A.J. Balkcum

Stable high-power TE/sub 01/ gyro-TWT amplifiers

The stability of high power gyro-TWT amplifiers operating in the low-loss TE/sub 01/ mode of cylindrical waveguide has been studied, Linear theory has been used to determine the threshold start-oscillation beam current for absolute instability in the operating mode and the critical section lengths for the dominant gyro-BWO interactions occurring at various cyclotron harmonics in other waveguide modes. The performance of the amplifier was evaluated with a nonlinear, self-consistent slow-timescale simulation code. Utilizing interaction sections whose lengths are less than the threshold start-oscillation length and are separated by attenuating severs for isolation, two stable three-section devices have been designed which are predicted to yield: (1) a peak output power of 230 kW at 35 GHz with an efficiency of 23%, a saturated gain of 46 dB and a constant-drive bandwidth of 6% for a 100 kV, 10 A electron beam with an /spl alpha/=/spl nu//sub /spl perp// //spl nu//sub z/=1.0 and an axial velocity spread /spl Delta//spl nu//sub z///spl nu//sub z/=5% and (2) 105 kW at 94 GHz with 21% efficiency, 45 dB saturated gain and 5% constant-drive bandwidth for a similar 5 A electron beam. In addition, the design of the 0 dB input/output couplers and the MIG electron gun are given. Due to the low loss of the TE/sub 01/ mode, both of these amplifiers can be operated continuously. >

Periodic permanent magnet focusing of an annular electron beam and its application to a 250 MW ubitron free-electron maser

Stable electron beam flow is shown to exist for an annular electron beam within a coaxial periodic permanent magnet wiggler and transmission line with zero guide field. When the beam is properly centered near the magnetic minimum of the wiggler, a condition exists where the magnetic radial force balances the space‐charge repulsion of the beam. The stability condition relating the electron beam current to the magnetic field strength in terms of the plasma frequency, wiggler cyclotron frequency and relativistic Lorentz factor is ω2p=(γ/2)Ω2w, which is the same stability condition found for Brillouin flow in a uniform magnetic field and for focused beam flow along periodic dipole magnetic fields. The radial scallop of an annular electron beam satisfying this condition can be extremely low. Although this propagation technique can be utilized for any high power coaxial linear‐beam device, it is most appropriate for free electron masers. Using parameters satisfying the stability condition, a 500 kV, 1 kA, TE01 ...

High-power coaxial ubitron oscillator: theory and design

The theory and design of a high-power coaxial ubitron oscillator is discussed. The device utilizes a high-current annular electron beam interacting with the radio frequency (RF) breakdown-resistant TE/sub 01/ mode in a coaxial cavity. Stable and laminar flow of the electrons is achieved without a confining axial field by using periodic permanent magnets to form the wiggler. A linear theory is derived for the cavitys start-oscillation condition using a Vlasov analysis and a nonlinear simulation code is described. Agreement is good between the linear theory, nonlinear code, and the 2-1/2-dimensional particle-in-cell code, MAGIC. A 1-GW, S-band oscillator design with 21% efficiency is presented. It was found that the interaction efficiency in the cavity could be increased to 46% using the technique of axial mode profiling.

35-GHz 25-kW CW low-voltage third-harmonic gyrotron

A 50-kV third-harmonic gyrotron is shown to be capable of high efficiency. Operation at the third harmonic allows the required magnetic field for 35 GHz generation to be supplied by a 4.5-kG permanent magnet. Two gyrotrons employing sliced circuits for mode control have been evaluated with a large-signal nonself-consistent particle-tracing simulation code and found to be capable of producing 25 kW continuously. The preliminary design of a third-harmonic TE/sub 41/ gyrotron utilizing a magnetron injection electron gun is predicted to yield a device efficiency of 17%, which can potentially be increased to 46% with an ideal single-stage depressed collector, while an axis-encircling electron beam from a Cusp electron gun is predicted to drive a third-harmonic TE/sub 31/ gyrotron with a device efficiency of 23%, which can theoretically be increased to 45% through the use of an ideal depressed collector.

