William Erwin Cohen

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.

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.

Microwave and electron-beam diagnostics of a rectangular cross section gyrotron

Diagnostic experiments and operation of multi-megawatt gyrotrons utilizing rectangular cross-section resonant cavities are currently under investigation. The goal of the experiments is to achieve gyrotron generation of high power- long pulse microwaves. A rectangular cross-section resonant cavity has the advantage that it is a linearly polarized source of microwaves. The output microwaves have a high degree of polarization control by changing the magnetic field in the interaction region. Diagnostics include cold tests of the microwave cavity, heterodyne mixer measurements of operating frequencies, and beam alpha (Vperp/Vparallel)/spatial distribution measurements using glass witness plates. Operation at microwave powers greater than 10 MW and high polarization power ratios (vertical/horizontal) greater than 100 have been observed in experiments. The electron beam was produced by MELBA (Michigan electron long beam accelerator) with the following parameters: -0.8 MV, 1 - 10 kA diode current, 0.5 - 1.0 microsecond pulse length.

Magnetron simulations and experiments

Summary form only given, as follows. Simulations and experiments are underway at UM to investigate the characteristics of GW-level, cold cathode, relativistic magnetrons versus kW-level, thermionic cathode magnetrons. The goal of the research is to understand why kW magnetrons exhibit 50-80% efficiencies, but relativistic (>100 MW) magnetrons are limited to /spl sim/30% efficiency. We present simulations relevant to relativistic magnetron experiments beginning at the University of Michigan. Planned MELBA experimental relativistic e-beam parameters are: voltage=-500 kV, current=1-10 kA, and pulselength from 100-500 ns. The main relativistic magnetron under study is the Maxwell Physics International L-band magnetron. Commercial oven magnetrons are also being studied. Mode competition and mode hopping are being addressed by utilizing time-frequency-analysis of the heterodyned microwave signals, we are planning to explore the effects of the vastly different impedances and space charge densities that characterize each device.