CW Klystron Research Articles

Development of a long pulsed RF test stand and its applications for performance studies of 1 MW CW klystron

Research and development activities of high power microwave sources for powering RF cavities of Indian Spallation Neutron Source and Accelerator Driven Subcritical Systems are underway at RRCAT. Front end accelerating structures such as Radio Frequency Quadrupoles and Drift Tube Linac demand pulsed RF power up to 1 MW. A 1 MW pulsed RF system based on TH 2089 klystron amplifier at 352.2 MHz with pulse width capability up to 1.5 ms has been developed and tested. A compact 100 kV, 20 A converter type modulator with pulse width capability up to 1.6 ms has been used to energize the klystron of RF test stand. The performance of the klystron in pulsed mode operation has been studied and presented. The variation in the RF output power was measured and it is within ±0.75%. The phase variation of RF output power within the pulse and the pulse to pulse is less than ±2.5°.

High voltage electrical system of 8.56 GHz CW klystron for electron cyclotron heating on QUEST spherical tokamak

A high voltage DC power supply for the cathode of the 8.56 GHz CW klystron has been set up for electron cyclotron heating (ECH) in steady state tokamak operation on QUEST spherical tokamak. The power supply is equipped with an IGBT array and a reactor for fast shutoff of the voltage in 10 μs, where the influx of electric energy at the short circuit is limited to 5 J. AC switches also have been installed in the three-phase power lines. High voltage relays are useful to save electric energy consumption. Fast three-phase AC switching by IGBT-stack is applicable to reduce the electric load of the components of the klystron power supply.

Stability analysis of a C-band 500-kW klystron with a multi-cell output cavity

A progogype 5-GHz 500-kW CW klystron (model E3762 provided by Toshiba Electron Tubes & Devices Co. Ltd.) has been operating as the RF source for the lower hybrid current drive (LHCD) system in the KSTAR tokamak. In order to investigate how the efficiency of the 5-GHz 500-kW CW klystron prototype could be enhanced, the cavity design study is being carried out with simulation code based on the main klystron operation parameters. This is being done by simulating klystron performances for various cavity parameters including the number of cavities, inter-cavity distance, and cavity tuning frequencies. The simulation has been done with the FCI (field charge interaction) code aided by a matlab script for scanning input parameters. Initial set of scan parameters was obtained by benchmarking the E3762 klystron. It was possible to obtain optimized design parameters with better efficiency for a cavity system adopting a multi-cell output cavity. However, the multi-cell output cavity is prone to produce a self-oscillation due to the prolonged (several half RF periods) beam-field interaction along its multiple gaps. We have checked the feasibility of the optimization by evaluating the stability of output cavity system. The stability is given by the ratio of a beam-loading conductance to the circuit conductance.

Advances In LHCD System For SST-1 Tokamak

The lower hybrid current drive (LHCD) system, which is a mature, robust, and reliable heating and current drive system in a large number of tokamaks, is designed, developed, and being commissioned on the steady-state superconducting tokamak (SST-1) for driving 220 kA of plasma current, noninductively, for 1000 s, at nominal plasma parameters (plasma density ∼2×1019 m−3, temperature ∼1 keV, toroidal magnetic field ∼3 T), using four 3.7-GHz, 500-kW continuous wave (cw) klystrons. It employs a conventional grill antenna to launch toroidal lower hybrid waves asymmetrically, with a parallel refractive index N‖ of approximately 2.25 at 90-deg relative phasing of adjacent channels.The system is very complex and requires interfacing with several subsystems such as high-power radio-frequency systems, high-voltage power supply systems, auxiliary power supply systems, efficient thermal management systems, complex networks of transmission line systems, and robust and reliable data acquisition and control systems.With the SST-1 LHCD system as a case study, this lecture gives a broad overview of the physics and design layout of LHCD systems. It addresses cutting-edge technologies employed in realizing the system and gives the present status and advances made for cw operation. The challenges and opportunities are also highlighted.

