T. T. Yang

An ultra high gain gyrotron traveling wave amplifier

Physics and technology issues of importance to the high gain gyrotron traveling wave amplifier (gyro-TWT) are investigated in theory and experiment. The gyro-TWT is known to be highly susceptible to spurious oscillations, especially in high gain operations. In the current study, oscillations of various origins are classified and characterized with detailed theoretical modeling. They are shown to be intricately connected to the interplay between the absolute/convective instabilities, circuit losses, and reflective feedback. Knowledge of these processes leads to the suggestion of an ultra high gain scheme which employs distributed wall losses for the suppression of spurious oscillations. An experimental Ka-band gyro-TWT stable at zero drive has produced 93 kW saturated peak power at 26.5% efficiency and 70 dB gain, with a 3 dB saturated output power bandwidth of 3 GHz.

An extended interaction oscillator based on a complex resonator structure

We report experimental investigation of a novel scheme for efficient interaction between a linear electron beam and an electromagnetic wave in a complex extended interaction structure. The structure of interest consists of a high R/Q, five-gap, coupled-cavity resonator that incorporates a coaxial section of a quarter plasma wavelength placed between the first and second cavities. In the coaxial section, beam and wave propagate in separate channels. The first cavity, strongly coupled to the other cavities through the wave channel of the coaxial section, serves as a buncher cavity. An inner channel running through the center conductor of the coaxial section provides a cutoff drift space for ballistic bunching of electrons, an effect that is shown to significantly enhance the interaction efficiency. Oscillation power of 2.2 kW at 16.6 GHz was demonstrated with an interaction efficiency of 30%. The total efficiency was further increased to 41% by incorporation of a two-stage depressed collector.

A mechanically tunable magnetron injection gun

The magnetron injection gun (MIG) has been the predominant choice as the electron-beam generator in gyrodevices and it can be tuned by externally installed magnetic-field trim coils. The double-anode MIG, traditionally employed in most experiments, has the intermediate anode as an internal tuning mechanism. The single-anode design, on the other hand, is simpler to construct but lacks an internal beam tuning mechanism. The current paper describes a method to allow mechanical tuning of the MIG. Since the tip of the cathode is located in the region of the highest electric-field gradient, its extension and retraction relative to the rest of the cathode assembly provide an effective means for local electric-field profile modification and, therefore, a tuning mechanism for the electron-beam property and quality. It is demonstrated, in computer simulations and experimental tests, that the tuning mechanism adds a useful degree of freedom to the single-anode MIG for beam optimization. This method may also be applied to the double-anode design.

Successful Operation of the 500 MHz SRF Module at TLS

A project to replace two existing room-temperature radio frequency (RF) cavities by one CESR-III 500 MHz superconducting radio frequency (SRF) module was initiated for the Taiwan Light Source (TLS) synchrotron ring in 1999. The goals are to double the photon flux of the synchrotron light by doubling the electron beam current and to increase the stability of the electron beam by taking the advantage of the ultra-weak high-order modes (HOM) of the SRF cavity. The SRF module has been routinely operated since February 2005. The NSRRC users have benefited from a very high photon flux stability (Δ I0/I0∼ 0.05%) that had never been achieved previously. Here, we report the initial operational experience of the SRF system.

Stabilization of the spectral intensity fluctuations with the higher order mode frequency tuners

A strong excitation of the longitudinal coupled-bunch instabilities, which is suspected to be driven by the TM/sub 011/-like mode of the DORIS-I cavities, was observed at the higher electron beam current in the storage ring of SRRC. Such instability leads to heavy fluctuations of the photon beam intensity in the horizontal plane and therefore restricted the maximal useful beam current for the user experiments. This restriction has been released by replacing the damping antennae with the additional tuners. Here, we report our experiences after one-year routine operations of the main RF cavities with the second tuners at SRRC.

Elastoplastic Buckling on the Bent Waveguide of CESR-Type SRF Cavity

A superconducting radio-frequency (SRF) cavity module of CESR-type has been adopted for some advanced accelerators. During operation, the pressure in the liquid-helium vessel acts on the cavity wall, but an ultrahigh vacuum must prevail inside the cavity; the cavity structure must be thus pressure-tested at ambient temperature as a standard procedure for safety. During the pressure test elastoplastic buckling might occur on this SRF cavity and its bent waveguide section, being a shell-like structure. A nonlinear finite-element model for computation is established to assess the mechanism of buckling and post-buckling of the bent waveguide. Some test results with copper bent waveguides are presented. The critical pressure and buckling pattern are strongly affected by the material property and the thickness of the structure and the U-channel.

Cryogenic‐related Performance of an SRF Cavity Module in NSRRC

A superconducting 500‐MHz cavity module has been installed into the electron storage ring of NSRRC. This SRF module is tested on both the RF and cryogenic performances, before and after installation into the electron ring. Calibrations and measurements on its cryogenic load at different operating helium bath pressures are described and concluded. The test results of unloaded quality factor are reported. Meanwhile the excellent regulation on helium bath pressure is so advantageous to all these measurements. During normal operation with RF power, fluctuations of the helium bath pressure and liquid helium level are +/− 1.38 mbar (0.02 psi) and +/−0.2%, respectively.

Commissioning and Operations Results of the Industry-Produced CESR-Type SRF Cryomodules

Upon signing a technology transfer agreement with Cornell University, ACCEL began producing turn-key 500 MHz superconducting cavity systems. Five such cryomodules have been delivered and commissioned to date. Four of them are installed in accelerators for operation (two in CESR and one each in Canadian Light Source and Taiwan Light Source) and one serves as an off-line spare at CLS. One more cryomodule is scheduled for testing in early 2005. It will be a spare unit for TLS. Three cryomodules for DIAMOND Light Source are being fabricated at ACCEL. The commissioning results and operational experience with the cryomodules in CESR, CLS and TLS are presented.

Design of a third harmonic Landau cavity for the SRRC storage ring

We present here our design of the SRRC third-harmonic Landau cavity to be operated on the first stage in a passive mode for bunch lengthening so as to increase the Touschek life-time. Its implementation will also be helpful for suppression of the longitudinal coupled-bunch instabilities observed in the storage ring of SRRC. The cavity profile was optimized with the 2D code URMEL with the following criteria: (1) maximization of the shunt impedance; (2) minimization of the maximal power flow density on the cavity surface; (3) feasibility of the mechanical constructions. A prototype is now under construction.

On the mechanical design of a 1.5 GHz Landau cavity

A third harmonic Landau cavity with the fundamental mode of about 1.5 GHz is under development at SRRC. The cavity consists of inner copper layer and outer stainless steel layer. The vacuum-brazing process is adopted for cavity construction. The maximal power flow density on the cavity surface will be about 52.19 W/cm/sup 2/, as the total thermal loading of 32 kW. To verify the cooling capacity, the finite element method is adopted to perform the thermal and stress analyses. Under 15 m/sup 3//hr water flow rate, the temperature rise on the cavity body will be less than 14/spl deg/C, and the maximal equivalent stress on the copper layer of 29.75 N/mm/sup 2/(MPa) is achieved, which is much less than the yielding stress of copper.