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Title: Angular distribution of the bremsstrahlung emission during lower-hybrid current drive on PLT

Abstract

The bremsstrahlung emission from the PLT tokamak during lower-hybrid current drive has been measured as a function of angle between the magnetic field and the emission direction. The emission is peaked strongly in the forward direction, indicating a strong anisotropy of the electron-velocity distribution. The data demonstrate the existence of a nearly flat tail of the velocity distribution, which extends out to approximately 500 keV and which is interpreted as the plateau created by Landau damping of the lower-hybrid waves.

Authors:
; ; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
Princeton Univ., NJ (USA). Plasma Physics Lab.
OSTI Identifier:
5642336
Report Number(s):
PPPL-2225
ON: DE85013762
DOE Contract Number:
AC02-76CH03073
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; BREMSSTRAHLUNG; ANGULAR DISTRIBUTION; PLT DEVICES; CURRENT-DRIVE HEATING; LOWER HYBRID HEATING; MAGNETIC FIELDS; DISTRIBUTION; ELECTRIC HEATING; ELECTROMAGNETIC RADIATION; HEATING; HIGH-FREQUENCY HEATING; JOULE HEATING; PLASMA HEATING; RADIATIONS; RESISTANCE HEATING; THERMONUCLEAR REACTORS; TOKAMAK TYPE REACTORS; 700104* - Fusion Energy- Plasma Research- Plasma Kinetics-Experimental- (-1987)

Citation Formats

von Goeler, S., Stevens, J., Bernabei, S., Bitter, M., Chu, T.K., Efthimion, P., Fisch, N., Hooke, W., Hill, K., and Hosea, J.. Angular distribution of the bremsstrahlung emission during lower-hybrid current drive on PLT. United States: N. p., 1985. Web. doi:10.2172/5642336.
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von Goeler, S., Stevens, J., Bernabei, S., Bitter, M., Chu, T.K., Efthimion, P., Fisch, N., Hooke, W., Hill, K., & Hosea, J.. Angular distribution of the bremsstrahlung emission during lower-hybrid current drive on PLT. United States. doi:10.2172/5642336.
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von Goeler, S., Stevens, J., Bernabei, S., Bitter, M., Chu, T.K., Efthimion, P., Fisch, N., Hooke, W., Hill, K., and Hosea, J.. Sat . "Angular distribution of the bremsstrahlung emission during lower-hybrid current drive on PLT". United States. doi:10.2172/5642336. https://www.osti.gov/servlets/purl/5642336.
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@article{osti_5642336,
title = {Angular distribution of the bremsstrahlung emission during lower-hybrid current drive on PLT},
author = {von Goeler, S. and Stevens, J. and Bernabei, S. and Bitter, M. and Chu, T.K. and Efthimion, P. and Fisch, N. and Hooke, W. and Hill, K. and Hosea, J.},
abstractNote = {The bremsstrahlung emission from the PLT tokamak during lower-hybrid current drive has been measured as a function of angle between the magnetic field and the emission direction. The emission is peaked strongly in the forward direction, indicating a strong anisotropy of the electron-velocity distribution. The data demonstrate the existence of a nearly flat tail of the velocity distribution, which extends out to approximately 500 keV and which is interpreted as the plateau created by Landau damping of the lower-hybrid waves.},
doi = {10.2172/5642336},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Sat Jun 01 00:00:00 EDT 1985},
month = {Sat Jun 01 00:00:00 EDT 1985}
}
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Technical Report:
View Technical Report (0.94 MB)
DOI:  10.2172/5642336
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  • ; ; ; ...
    Lower hybrid current drive requires the generation of a high energy electron tail anisotropic in velocity. Measurements of bremsstrahlung emission produced by this tail are compared with the calculated emission from reasonable model distributions. The physical basis and the sensitivity of this modeling process are described and the plasma properties of current driven discharges which can be derived from the model are discussed.

  • ; ;
    During lower-hybrid heating in low-density-tokamak discharges, a nonMaxwellian tail of high-energy electrons is formed. This tail carries the plasma current. Utilizing the fact that relativistic electrons emit bremsstrahlung predominantly in the forward direction, we investigate the shape of the electron distribution by measuring the dependence of the x-ray emission on the angle between the magnetic field and the line of sight. The experimental data indicate that the distribution function is predominantly peaked in the forward direction, although a small fraction of the electrons is in the backward cone. The energy dependence of the x-ray spectra is consistent with that ofmore » a velocity distribution which has a plateau extending out to several hundred kiloelectron volts. Radial profiles show that the hot electrons are located in the central plasma region and form a high-conductivity plasma with the current profile frozen in. The slope of the spectrum depends on the rf power and on the phasing of the waveguide grill, but not on the externally applied plasma voltage. Relaxation oscillations occur shortly after switching the rf off. They also appear during the rf for low rf power and at the high-density limit of the lower-hybrid current drive. The x-ray spectra confirm that parallel energy is transferred to perpendicular energy during the instability, suggesting an instability due to the anomalous Doppler effect.« less

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    Two distinct conditions where the rf phase velocity is directed opposite to the ohmically induced electron drift have been studied experimentally: (1) rf current ramp-up (f/sub p/ > 0), where an induced electric field opposes the current increase; (2) rf current ramp-down, where waves are launched opposite the electron drift direction for a decaying plasma current. The time behavior of the electron tail is inferred from hard x-ray (30 to 750 keV) emission as a function of angle to B. In cases (1) and (2) we find that the emission amplitude in the reverse (opposite the LH phase velocity) directionmore » increases throughout the rf pulse, while it is constant in the forward direction. This increase indicates that some high energy electrons are accelerated or are even running away in the direction opposite to the main rf produced tail. It also indicates for case (1) that the electric field in the plasma center has been reversed by the rf current drive.« less

  • ; ;
    Order of magnitude improvements in the level and duration of current driven by lower hybrid waves have been achieved in the PLT tokamak. Steady currents up to 175 kA have been maintained for three seconds and 400 kA for 0.3 sec by the rf power alone. The principal current carrier appears to be a high energy (approx. 100 keV) electron component, concentrated in the central 20 to 40 cm diameter core of the 80 cm PLT discharge.

  • ; ;
    Steady currents up to 165 kA for 3.5 seconds and 420 kA for 0.3 seconds have been maintained by 800 MHz lower hybrid waves. For line-averaged densities up to 7 x 10/sup 12/ cm/sup -3/ the current is maintained with no input power from the ohmic heating transformer. The waves are launched with an array of six waveguides. Measurements of X rays and electron cyclotron radiation show that the rf power produces and maintains a suprathermal tail of electrons apparently independent of the number of fast electrons in the plasma prior to turning on the rf power. Measurements of current-drivemore » efficiency and the electron tail provide direct evidence for a resonant wave-particle interaction. The radial profile of the rf-sustained current inferred from x-ray measurements is peaked in the center of the plasma and appears to obey the same q-value restraints as the inductively driven ohmic heating current. Current drive is observed to be accompanied always by radiation at frequencies greater than or equal to ..omega../sub ce/ and less than or equal to ..omega../sub pe/. The connection between this radiation and the current-drive mechanism is under study.« less