Sequence Control System of 1 MW, 352.2 Mhz CW Klystron Amplifier

A 1 MW, 352.2 MHz, RF test set up based on Thales make TH 2089 klystron amplifier, having both CW and pulsed capability is being developed at Raja Ramanna Centre for Advanced Technology (RRCAT), Indore for characterization and qualification of RF components, cavities and related subsystems. Provision to vary RF power from 50 kW to 1 MW with adequate flexibility for testing wide range of RF components has been incorporated in this test set up. A high power radio frequency (RF) system is always equipped with an interlock and protection circuit for its reliable and safe operation. In an RF system, RF input, all the DC bias power supplies and water cooling parameters like flow and temperature need to be switched ON/OFF in a certain predefined timing sequence to prevent damage to the high power RF device (in this case klystron) and its system. So sequence control system is basically for protecting the klystron amplifier. The sequence control system of 1-MW CW klystron has been developed in order to drive the 1-MW, 352.2 MHz klystron amplifier (TH 2089). The system is able to control the operation of several power supplies and many environment conditions. The hardware of sequence control and the interlock system are based on the NI cRIO 9081(Integrated 1.06GHz Dual Core Controller). Proposed application software is built on LabVIEW 2011 platform and designed to work on Microsoft window environment. The basic function of the software is to check and indicate all the interlocks & log the data before providing power to klystron. The data in this system is acquired with real time visualization. The software will acquire system parameters like status of various mains and power supplies, RF status, various channel status and selection, interlocking system and other incoming status signal as per predefined test sequence. It process acquired data and control the system to achieve user define goals. Additionally, a protection circuit senses the fault conditions such as overvoltage, over-current, high water/air temperature, low water flow, arcing etc. and generates trips which in turn switch off the supplies in a controlled sequence. Communication between PC and real-time controller is established through standard Ethernet. Provision for offline fault analysis based on settable pre and post trigger mechanism has also been incorporated.

Design and development of collector for C-band 250 kW CW Klystron

The paper presents the design and development of collector for C-band 250 kW high power klystron. The design criteria for the collector assembly is selection of material, vacuum and high temperature compatibility, proper electron beam dispersion, minimum back scattering of electrons and thermal design for proper cooling at high power dissipation. All these aspects have been discussed for collector development in details. The collector has been designed in TRAK and then beam propagation has been analyzed in MAGIC 2D software. The thermal simulation has been done using ANSYS 11.0 (multi-physics). The outer surface of the collector has been grooved to facilitate its proper cooling. Design results are presented for water cooling with different flow rates and channel dimensions. OFHC copper material is chosen for collector which is suitable for vacuum and hydrogen brazing operations and good thermal properties for efficient cooling.

Test result of 5 GHz, 500 kW CW prototype klystron for KSTAR LHCD system

A 5 GHz LHCD system is being designed for current drive and profile modification necessary for AT mode and steady-state operation of the KSTAR tokamak. A prototype 500 kW CW klystron operating at 5 GHz was developed for the steady-state RF source. In this klystron, a multi-cell cavity is introduced to reduce cavity voltage and ohmic power loss. The klystron is designed with a triode system for optimization of gain, efficiency and beam control. The high voltage for the cathode is turned by using a thyristor switching system at the low voltage transformer unit. For anode voltage control, a mod-anode voltage divider system is used which utilize the parallel-circuit of the FET switch and Zener diodes. The RF output power of the klystron was 300 kW for 800 s and 450 kW for 20 s. The maximal temperature at collector top surface was 83 °C and power loss at the tube body did not exceed 10 kW, the interlock level for the protection of the klystron. Detailed results of the klystron system test and commissioning are presented.

Development status of KSTAR 5 GHz LHCD system

A steady-state lower hybrid current drive (LHCD) system is under development for advanced tokamak experiments of the Korea Superconducting Tokamak Advanced Research (KSTAR) device. The KSTAR 5 GHz steady-state LHCD system is being designed to couple an input power of 2 MW for 300 s generated by four 5 GHz klystrons. For the development of this system, there are two critical issues. One is the development of a 5 GHz CW klystron for the RF source of the system. The other is the design of a steady-state LH launcher with active water cooling. In this paper, the current status of the development and design for the KSTAR steady-state LHCD system is described. For the LHCD system, aiming at a basic experimental study of 5 GHz LH wave propagation and operational experience with an LHCD system, the installation of an initial LHCD system with a capacity of 0.5 MW for 2 s is scheduled in 2010 using a 5 GHz prototype klystron and an un-cooled 1 MW launcher. The design and progress for the initial LHCD system are also presented.

