Jun-Chieh Wang

Intense Electron Beam Cyclotron Masers with Microsecond Pulselengths

Experiments are underway to investigate the generation of MW to GW microwave power levels utilizing intense (1-10 kA), relativistic (0.4-1 MV), electron beams with pulselengths from 0.4 to 1 μs. Significant research issues concern the effects of beam voltage fluctuations and space charge on long-pulse microwave emission power and frequency stability. Two prototype masers have been developed utilizing a moderate energy pulseline generator with peak parameters: 0.4 MV, 1.2 kA, and 0.4 μs. Prototype I utilized a nonadiabatic step in the diode magnetic field to impart a large α (V⊥/V11≈1-2) to a relatively low transported current of 30-60A. With a uniform waveguide resonator this maser generated peak microwave power levels of 0.6 MW and pulselengths of 0.3 μs over 85% voltage droop. The emission frequency was close to the relativistic cyclotron frequency. The Prototype II maser employed an improved magnetic field profile which transported 500-1000 A of beam current, but at a lower value of α(0.2 - 0.3). A series of permanent magnet wiggler/kickers were used in preliminary tests. Power levels were achieved in the range of 2 MW in the X-band and at MW levels in the K-band. These emission frequencies apparently correspond to the CRM forward-wave and backward-wave interaction with the TE11n cavity modes. By lowering the magnetic field below a critical value the microwave emission was cut off. Design studies have been performed for a long-pulse MW-GW cyclotron maser on the MELBA accelerator with parameters: 0.8 - 1 MV, 1 - 10 kA, and 1 - 1.5 μs.

Frequency-tunable, high-power microwave emission from cyclotron autoresonance maser oscillation and gyrotron interactions

The authors report experimental observations of high-power microwave emission from both the high-frequency cyclotron autoresonance maser (CARM) oscillation and the low-frequency gyrotron interaction. High-power (3-10 MW) microwave emission is attributed to the CARM mechanism, which is magnetically tunable through discrete axial cavity modes from 15 through 16.7 GHz. For the same experimental parameters, megawatt-level microwave emission is observed, which is magnetically tunable from 10 through 14 GHz, indicating low-frequency gyrotron oscillation. High-frequency microwave pulse-lengths of up to 0.2 mu s have been generated, which is a factor of five greater than those achieved in previous CARM experiments. >

Glow-like atmospheric pressure micro-discharges produced by charge rollers

Summary form only given. Conductive charge rollers (CR) are used in print engines for surface charging of the cylindrical photoconductor (PC) at atmospheric pressure. The charging process is essentially a dielectric-barrier-discharge (DBD). Microplasmas are produced in the narrowing gap between the CR and PC, which then charges the PC. The streamer-like plasmas can be terminated by surface charging of the PC if operated with a dc or quasi-dc voltage on the CR. From a practical matter, the surfaces of both the CR and PC are rotating. The rotation of the PC brings in uncharged surface which reestablishes the voltage between the CR and PC, and re-ignites the plasma. As a result, a periodic charging pattern on the PC surface may be formed. Under certain operating conditions, a glow-like discharge was simulated in the CR and a quasi-dc current was collected on the PC surface. These behaviors and the uniformity of surface charging are sensitive to the speed of the PC and applied voltage.In this presentation, we will discuss the behavior of atmospheric pressure microplasmas sustained in air between the CR and PC, and the charging properties on the PC surface using results from a 2-dimensional simulation. The model, nonPDPSIM, solves Poissons equation and transport equations for charge and neutral species and the electron energy conservation equation for electron temperature. A Monte Carlo simulation is used for tracking sheath accelerated electrons. Rate and transport coefficients for bulk electrons are obtained from local solutions of Boltzmanns equation for the electron energy distribution. Radiation transport is addressed using a Greens function approach.

Electron Current From an RF Microdielectric Barrier Discharge

Nonarcing microdielectric barrier discharges (mDBDs) using radio-frequency excitation are attractive in that the arrays of devices can be inexpensively produced to generate surface sources of plasma or radicals. Images of the time evolution of electron extraction from an mDBD sustained in atmospheric-pressure N2 are presented.

Modeling of micro-dielectric barrier discharges

Summary form only given. Arrays of micro-plasmas having dimensions of tens to hundreds of microns are finding use as sources of radicals and charged particles in addition to their conventional use as photon sources. In one variant of these devices, the electrodes are fully or partially covered by dielectrics, and so they operate as dielectric barrier discharges (micro-DBDs). As such, the devices must be pulsed or driven with high frequency (HF) waveforms. When operating at atmospheric pressure in air, the plasma formation and decay times can be as short as tens of ns, and so the plasma may need to be re-ignited with each discharge pulse. In this situation, the physical structure of the micro-DBD and the electron emitting properties are important to its operation.In this presentation, we will discuss the properties of microDBDs sustained in atmospheric pressure N2 and air using results from a 2-d plasma simulation. The micro-DBDs are sandwich structures with openings of tens-of-microns excited with HF voltage waveforms. The model includes solution of Poissons equation, transport of charged and neutral species, radiation transport, electron photo-emission from surfaces, and beam transport of secondary electrons. We found that, depending on the details of the voltage waveform and surrounding structures, the plasma can be expelled from the micro-DBD cavity during one part of the HF cycle, thereby requiring the plasma to be reformed later in the cycle. This expulsion is partly facilitated by the Debye length being larger (in some cases) than the DBD cavity. Long lived neutral species in the plasma can facilitate restart by production of secondary electrons from surfaces. For example, UV photon emission from long lived states continuously provides seed secondary electrons at surfaces until the potential is favorable to generate the plasma.

Intense Electron Beam Cyclotron Masers With Microsecond Pulselengths

Experiments have been performed in which MW microwave power levels have been produced by means of the cyclotron maser resonance on high current, relativistic electron beams from a generator with parameters: 0.4 MV,1.2 kA, and 0.4 gs. In the first series of experiments, a nonadiabatic dip in the magnetic field imparted large ratios of perpendicular to parallel momentum (a >1) in a transported beam current of typically 20 - 60 A. X-Band microwave power (>0.6 MW) was sustained over nearly the full e-beam pulselength and 85% variation in electron beam generator voltage. In more recent experiments, MW power levels in X-band have been generated using an improved magnetic field profile which increased the transported electron beam current to 500 - 1000 A, but with lower a (0.2 -0.3). A wiggler has been utilized to increase the ratio of perpendicular to parallel momentum. Data show a strong (up to a factor of 40) enhancement in peaks of K-band microwaves when the e-beam energy is near a resonance condition with the wiggler. Design studies are underway for a multimegawatt, microsecond, intense electron beam cyclotron maser which will operate on the Michigan Electron Long Beam Accelerator (MELBA) at parameters: 0.7-1 MV, 1-10 kA, and voltage compensated pulselength of 1-1.5 gs.