Reginald Lamar Jaynes

Effects of tapering on gyrotron backward-wave oscillators

Computer modeling has been utilized to guide gyrotron backward-wave oscillator (gyro-BWO) experiments at the University of Michigan over a wide range of tapered interaction regions and tapered magnetic fields. E-GUN code is used to examine beam and diode characteristics, while MAGIC is used to analyze the dynamics of the problem, such as particle kinematics and microwave power production. Several innovative techniques are used to create matching boundary conditions for a backward propagating wave. MAGIC simulations predict optimum performance of the gyro-BWO operating in a TE/sub 01/ mode within a combination of uniform interaction region and a tapered axial magnetic field which increases 7.5% in the direction of beam propagation. Experiments have been performed to investigate the effects of tapering magnetic fields and tapered interaction region radii on the high-power microwave emission from the gyro-BWO over the frequency range from 4.0 to 6.0 GHz. These experiments were performed on the Michigan Electron Long-Pulse Accelerator (MELBA) with parameters: V=-0.7 to -0.9 MV, I/sub diode/=1-10 kA, I/sub tube/=1-4 kA, T/sub e-beam/=0.4-1.0 /spl mu/s. Tapered interaction regions of 37%, 23%, 9.4%, and 6.4% were built and tested to determine their effect on microwave power, pulselength, and inferred energy compared to the uniform interaction region. Magnetic tapering trim coils with a range of -10.6%</spl Delta/B/B/sub 0/<+15.0% were constructed which allow the orientation of the field taper to be changed without breaking the vacuum. The peak microwave power from individual shots was from 30 to 55 MW. Experiments on magnetic field tapering indicate that positive tapered fields improve microwave power and energy output.

Long-pulse, high-power, large-orbit, coaxial gyrotron oscillator experiments

Long-pulse, large-orbit, coaxial gyrotrons are currently under investigation. The electron beam is generated by the Michigan Electron Long Beam Accelerator (MELBA) with parameters: V=-0.8 MV, I/sub anode//spl les//spl sim/4 kA, I/sub tube/=0.2-2 kA, and pulse length=0.5-1 /spl mu/s. Large-orbit, axis-encircling electron beams are generated by a magnetic cusp. Experimental gyrotron performance with coaxial cavities (unslotted and slotted) is compared to a noncoaxial cavity. The coaxial gyrotron demonstrated superior current transport and microwave production over the noncoaxial gyrotron. The coaxial rod apparently raises the limiting electron beam current in the diode, allowing higher currents to be extracted. The unslotted, coaxial gyrotron showed microwave power levels of 20-40 MW with pulse lengths of 10-40 ns, This coaxial gyrotron operated in two main modes: TE/sub 111/ and TE/sub 112/ with frequencies of 2.34 and 2.5 GHz, respectively. The gyrotron frequency is tunable between the respective modes by changing the magnetic field. The slotted, coaxial gyrotron showed the highest power of 60-90 MW and extremely short pulse lengths of 10-15 ns. For all three gyrotrons, the microwave pulse-shortening mechanisms of mode hopping and mode competition are definitively identified by time-frequency analysis of heterodyned microwave data.

Polarization control of microwave emission from high power rectangular cross-section gyrotron devices

Results are summarized of experiments on a gyrotron utilizing a rectangular-cross-section (RCS) cavity region. The major issue under investigation is polarization control of microwave emission as a function of magnetic field. The electron beam driver is the Michigan Electron Long Beam Accelerator (MELBA) at parameters: V=0.8 MV, I/sub diode/=1-10 kA, I/sub tube/=0.1=0.5 kA, and t/sub e/-beam=0.4-1.0 /spl mu/s. The annular e-beam is spun up into an axis-encircling beam by passing it through a magnetic cusp prior to entering the RCS interaction cavity. Experimental results show a high degree of polarization in either of two orthogonal modes as a function of cavity fields. The RCS gyrotron produced peak powers of 14 MW in one polarization (TE/sub 10/) and 6 MW in the cross-polarized mode (TE/sub 01/). Electronic efficiencies for this device reached as high as 8% with transverse efficiency of 16%. Experimental results on the beam alpha (/spl alpha/=V/sub /spl perp///V/sub /spl par//) diagnostics, where alpha is the ratio of the e-beams transverse velocity to its parallel velocity, agree well with the single electron trajectory code. MAGIC code results are in qualitative agreement with microwave measurements. Microwave emission shifts from the dominant fundamental mode polarization (TE/sub 10//spl square/ ), to the next higher order mode polarization (TE/sub 01//spl square/) as the solenoid magnetic field is raised from 1.4-1.9 kGauss. Frequency measurements using heterodyne mixers support mode identification as well as MAGIC code simulations.

Experiments on Slotted, Coaxial, High Power Gyrotrons

This research program investigates high power microwave generation utilizing a microsecond electron beam accelerator to study means of eliminating microwave pulse-shortening. The particular device under study is the coaxial gyrotron oscillator in the S-band frequency range. Experiments have concentrated on three types of gyrotron cavities: (1) coaxial, unslotted, (2) coaxial, slotted, and (3) noncoaxial, unslotted. The first major result is that the coaxial rod raises the limiting current in the e-beam diode, permitting reliable, higher current extraction into the microwave tube. The second major finding is that the slotted cavity gives the highest peak powers (approximately 90 MW) but very short pulselengths (approximately 10 - 20 ns). The unslotted coaxial gyrotron emits power levels of 20 - 40 MW with longer pulselengths (up to 40 ns). The noncoaxial gyrotron radiates lower peak power levels (approximately 20 MW). All of the gyrotron types exhibited signs of the pulse shortening mechanisms of mode hopping and mode competition as diagnosed by time-frequency-analysis (TFA). TFA also shows that lower power microwave oscillation is maintained over some 300 ns, but the power level may be modulated due to e-beam voltage fluctuations.