Design, fabrication and testing of UHV compatible high power RF devices for lower hybrid current drive system on SST-1 tokamak

Maintaining plasma current under steady state conditions is one of the most important pre-requisites for a tokamak-based reactor. Lower hybrid current drive (LHCD) system aims to drive tokamak plasma current by means of RF power. The LHCD system on SST-1 tokamak is based on two 500 kW, CW klystrons operating at 3.7 GHz. A waveguide transmission line transmits power from source to the antenna. A phased array waveguide antenna is used to couple power to the plasma. The antenna side of the transmission line is placed inside the tokamak vacuum vessel. The design and fabrication of this In-Vessel system has to satisfy the demands of high power RF as well as ultra high vacuum (UHV) compatibility. This paper describes some of the critical UHV compatible In-Vessel RF devices, their design, fabrication, and test results.

Commissioning of 3.7 GHz LHCD system on ADITYA tokamak and some initial results

To drive plasma current non-inductively, a lower hybrid current drive (LHCD) system has been designed, fabricated and successfully installed on ADITYA tokamak. The system is designed to launch 120 kW of RF power, at a frequency of 3.7 GHz. The system mainly consists of a high power CW klystron source, a long waveguide transmission line of about 100 m length, a UHV compatible modular waveguide line of about 2.65 m, and a conventional grill type antenna. Independent phase shifters, one each in the eight lines, are used to adjust the antenna phasing and also provides the flexibility to launch a composite spectrum. The antenna is designed to launch lower hybrid waves (LHW) with parallel refractive index ( N ||), in the range, 1 < N || < 4.5, by appropriately phasing the antenna. Antenna is positioned in the shadow of the poloidal limiter and is provided with 100 mm radial movement to achieve optimum coupling conditions. The complete system development includes design, fabrication and testing of number of waveguide components, modular waveguide lines and their integration. Different cost effective fabrication techniques are adopted to achieve good RF performance. Special attention is paid on the flanged joint seals in the long transmission line to minimize the RF losses. The entire LHCD system is calibrated, especially, in terms of phase, insertion loss and return loss measurements. After the successful integration of the system on ADITYA tokamak, some initial experiments have been carried out to assess the system commissioning and its performance. The experiments were done with a plasma (hydrogen) density of 2–5 × 10 12 cm −3 at a toroidal magnetic field of 0.8 T with 10–25 kA of plasma current. Initial results indicate that, good coupling is achieved in the presence of proper edge density. Measurements obtained from second harmonic electron cyclotron emission (ECE) and hard X-ray diagnostics suggest generation of suprathermal electrons in the presence of LH pulse. Plasma current pulse elongation with LH power is observed but needs further investigation to derive conclusions. This paper presents the design, fabrication, testing and integration of the waveguide lines, waveguide components and UHV compatible modular transmission lines of the LHCD system on ADITYA tokamak and discusses some of the initial results.

Progress in LHCD: a tool for advanced regimes on ITER

The recent success in coupling lower hybrid (LH) waves in high performance plasmas at JET together with the first demonstration on FTU of the coupling capability of the new passive active multijunction launcher removed major concerns on the possibility of using LH on ITER. LH exhibits the highest experimental current drive (CD) efficiency at low plasma temperature thus making it the natural candidate for off-axis CD on ITER where current profile control will help in maintaining burning performance on a long-time scale. We review recent LH results: long internal transport barrier obtained in JET with current profile sustained and controlled by LH acting under real time feedback together with first LH control of flat q-profile in a hybrid regime with Te ∼ Ti. Minutes long fully non-inductive LH driven discharges on Tore Supra (TS). High CD efficiency with electron cyclotron in synergy with LH obtained in FTU and TS opening the possibility of interesting scenarii on ITER for MHD stabilization. Preliminary results of LH modelling for ITER are also reported. A brief overview of ITER LH system is reported together with some indication of new coming LH experiments, in particular KSTAR where CW klystrons at the foreseen ITER frequency of 5 GHz are being developed.

