Study of instabilities in long scale-length plasmas with and without laser-beam-smoothing techniques

Journal Article · Fri Jun 01 04:00:00 EDT 1990 · Physics of Fluids B; (USA)

DOI:https://doi.org/10.1063/1.859549· OSTI ID:6756719

Willi, O; Afshar-rad, T; Coe, S ^[1]; Giulietti, A ^[2]

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1. Imperial College of Science, Technology, and Medicine, Prince Consort Road, London, SW7 2BZ, England (GB)
2. Istituto di Fisica Atomica e Molecolare del C.N.R., 56100 Pisa (Italy)

Experimental results are presented for the interaction of an intense laser beam in large preformed, homogeneous underdense plasmas. Parametric instabilities, including filamentation, stimulated Raman (SRS) and stimulated Brillouin scattering (SBS), have been extensively studied. In this paper comparisons are made between coherent and smoothed laser interaction beams for their effectiveness in suppressing these instabilities. Direct observations of the enhanced breakup of the coherent interaction beam due to filamentation are presented. Measurements of the transverse scale lengths of the hot spots, and the level of the intensity modulations of the filamentary structures are given. Further, direct experimental observations of SRS and SBS generated in filaments are presented. These provided conclusive evidence that this instability provides the primary mechanism for the generation of SRS and SBS under such plasma conditions. With the smoothed laser interaction beams, using random phase plates (RPP) and induced spatial incoherence (ISI), the level of filamention was significantly reduced. The backscattered fractions of SRS and SBS with respect to the irradiance of the interaction beam were measured for coherent, RPP and ISI. Reduction of typically a factor of 20 and 500 were recorded using RPP and ISI, respectively, in the backscattering levels of SRS and SBS when compared with the coherent interaction beam. The reductions are attributed primarily to the effectiveness of the beam-smoothing techniques in suppressing the filamentation instability.

• OSTI ID: 6756719
• Journal Information: Physics of Fluids B; (USA), Journal Name: Physics of Fluids B; (USA) Vol. 2:6; ISSN 0899-8221; ISSN PFBPE
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
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ALUMINIUM
BACKSCATTERING
BRILLOUIN EFFECT
COHERENT SCATTERING
CONFINEMENT
ELEMENTS
FOILS
HOMOGENEOUS PLASMA
INERTIAL CONFINEMENT
INHIBITION
INSTABILITY
LASER-PRODUCED PLASMA
METALS
NUCLEAR REACTIONS
NUCLEOSYNTHESIS
PARAMETRIC INSTABILITIES
PLASMA
PLASMA CONFINEMENT
PLASMA DENSITY
PLASMA DIAGNOSTICS
PLASMA INSTABILITY
PLASMA MACROINSTABILITIES
RAMAN EFFECT
SCATTERING
SYNTHESIS
THERMONUCLEAR REACTIONS