Optical spectroscopy of plasma in high power microwave pulse shortening experiments driven by a /spl mu/s e-beam

Microwave pulse shortening experiments have been performed on a rectangular-cross-section (RCS) gyrotron driven by the Michigan Electron Long Beam Accelerator (MELBA) at parameters V=-800 kV, I/sub tube/=0.3 kA and pulselengths of 0.5-1 /spl mu/s. Pulse shortening typically limits the highest (10 MW level) microwave power pulselength to 100-200 ns. Potential explanations of pulse shortening are being investigated, particularly plasma production inside the cavity and at the e-beam-collector. We report the first optical spectroscopy diagnostic measurements inside an operating gyrotron as a means of exploring plasma effects on pulse shortening. Plasma hydrogen H-/spl alpha/ line radiation has been characterized in both time-integrated and temporally-resolved measurements and correlated with microwave power/cutoff. Hydrogen is believed to originate from water absorbed on internal tube surfaces in the gyrotron.

Radio-frequency plasma cleaning for mitigation of high-power microwave-pulse shortening in a coaxial gyrotron

Results are reported demonstrating that radio-frequency (rf) plasma cleaning is an effective technique for mitigating microwave-pulse shortening (i.e., lengthening the pulse) in a multimegawatt, large-orbit, coaxial gyrotron. Cleaning plasmas were generated by 50 W of rf power at 13.56 MHz in nitrogen fill gas in the pressure range 15–25 mTorr. Improvements in the averaged microwave energy output of this high-power-microwave device ranged from 15% to 245% for different initial conditions and cleaning protocols. The mechanism for this improvement is believed to be rf plasma sputtering of excess water vapor from the cavity/waveguide and subsequent removal of the contaminant by cryogenic vacuum pumps.

Radio frequency plasma processing effects on the emission characteristics of a MeV electron beam cathode

Experiments have proven that surface contaminants on the cathode of an electron beam diode influence electron emission current and impedance collapse. This letter reports on an investigation to reduce parasitic cathode current loss and to increase high voltage hold off capabilities by reactive sputter cleaning of contaminants. Experiments have characterized effective radio frequency ~rf! plasma processing protocols for high voltage anode‐cathode ~A‐K! gaps using a two-stage argon/ oxygen and argon rf plasma discharge. Time-resolved optical emission spectroscopy measures contaminant ~hydrogen! and bulk cathode ~aluminum! plasma emission versus transported axial electron beam current turn on. Experiments were performed at accelerator parameters: V520.7 to 21.1 MV, I~diode!53‐30 kA, and pulse length50.4‐1.0 ms. Experiments using a two-stage low power ~100 W! argon/oxygen rf discharge followed by a higher power ~200 W! pure argon rf discharge yielded an increase in cathode turn-on voltage required for axial current emission from 6626174 kV to 981697 kV. The turn-on time of axial current was increased from 100622 to 175642 ns.

Velocity ratio measurement diagnostics and simulations of a relativistic electron beam in an axis encircling gyrotron

This paper reports on diagnostic experiments and simulations of a large-orbit, axis-encircling gyrotron. The electrons perpendicular to parallel velocity ratio /spl alpha/ is measured. The experimental diagnostic consists of an apertured portion of the beam which is passed through a cusped magnetic field then collected on a glass plate. The cross section of the beam is recorded on the glass plate in a radiation darkened pattern. From the gyro-radius, B-field, and beam energy, /spl alpha/ can be calculated. Particle distributions of the radiation darkened plate are in excellent agreement with numerical simulations of single particle orbits derived from the relativistic equations of motion. The experimental measurement of /spl alpha/ yields values from 0.9 to 1.4. The simulations predict an /spl alpha/ of 1.0 to 1.5.

Rectangular-cross-section high-power gyrotron

Experiments are underway to generate high power, long-pulse microwaves by the gyrotron mechanism in rectangular-cross- section interaction tubes. Long-pulse electron beams are generated by MELBA (Michigan Electron Long Beam Accelerator), which operates with parameters: -0.8 MV, 1 - 10 kA diode current, and 0.5 - 1 microsecond pulselength. Multimegawatt range microwave power levels have been generated. Adjustment of the solenoidal magnetic field is being studied for polarization control. Polarization power ratios up to a factor of 15 have been achieved. Electron beam dynamics, i.e. beam alpha (the ratio of the beam perpendicular velocity to the parallel velocity, vperp/vpar, are being measured by radiation darkening on glass plates. Computer modelling utilized the MAGIC Code and a single particle orbit code into which are injected a distribution of electron angles or energies. Both small- orbit and large orbit (rotating) e-beams are being investigated.

Application of time-frequency analysis to high-power microwave devices

Research is being conducted on high power microwave devices (e.g., gyrotrons) at the University of Michigan. Of utmost concern is the phenomenon of pulse shortening, that is, the duration of the microwave pulse is shorter than the duration of the cathode voltage. For years researchers have applied the Fourier transform to the heterodyned microwave signals. The problem with this technique is that a signal with multiple frequency components has the same spectrum as that of a signal with frequency components emitted at different times. Time-frequency analysis (TFA) using Reduced Interference Distributions provided an entirely different outlook in the community when it was recently applied to heterodyned microwave signals. Results show, with unprecedented clarity, mode hopping, mode competition, and frequency modulation due to electron beam voltage fluctuations. The various processes that lead to pulse shortening may finally be identified. Time resolved maximum intensity of the TFA has produced results very similar to the microwave power signal, verifying the utility of TFA in the analysis of the temporal evolution of power in each mode.