Preliminary design of high-power wave-guide/transmission system for multimegawatt CW requirements of 100 MeV proton LINAC

Development of a 100 MeV CW proton LINAC has been planned at CAT. This LINAC will be needing CW rf power in the frequency ranges of 350 MHz and 700 MHz for its RFQ and DTL/CCDTL/SFDTL structures respectively. The power to the accelerating structures will be produced by either 1 MW CW or 250 kW CW klystrons/inductive output tubes (HOM IOTs). The power needed by respective feed points in the structure is max. 250 kW which will be powered by splitting the power from 1 MW klystron/klystrode into four channels by using a wave-guide system. In case of using 250 kW tubes the power to the structures will be provided directly from each tube. Two types of wave-guide transmission system have been considered, viz, WR 2300 for 350 MHz rf needs and WR 1500 for 700 MHz rf needs. The typical wave-guide system has been designed using the 1 MW CW klystron followed by wave-guide filter, dual directional coupler, high-power circulator, three 3 dB magic TEE power dividers to split the main channel into four equal channels of 250 kW each. Each individual channel has dual directional couplers, flexible wave-guide sections and high power ceramic vacuum window. The circulator and each power divider is terminated into the isolated ports by high power CW loads. Out of the four channels three channels have phase shifters. Present paper describes the technological aspects and design specifications-considerations for these stringent requirements.

Application of M-type cathodes to high-power cw klystrons

Two types of high-power cw klystrons have been widely used at KEK in both TRISTAN and KEKB e +e − collider projects: one is a 0.8 MW/1.0 MW tube, called YK1302/YK1303 (Philips); the other is a 1.2 MW tube, called E3786/E3732 (Toshiba). Normally, the dispenser cathodes of the `B-type' and the `S-type' have been used, respectively, but for improved versions they have been replaced by low-temperature cathodes, called the `M-type'. An Os/Ru coating was applied to the former, whereas an Ir one was applied to the latter. Until now, all upgraded tubes installing M-type cathodes, 9 and 8 in number, respectively, have worked successfully without any dropout. A positive experience concerning the lifetime under real operation conditions has been obtained. M-type cathodes are, however, more easily poisoned. One tube installing an Os/Ru-coated cathode showed a gradual, and then sudden decrease in emission during an underheating test, although the emission could fortunately be recovered by aging at the KEK test field. Once sufficiently aged, the emission of an Ir-coated cathode proved to be very high and stable, and its lifetime is expected to be very long. One disadvantage of this cathode is, however, susceptibility to gas poisoning and the necessity of long-term initial aging. New techniques, like ion milling and fine-grained tungsten top layers, were not as successful as expected from their smaller scale applications to shorten the initial aging period. A burn-in process at higher cathode loading was efficient to make the poisoned cathode active and to decrease unwanted Wehnelt emission. On top of that, the emission cooling, and thus thermal conductivity near the emitting layer could play an important role in such large-current cathodes as ours.

On the theory of ion noise in microwave tubes

This work examines from a theoretical perspective recent experimental work regarding ion noise in the CPI continuous wave illuminator (CWI) and also earlier related ion measurements on a CW klystron. The measurements are to a large extent complementary, the former doing only microwave measurement, the latter doing only ion current measurements. The measurements are compared to theory which seems applicable. Many results do agree at least qualitatively with theory.

Direct Energy Conversion from Spent Electron Beam in High-Power CW Klystrons

Design of a multistage depressed collector for a 1.2MW L-band CW klystron was accomplished by use of numerical simulations. To obtain information on spent electrons (i.e. input condition for collector design), a klystron simulation code was newly developed. 5 refs., 9 figs., 2 tabs.

