Joshua Ian Rintamaki

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.

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.

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.

Rectangular interaction structures in high power gyrotron devices

Summary form only given, as follows. Gyrotron devices utilizing rectangular interaction waveguide are currently being investigated. Some current issues under investigation include the control of polarization, and power versus pulselength of microwave emission. Annular long-pulse electron beams are generated by the Michigan Electron Long Beam Accelerator (MELBA) at parameters: electron beam voltage of 750 kV, injected current of 1-3 kA, and /spl tau/=0.5-1.0 /spl mu/s. Initial experimental results of polarized microwave radiation from a rectangular cross-section (RCS) gyrotron will be presented for small-orbit electron beams. Frequency measurements and annular e-beam generation and transport experimental results will be presented. Simulations using the E-gun code and MAGIC code will be presented.

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.

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.

Laser heated LaB/sub 6/ thermionic cathode on a MV electron beam accelerator

Summary form only given. One of the major problems in high voltage/current and long pulse e-beam generation is plasma diode closure. LaB/sub 6/, thermionic cathodes can eliminate plasma and are more robust from poisoning than oxide cathodes. To heat the LaB/sub 6/ disk on the MELBA cathode stalk at -1 MV we employ a 100-650 W CW Nd:YAG laser. Depending upon the LaB/sub 6/ disk diameter (1 cm or 2.2 cm), temperature, and cathode shape, preliminary analysis of experiments suggests three e-beam modes: 1) Thermionic mode at electron beam current density of 100-150 A/cm/sup 2/, with current shape which matches the voltage flatness, 2) Plasma mode in which 2-6 kA of current is drawn with rapid current ramping, and 3) Combined thermionic and plasma mode with initial thermionic current turn-on which is later overtaken by plasma current. It is believed that the plasma modes may involve explosive edge emission or outgassing from a graphite cathode mounting structure.

Effects of RF plasma processing on the impedance and electron emission characteristics of a MV beam diode

Summary form only given. Experiments have proven that both the surface contaminants and microstructure topography on the cathode of an electron beam diode influence impedance collapse and electron emission current. Experiments have characterized effective RF plasma processing protocols for high voltage A-K gaps using argon and argon/oxygen gas mixtures. RF processing time, feed gas pressure, and RF power were adjusted. Time resolved optical emission spectroscopy measured contaminant (hydrogen) and bulk cathode (aluminum) plasma emission versus transported axial electron beam current. Experiments utilize the Michigan Electron Long Beam Accelerator (MELBA) at parameters: V=-0.7 to -1.0 MV, I(diode)=3-30 kA, and pulselength=0.4 to 1.0 microseconds. Microscopic and macroscopic E-fields on the cathode were varied to characterize the scaling of breakdown conditions for contaminants versus the bulk material of the cathode after plasma processing. Electron emission was suppressed for an aluminum cathode in a high voltage A-K gap after RF plasma processing. 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 turn-on voltage required for axial current emission from 662/spl plusmn/174 kV to 981/spl plusmn/97 kV. After two-stage RF plasma processing axial current emission turn-on time was increased from 100/spl plusmn/22 nanoseconds to 175/spl plusmn/42 nanoseconds. Aluminum optical emission was delayed >150 nanoseconds after the overshoot in voltage after two-stage RF plasma processing.

Optical spectroscopy of plasma and plasma processing in high power microwave pulse shortening experiments

Summary form only given, as follows. Spectroscopic measurements have been performed to characterize the undesired plasma in a multi-megawatt coaxial gyrotron. This gyrotron is 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 (/spl sim/40 MW) microwave power pulselength to 50-100 ns. Potential explanations of pulse shortening are being investigated, particularly plasma production inside the cavity and at the e-beam collector. The source of this plasma is believed to be due to water vapor. Plasma H-/spl alpha/ line radiation has been characterized and correlated with microwave power and microwave cutoff. Experiments are underway to determine the effects of RF plasma processing of the coaxial cavity and collector. A collaborative effort is underway with the Stanford Linear Accelerator Center/U.C. Davis to study RF cavity breakdown. A SEM is being used to examine the surface effects of RF processing cavity parts.

Microwave production and beam transport in a multi-MW large-orbit, axis encircling, coaxial gyrotron oscillator

Summary form only given, as follows. Large orbit, coaxial gyrotrons are currently under investigation with microwave power up to 40 MW. The electron beam is produced by MELBA (Michigan Electron Beam Accelerator) with the following parameters: 0.75-1.0 MV, 1-10 kA diode current, 0.2-1.5 kA tube current, 0.5-1.0 microsecond pulselength. The axis encircling electron beam is generated by a cusp magnetic field. Electron beam current transport through the magnetic cusp and through the microwave cavity are measured. The frequency of the fundamental coaxial mode, TE/sub 111/, is observed to be 2.34 GHz. A novel method of time-frequency analysis (TF) using reduced interference distributions is used to analyze heterodyne mixer data, TF analysis shows microwave frequency modulation with e-beam voltage modulation. Mode competition and mode hopping between TE/sub 111/ and TE/sub 112/ modes are also observed by TF analysis. Microwave cold test data are compared to operating modes of the gyrotron.