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Comparative Overview of Inductive Output Tubes

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Comparative Overview of Inductive Output Tubes ! 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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