Anomalies in Universal Intensity Scaling in Ultrarelativistic Laser-Plasma Interactions

Laser light incident on targets at intensities such that the electron dynamics is ultrarelativistic gives rise to a harmonic power spectrum extending to high orders and characterized by a relatively slow decay with the harmonic number m that follows a power law dependence, m^{-p}. Relativistic similarity theory predicts a universal value for p = 8/3 up to some cut-off m = m*. The results presented in this work suggest that under conditions in which plasma effects contribute to the emission spectrum, the extent of this contribution may invalidate the concept of universal decay. We report a decay with harmonic number in the ultrarelativistic range characterised by an index 5/3 < p < 7/3, significantly weaker than that predicted by the similarity model.Comment: 5 pages, 4 figure

Raman gain against a background of non-thermal ion fluctuations in a plasma

A complex stimulated Raman scattering event against a background of non-thermal ion acoustic waves in an inhomogeneous plasma is described. We obtain analytic forms for the Raman gain due to a five-wave interaction consisting of conventional three-wave Raman scattering followed by the decay of the Raman Langmuir wave into a second Langmuir wave (or a second scattered light wave) and an ion acoustic wave. Very modest levels of ion waves produce a. significant effect on Raman convective gain. A combination of plasma inhomogeneity and suprathermal ion fluctuations may offer a means for the control of Raman gain

Wire tomography in the H-1NF heliac for investigation of fine structure of magnetic islands

Electron beam wire tomography in the H-1NF heliac enables high resolution mapping of vacuum flux surfaces with minimal disruption of the plasma operations schedule. Recent experimental results have proven this technique to be a highly accurate and high resolution method for mapping vacuum magnetic islands. Islands of width as small as delta approximately 8 mm have been measured, providing estimates of the internal rotational transform of the island. Point-to-point comparison of the mapping results with computer tracing, in conjunction with an image warping technique, enables systematic exploration of magnetic islands and surfaces of interest. Recent development of a fast mapping technique significantly reduced the mapping time and made this technique suitable for mapping at higher magnetic fields. This article presents recent experimental results and associated techniques.with support from the Australian Research Council Grant No. DP0344361