High-Power RF Controls for the NBS-Los Alamos Racetrack Microtron

The high-power rf system for the National Bureau of Standards (NBS)-Los Alamos racetrack microtron (RIM)l uses waveguide power splitters2 and waveguide phase shifters to distribute rf power from a single 500-kW cw klystron to four side-coupled accelerating structures. The amplitude and phase of each structure is controlled by a feedback system that uses the waveguide variable power splitters, waveguide phase shifters, and klystron drive as the active control elements. A block diagram of this system is shown in Fig. 1. Figure 2 shows a subset of the complete system on which the measurements reported in this paper were performed. The feedback controls on the capture section use low-level rf amplitude and phase controls on the rf drive to the klystron. These controls are very fast with an open loop gain bandwidth of approximately 40 kHz. The feedback loop is identical to the feedback loop used in the chopper/buncher system described in another paper at this conference. 3

Higher Frequency Operation of Linear Accelerator

The high frequency operation of linacs is discussed and several trial designs for fusion material irradiction test facility in Japan are given. The size of linac namely the construction cost may be reduced by using 1 MW CW klystrons with the frequency of about 1 GHz. Preliminary results of the fabrication technique for the linac drift tube by copper coating are shown.

The NBS-LASL CW Microtron

The NBS-LASL racetrack microtron (RTM) is a joint research project of the National Bureau of Standards and the Los Alamos Scientific Laboratory. The project goals are to determine the feasibility of, and develop the necessary technology for building high-energy, high-current, continuous-beam (cw) electron accelerators using beam recirculation and room-temperature rf accelerating structures. To achieve these goals, a demonstration accelerator will be designed, constructed, and tested. Parameters of the demonstration RTM are: injection energy - 5 MeV; energy gain per pass - 12 MeV; number of passes - 15; final beam energy - 185 MeV; maximum current 550 ?A. One 450 kW cw klystron operating at 2380 MHz will supply rf power to both the injector linac and the main accelerating section of the RTM. The disk and washer standing wave rf structure being developed at LASL will be used. SUPERFISH calculations indicate that an effective shunt impedance (ZT2) of about 100 M?/m can be obtained. Thus, rf power dissipation of 25 kW/m results in an energy gain of more than 1.5 MeV/m. Accelerators of this type should be attractive for many applications. At beam energies above about 50 MeV, an RTM should be considerably cheaper to build and operate than a conventional pulsed rf linac of the same maximum energy and time-average beam power. In addition, the RTM provides superior beam quality and a continuous beam which is essential for nuclear physics experiments requiring time-coincidence measurements between emitted particles.

Performance of a High Efficiency High Power UHF Klystron

A 500 kW CW klystron has been designed for the PEP storage ring at SLAC. The tube operates at 353.2 MHz, 62 kV, a microperveance of 0.75, and a gain of approximately 50 dB. Stable operation is required for a VSWR as high as 2:1 at any phase angle. The design efficiency is 70 percent. To obtain this value of efficiency a second harmonic cavity is used in order to produce a very tightly bunched beam in the output gap. At the present time it is planned to install 12 such klystrons in PEP. A tube with a reduced size collector was operated at 4 percent duty at 500 kW. An efficiency of 63 percent was observed. The same tube was operated up to 200 kW CW for PEP accelerator cavity tests. A full-scale CW tube reached 500 kW at 65 kV with an efficiency of 55 percent. In addition to power and phase measurements into a matched load, some data at various load mismatches are presented.

High Efficiency, CW, High Power Klystrons for Storage Ring Applications

The 125 kW CW klystrons to be described in the first part of this paper were specifically designed for energy upgrading of the electron-positron storage ring, SPEAR II, at SLAC. Four tubes are required with an efficiency greater than 50 percent. The operating frequency is 358.54 MHz. The gain is approximately 50 dB, the operating voltage is 41 kV and the microperveance is 0.7. The klystron must be stable when working into an accelerator cavity VSWR as high as 2:1 at any phase angle. The calculated and experimental data to be presented include saturation and phase characteristics. A maximum efficiency of 57 percent was obtained and the highest output power was 160 kW CW at 42.5 kV. The second part of this paper deals with the design of a higher power version of this tube. The RF power is 500 kW CW at an efficiency exceeding 70 percent. The operating voltage and current are 62 kV and 11. 5 A, respectively. The higher efficiency is obtained by inserting an additional cavity to make use of second harmonic bunching. Eighteen such klystrons are to be used in the proposed PEP storage ring.