High-beta, hot-electron plasmas have been produced by electron-cyclotron heating in the SM-1 axisymmetric mirror using closely-spaced multiple frequencies. The relativistic electrons produce annular distributions (ELMO rings) with as much as ten times more stored energy than with single-frequency heating. While larger frequency separations (Δf/f∼0.1) provide some control of the ring size, the dominant effects are associated with an improvement in heating efficiency which persists to very small frequency separations (Δf/f∼10−3). Details of the reconstruction of the ring distribution (both in steady state and during build-up), the influence of multiple frequency heating on fluctuations, axial electron losses, and a scaling of these effects with power are presented.

An rf driven multicusp source capable of generating 1-ms H− beam pulses with a repetition rate as high as 150 Hz has been developed. This source can be operated with a filament or other types of starter. There is almost no lifetime limitation and a clean plasma can be maintained for a long period of operation. It is demonstrated that rf power as high as 25 kW could be coupled inductively to the plasma via a glass-coated copper-coil antenna. The extracted H− current density achieved is about 200 mA/cm2.

This Letter reports the first observations of stimulated Brillouin scattering in microwave interaction with a plasma, as verified by the satisfaction of the frequency-and wavelength-matching rules and growth rate. A small amount of chamber reflectivity causes ion fluctuations due to the standing-wave ponderomotive force, which then serve as an enhanced noise level for the initiation of the instability. PACS numbers: 52.25.Ps, 52.40.Mj Stimulated Brillouin scattering (SBS) in a plasma is due to the interaction of an electromagnetic wave and an ion acoustic wave. The resultant wave may be scattered at various angles, but the scattering maximizes in the backward direction, with a characteristic frequency downshift of the ion acoustic frequency, oo s = kc s , where c s is the ion sound speed and k is given by momentum considerations as 2k 0 , where k Q is the incident wave number. This process is important in laser fusion, since the scattered energy is not available to the compression process. SBS has been observed in laser-plasma interactions, 1 * 2 identified by the characteristic oo s frequency shift. Here we report the first observations of SBS in microwave interaction with plasma, finding not only the frequency downshift in the reflected wave, but directly measuring the wavelength of the ion acoustic fluctuations. An interesting feature of these experiments is that a small chamber reflectivity sets up ion fluctuations which then serve as an enhanced noise level for the initiation of the stimulated Brillouin-scattered wave. After an initial fast growth time, we find the SBS reflectivity growth rate to agree with the classical calculations. 2The experiments were performed in an unmagnetized plasma of 75 cm diameter and 200 cm length, produced by a multifilament dc discharge with surface multidipole confinement. Typical operating parameters were gas filling pressure (1-3) xlO" 4 Torr, electron density n e = 10 10 -10 n cm" 3 , electron temperature T e -2 eV and electron-ion temperature ratio T e /Ti= 10-12. Various gases were used including hydrogen, helium, neon, and argon. Experiments were performed at a number of microwave pump frequencies in the range 3-16 GHz, with peak powers up to 1 MW and pulse widths, T P , from 0.1-20 p,s. The wave was launched along the chamber axis by a high-gain (-20 dB) gridded horn, the radiation pattern of which gave an effective interaction length of 70-120 cm. In order to avoid possible complications introduced by effects other than SBS, e.g., the parametric decay instability, the electron density was adjusted to 0.1 of the critical density, n c1 for all experiments.The incident and backscattered electromagnetic waves were separated for detection with use of either a circulator or directional coupler, and fed into a square law detector for power measurements or a spectrum analyzer for frequencyshift measurements. Ion waves were detected with movable Langmuir probes, after the microwave pump is off, thus avoiding the problem of rf pickup.Since T e /T i »l, Landau damping is rath...

The laser plasma interaction is simulated by the interaction of a microwave beam with an inhomogeneous, collisionless plasma. Magnetic fields are generated perpendicular to the density gradient and to the incident-wave polarization. The observed sharp reversal of field direction in the resonant region supports resonant absorption as the generation mechanism.