W. Lawson

Gyro-amplifiers as candidate RF drivers for TeV linear colliders

The feasibility of future electron-positron colliders operating at energies >1 TeV will depend on both operating efficiency and cost of the microwave amplifiers that can be developed to drive the collider. To zeroth order, the required number of amplifiers depends inversely on the parameter A=P/sub p/T/sub p///spl lambda//sup 2/, where /spl lambda/ is the wavelength, and P/sub p/ and T/sub p/ are, respectively, the power and duration of the amplifier output pulses. Thus, one goal of amplifier research is to maximize A while keeping other parameter values within bounds so as not to excessively increase the cost of either the individual amplifier system or the collider structure (e.g., amplifier voltage V/spl lsim/500 kV, wavelength /spl lambda//spl gsim/1 cm). Operating within these bounds, gyro-amplifiers have demonstrated values of A=11/spl times/10/sup 4/ W s/m/sup 2/, which compares favorably with the best values of A achieved by klystrons. The gyro-amplifier program which led to this accomplishment is reviewed. Some 20 different gyro-amplifier configurations have been examined on our 450 kV gyro-amplifier test facility during the past several years. These tubes fall into five major categories: first-, second-, and third-harmonic gyroklystrons, as well as first- and second-harmonic gyrotwystrons. Peak powers in excess of 30 MW with pulse duration of 0.8 /spl mu/s have been achieved at 19.76 GHz in the TE/sub 02/ mode via a two-cavity second-harmonic gyroklystron with a first-harmonic drive cavity. The peak efficiency and gain were 28% and 27 dB, respectively. At present, there is ongoing construction of a new three-cavity second-harmonic coaxial gyroklystron, driven by a 500-kV 720-A beam, which is predicted to have an output power well above 100 MW at 17.136 GHz with an intrinsic efficiency in excess of 40%. With the use of a depressed collector, achievable overall amplifier efficiency, which is of very great importance in the collider application, could be in the range of 50-65%.

Experimental studies of a high power, X-band, coaxial gyroklystron

Gyroklystrons are possible candidates for the RF sources in future linear colliders. A two-cavity and a three-cavity X-band gyroklystrons have been designed, built, and tested on the new 100 MW test bed at the University of Maryland. The theoretical design of the three-cavity X-band gyroklystron called for a peak power of 96 MW at an efficiency near 40% at a frequency of 8.568 GHz. In experiments, peak output power in the range of 75-85 MW at a frequency of 8.6 GHz has been measured with the three-cavity gyroklystron tube. The three-cavity tube was operated at electron beam voltage and current of 470 kV and 500 A, respectively. The efficiency is approximately 32% and the gain is approximately 30 dB. The pulse width of the output power signal is 1.7 /spl mu/s (full width at half maximum), while the pulse repetition rate was limited to 2 Hz in the proof-of-principle study. The paper details the design, performance, and diagnostics of these two X-band gyroklystrons operating at the fundamental cyclotron resonance.

Current Status of Gyroklystron Research at the University of Maryland

At the University of Maryland, we have been developing high‐power coaxial gyroklystrons. Our present work is focused on the development of a 17.136 GHz four‐cavity frequency‐doubling gyroklystron amplifier. This device will then be used to drive a high gradient linear accelerator structure recently developed by the Haimson Corporation. Our work has been afflicted by many technical challenges, most arising from thermal imperfections in the custom‐made high current emitter of our electron gun. In our latest experimental run, instabilities were detected in the input cavity of our amplifier tube. These instabilities appear when the beam pitch ratio (α) is approximately 1, thus impeding our search of domains with higher α values (note that the circuit was designed to operate at α=1.4). In order to remedy this problem, we have radically redesigned the input cavity, changing both its geometry and Q factor. The new input cavity has been fabricated and cold‐tested. It will soon undergo hot‐test in the next run of ...

