Three-Halves-Harmonic Radiation from Long-Scale-Length Plasmas Revisited

Introduction Starting with the very early experiments relating to laser fusion, observations have shown acharacteristic spectral emission near the 312 harmonic of the irradiating laser ~ a v e 1 e n ~ t h . l For simplicity we will name this emission "312 self-emission" to distinguish it from similar spectral features that may be induced by a separate probe beam. This spectral signature was immediately recognized as being related to the underlying t w ~ ~ l a s m o n d e c a ~ ~ ~ ITPD) instability near quarter critical density. The detailed generation mechanism for this emission is, however, still a subject of considerable controversy and active research. This article represents another attempt to explain the detailed origin of this emission and its consequences for diagnostic applications. These conclusions can be succinctly summarized as follows: ( 1 ) The observed threshold for this 312 emission is not a good indicator of the threshold of the underlying TPD instability; (2) the absence of 312 emission is not a reliable indicator of the absence of the TPD instability; and (3) the value of the spectral features as a temperature diagnostic is very limited at best.