John Lohr

Experimental determination of the nonlinear interaction in a one dimensional beam‐plasma system

Recent theories have shown that the instability driven by a cold, weak electron beam in a one‐dimensional plasma is stabilized when the unstable wave grows to an amplitude sufficient to trap the electron beam. Quantitative measurements of the instability confirm every prediction of the theory: the maximum wave amplitude, monochromaticity of the unstable mode, harmonic content of the mode, and wavelength of the slow oscillation in wave amplitude. The electron beam goes through the same patterns in phase space and time‐averaged f(v) as predicted. Both scaling and absolute magnitudes agree with theory.

Wave-particle interaction in the late beam-plasma instability

Experiments are reported on the beam‐plasma instability at times after saturation when the electron distribution function exhibits a high energy tail. The results show that high energy electrons are produced by a velocity resonant wave‐particle interaction. The experiments were formed on quite different devices at Dartmouth College and The University of Texas; however, the results from the two laboratories are consistent.

Electron density measurements from cutoff of electron cyclotron emission in the DIII-D tokamak

Cutoff of microwave emission at the second harmonic of the electron cyclotron frequency has been used to infer the local electron density profile in the DIII‐D tokamak. Modeling the density profile during the uncontrolled density increase associated with the transition to H‐mode operation permits the profile shape factor and line average to be estimated. In special cases, the density fluctuations associated with sawteeth can also be inferred. Independent corroboration of the standard density diagnostics has been performed with this technique.

X-Ray Energy Analysis for Radio Frequency Current Drive Experiments in the DIII-D Tokamak

The X-ray energy spectra were measured during electron cyclotron current drive (ECCD) and fast wave current drive (FWCD) experiments in the DIII-D tokamak. High energy electrons, over 100 keV, were observed during ECCD near the plasma center and enhanced radially by the effect of downshifted frequency. Tail formation with the maximum energy of 50 keV was observed clearly during 2 T, 0.8 MA FWCD experiments. For combined FWCD and ECCD, the bulk heating efficiency was increased over the case with FW only, with ηe~2.3×1019 m-3 keV/MW, and it was stronger for the counter (CTR) case than the co (Co) case. (Here Co corresponds to the direction of launched wave which will enhance the plasma current. CTR refers to the opposite direction.) However, the coupling of FWs to high energy electrons produced by ECH was stronger for the Co case.

Grating spectrometer installation for electron cyclotron emission measurements on the DIII‐D tokamak using circular waveguide and synchronous detection

The grating spectrometer installation on the DIII‐D tokamak uses fundamental circular waveguide propagating the TE11 lowest‐order mode followed by oversized circular guide carrying the low‐loss TE01 mode. The short section of fundamental guide permits use of an electronic chopper operating at 100 kHz for both calibration and plasma operation. By using ac‐coupled amplifiers tuned to the chopping frequency, the background signal generated in the indium antimonide detectors by neutrons and x rays is automatically subtracted and the system noise bandwidth is reduced. Compared with a quasi‐optical system, the much smaller fundamental horn and front‐end waveguide allow the waveguide system to be located outside a gate valve. With this configuration the entire waveguide run, including the actual horn and vacuum window used during plasma operations, can be included in the calibration setup.

Lower hybrid wave heating in Doublet IIA

Doublet IIA electron heating experiments utilize lower hybrid waves launched by slow wave structures with various n/sub parallel/ = ck/sub parallel//..omega.. (11, 14, 16) to achieve spatially localized heating by electron Landau damping. Radiofrequency power of 350 kW at 800 MHz and 200 kW at 915 MHz is available to heat circular discharges with ohmic input power of 100 kW. Significant increases in the electrical conductivity have been observed, but plasma response to the rf power is sensitive to the impurity level of the discharge. Power absorption mechanisms and energy balance will be discussed.

High time resolution electrostatic electron analyzer

A gridded electrostatic electron energy analyzer is described which has been used successfully to time resolve trapped electron distributions at 200 MHz in a beam plasma device. Examples of raw data and extracted phase space plots are presented.

Neutral flux measurements during neutral‐beam injection on Doublet III (abstract)

Neutral particle flux parallel and perpendicular to the plasma current and at energies equal to and below the injection energy has been observed with the parallel and perpendicular charge exchange spectrometers on Doublet II. Initial neutral injection is marked by a rapid increase in the high-energy neutral efflux which usually relaxes over tens of milliseconds to a lower equilibrium value. As injected power is increased by the stepped addition of more neutral beams, the measured flux is not linearly additive, often increasing slightly or not at all. For very high values of injected power, in the 5-7-MW range, the neutral particle flux is often observed in bursts correlated with the rapid fall of giant sawteeth on soft x-ray signals. In many of these shots the fishbone instability is present and is observed with the perpendicular spectrometer, however the parallel flux does not correlate with fluctuations observed with magnetic probes unless the instability terminates rapidly, in which case both a soft x-ray drop and a burst of neutrals occurs. Some aspects of these measurements have been modeled by a particle transport code with particular emphasis on changes in the neutral density profile during high-power neutral injection.

Phase locking the auto-resonant accelerator

The normal development of the beam‐plasma instability to a broadened fluctuation spectrum can be retarded by the injection of a plasma wave at the most unstable frequency.