Design of a 7 MW, 95 GHz, three-cavity gyroklystron

In this paper we present a three-cavity design of a gyroklystron amplifier at 95 GHz. We present the design of the magnetron injection gun (MIG), the magnetic field coils, and the three-cavity microwave circuit. The MIG produces a 500 kV, 45 A small orbit annular beam with an average perpendicular-to-parallel velocity ratio of 1.5 and a parallel velocity spread below 5% (RMS). The MIG requires a control anode with a voltage of about 65 kV, a magnetic compression of about 30, and a cathode loading near 10 A/cm/sup 2/. The circuit magnetic field is about 28.7 kG. The microwave circuit has a first-harmonic TE/sub 011/ input cavity which is driven at 47.5 GHz, and second-harmonic TE/sub 021/ buncher and output cavities which are resonant at 95 GHz. A peak power of 7.56 MW is obtained with 51.6 dB gain and 33.6% efficiency. A complete description of the system is presented along with a systematic study of the sensitivity of the device to parametric variations.

Design and operation of a two‐cavity third harmonic Ka‐band gyroklystron

We present the operating characteristics of a two‐cavity third harmonic gyroklystron experiment. The input cavity utilizes a 9.854 GHz TE011 mode which is driven by a 100 kW magnetron. The TE031 output cavity has a resonant frequency of 29.57 GHz. The nominal beam voltage and current are 435 kV and 210 A, respectively. The pulse length is about 1 μs and the average ratio of parallel to perpendicular velocity is near one. Peak powers above 1.8 MW are achieved with an efficiency of about 2% and a gain of 14 dB. The theoretical simulations are in good agreement with the experimental results.

100 MW gyroklystron development for linear collider applications

Designs for gyroklystron amplifiers capable of producing 100–150 MW of output power in 1–2 μs pulses will be described in this paper. For accelerator applications we plan to employ a second harmonic output cavity operating at 17.136 GHz. Initial experiments to test our new beam production and transport facilities will involve energy extraction from the fundamental cyclotron harmonic at 8.568 GHz. In both cases the microwave circuits employ coaxial cavities and drift tubes to limit spurious oscillations and cavity cross‐talk.

Operating characteristics of 17.14 GHz frequency-doubling coaxial gyroklystrons

In this paper we describe the present status of the gyroklystron program at the University of Maryland. We are presently studying multi-cavity gyroklystron microwave tubes as possible drivers for future linear colliders. A 3 cavity second harmonic circuit has produced about 28 MW of peak power with an efficiency of about 13% and gain of approximately 26 dB. Further investigation of this circuit was impeded by technical problems relating to the performance of a defective annular emitter. A 4 cavity circuit has been designed, constructed and is about to undergo testing. Computer simulations performed on this design yield a theoretical prediction of 80 MW of peak power with an efficiency of 35% and a gain of 60 dB. Additionally, a new output waveguide system has been designed. to allow coupling of our 4 cavity circuit to an accelerator structure.

Experimental gyroklystron research at the University of Maryland for application to TeV linear colliders

X‐Band and K‐Band gyroklystrons are being evaluated for possible application to future linear colliders. So far we have examined then different two‐ and three‐cavity configurations. We have achieved a maximum peak power of 27 MW in ∼1 μs pulses at a gain of 36 dB and an efficiency exceeding 32%. The nominal parameters include a 430 kV, 150–200 A beam with an average perpendicular to parallel velocity ratio near one. In this paper, we detail our progress to data and describe our plans for future experiments that should culminate in amplifier outputs in excess of 100 MW in 1 μs pulses.

Study of an energy recovery system for a large-orbit gyrotron

An approach to energy recovery from the spent beam of large-orbit gyrotron is outlined that uses a double magnetic cusp arrangement. The first cusp produces a large rotational velocity, while the second cusp unwinds the axis-encircling beam before it proceeds toward depressed collectors. The results of numerical simulations for beam transmission through the two cusps are summarized. An experimental setup for proof of principle is described. Results are presented in the form of the depressed potential on a Faraday cup versus the current collected by it. Current collection is observed up to larger depressed potentials when the second cusp is introduced, as compared with the case where field reversal is not introduced, thus verifying the concept. A design for multiple depressed collectors is presented along with the trajectories in the collector region.<<ETX>>

Initial operation of a 100 MW fundamental frequency gyroklystron

A 100 MW first harmonic gyroklystron is currently being implemented on the newly upgraded test bed at the University of Maryland. This initial experiment will serve as a diagnostic and precursor to the second harmonic systems that are in the designing stages. This paper details the specifics of the first and second harmonic two-cavity tube designs. Comparisons of two-cavity and three-cavity designs are also presented.