As an estimate this could be achieved in less than a year for the production of a prototype 352 MHz 200KW IOT.
The potential benefits of the IOT amplifier would have a huge impact on the ability of the ESS in reaching, and sustaining, it’s over arching principle of efficiency, reliability and cost-effectiveness. Especially when compared to the current chosen technological options of the tetrode and the klystron.
[1] Eichmeier and Thumm Editors, Vacuum Electronics, Springer 2008, ISBN 978-3-540-71938
[2] I. Tahir, A.C. Dexter, and R.G. Carter, IEEE Trans. Electron Devices, vol. 52, No.9, (2005).
[3] Gilmour, A. S. (1994). Principles of traveling wave tubes. Artech House Radar Library. Boston: Artech House.
[4] Note the TH391 operates up to 450MHz, the TH781 up to 200MHz, over 200MHz the TH781 with effects such as decreased effeciency
[5] Heppinstall and Clayworth, EBU Technical Review (1997) ; and private communication with CPI
[6] https://espace.cern.ch/acc-tec-sector/Chamonix/Chamx2011/talks/OB_23_11_talk.pdf
[7] Lyles et. al., EPAC 2004
[8] H. Bohlen, CPI report 2012
[9] Ives, et.al. , IEEE. Proc. IVEC 2010, page 214; http://www.calcreek.com/hardware.html#inductiveoutputtube;
[10] Grant et. al., IVEC 2002