Measurements of Strong Stimulated Brillouin Scattering in a Microwave-Plasma Interaction Experiment.
Abstract
Strong Stimulated Brillouin Scattering (SBS) is measured in the U. C. Davis CEREBERUS tank, a device that models SBS in laser driven pellets. A large diameter (D/(lamda)(,0) (DBLTURN) 2), semi-parallel (L/(lamda)(,0) (DBLTURN) 10) microwave beam (2.9 GHz, (v(,0s)/v(,e))('2)(, )<(, )1.0) is normally incident onto an argon plasma (n/n(,cr) < 1). The plasma is essentially collisionless, unmagnetized, inhomogeneous (L(,n) (DBLTURN) 10(lamda)(,0)) and quiesent. To model specular reflection from the critical surface, the microwaves are initially reflected by a grounded screen. The resulting standing microwaves drive a resonant ion wave (k(,i) (DBLTURN) 2k(,0)) that grows for powers above a clearly defined threshold, (v(,0s)/v(,e))('2)(, )>(, )0.1. This threshold and the growth rate are in good agreement with theory. Well above threshold, greater than 90% of the incident microwaves are scattered by ion quasi-modes. The phase velocity of these ion waves increases linearly with power and is very high, V(,phase) (DBLTURN) 5C(,s), at high powers, (v(,0s)/v(,e))('2) (DBLTURN) 1. Near threshold the phase velocity is that of a normal mode, V(,phase) (DBLTURN) C(,s). Well above threshold, the ion wave amplitude is believed to be limited only by pump depletion. A simple theory is constructed to show that ion trapping is ineffective as a saturation mechanism for these ion waves when V(,phase )>(, )2 C(,s). When the ion wave driver is removed, either by switching off the microwaves or by pump depletion, the ion waves quickly ((omega)(,i)('-1)) become normal modes. The waves then collapse due to ion trapping and heated ions are generated. The hot ion temperature and density are combined with conservation of energy and momentum to estimate the ion wave amplitude just before wave collapse. This estimate, (DELTA)n/n (TURN) 1, is consistant with a scattering region, L(,s), which is found to be a few wavelengths, L(,s) (DBLTURN) 2(lamda)(,0), and independent of the incident power level. This process does not reach steady state. Instead the scattering region moves down the density gradient, toward the microwave source. Large amplitude ion quasi -modes are driven in the plasma in front of the reflection point, the point where the transmitted power approaches zero, and behind it the ion waves collapse due to pump depletion and ion trapping.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1983
- Bibcode:
- 1983PhDT........26J
- Keywords:
-
- Physics: Fluid and Plasma