94-GHz 25-kW CW low-voltage harmonic gyrotron

A low-voltage second-harmonic gyrotron intended as a compact lightweight source has been designed and evaluated with a particle-tracing code and the particle-in-cell code MAGIC. The two codes are shown to be in good agreement when applied to a conventional fundamental-frequency gyrotron and also to the novel second-harmonic gyrotron. The 25-kW continuous wave (CW) 94-GHz gyrotron with a predicted conversion efficiency of 32% and device efficiency of 22.5% is driven by a 25-kV 4.5-A (/spl upsi//sub /spl perp////spl upsi//sub 2/=1.5, /spl Delta//spl upsi//sub z///spl upsi//sub z/=7%) electron beam from a magnetron injection gun and employs a low-loss TE/sub 021//TE/sub 031/ complex cavity for mode control. Although the 17-kG CW gyrotron will use a cryogen-free high-T/sub c/ superconducting magnet, a 94-GHz prototype will be tested at low duty with a conventional low-T/sub c/ superconducting magnet.

250-MW X-band TE/sub 01/ ubitron using a coaxial PPM wiggler

A highly efficient 250-MW X-band ubitron amplifier to drive high gradient linear accelerators is described. The ubitron utilizes periodic permanent magnets with zero guide field and a coaxial circuit operating in the breakdown resistant TE/sub 01/ mode. Periodic wiggler focusing in a coaxial transmission line allows for stable transport of high-current annular electron beams while producing a strong quiver velocity to drive the ubitron. A large-signal self-consistent simulation code predicts that an efficiency of 37% and a gain of 53 dB can be achieved in a length of 100 cm with a 500-kV 1-kA electron beam, while a tapered wiggler will yield an efficiency as high as 56%.

Theory and design of a high-harmonic gyrofrequency multiplier

In a gyrofrequency multiplier, the entire electron beam enters the output cavity optimally phased to lose energy in a first-order interaction (in contrast to the second-order standard gyrotron process) resulting in a high conversion efficiency. Because the electrons radiate at a high harmonic of the cyclotron frequency, a gyrofrequency multiplier is capable of efficiently generating power in the millimeter to submillimeter wave region. The operational principles of the gyrofrequency multiplier are described in this paper. Results from nonlinear simulation show that radial guiding-center spread of the beam can dramatically affect the devices efficiency while velocity spread effects are negligible. The design is presented for a 150 kW, 17.4 GHz, sixth-harmonic gyrofrequency multiplier which is under construction and is predicted to yield 31% conversion efficiency of the electron beam into the output wave. The 300 keV, 1.67 A axis-encircling beam is produced by a 1 MW, 2.9 GHz gyroresonant RF accelerator. >

94 GHz, 25 kW CW, low-voltage harmonic gyrotron

Summary form only given. A low-voltage second-harmonic gyrotron intended as a compact, lightweight source is under construction at UCD. The fieldable gyrotrons cryogen-free 17 kG solenoid magnet with a closed-loop cryocooler utilizes the tremendous advances made in high-T/sub c/ superconducting technology. The design is for CW operation. However, the 94 GHz prototype will be tested at low duty in pulse mode with an existing conventional low-T/sub c/ superconducting magnet. The second-harmonic gyrotron has been designed and evaluated with a particle-tracing code and the particle-in-cell code MAGIC. The 25 kW CW, 94 GHz gyrotron with a predicted conversion efficiency of 32% and device efficiency of 22.5% is driven by a 25 kV, 4.5 A, v/sub /spl perp///v/sub z/=1.5, electron beam from a magnetron injection gun (MIG) with 7% axial velocity spread. A MIG from a CPI/Varian 60 GHz gyrotron will be employed. The electron beam center radius of the new 94 GHz second-harmonic gyrotron is nearly the same as in the 60 GHz gyrotron. The long-term goal of this activity is to extend the power level to 100 kW CW.

Operation of a high-harmonic gyrofrequency multiplier

The operation of a sixth-harmonic gyrofrequency multiplier is described. The initial experimental results produced 3.5 kW of 17.23 GHz radiation from a 260 keV, 0.3 A, α=v⊥/v∥=4.8 axis-encircling electron beam in a near uniform magnetic field. The output cavity conversion efficiency was measured at 4.5% compared to the predicted value of 8% from a nonlinear simulation code. The design for a higher power, higher efficiency device utilizing magnetic tapering is also presented.

High-harmonic gyrofrequency multiplier

The design of a sixth-harmonic gyrofrequency multiplier driven by a 300 keV, 1.7 A, prebunched axis-encircling electron beam produced by a 1 MW, 2.9 GHz gyroresonant rf accelerator is presented. A nonlinear simulation code predicts that the device, which is currently being tested, will emit 150 kW at 17.4 GHz with an efficiency of 31%.