Measurement of ultra-high magnetic fields in petawatt laser experiments

Summary form only given, as follows. Ultra-high magnetic fields are one of the more interesting and far-reaching physical phenomena that can be generated with high intensity laser radiation. Vacuum B-fields of 2,900 MGauss accompany focused laser intensities of 10/sup 21/ W/cm/sup 2/; moreover, when solid targets are irradiated, relativistic electron jets are generated and transported into the target, producing, in turn, poloidal magnetic fields of several hundred MGauss. These fields are particularly important in the "fast ignitor" concept for inertial fusion, since they may ultimately dominate laser energy deposition by influencing electron transport. Direct measurement of such large B-fields are intrinsically difficult since they are generated in over-dense plasma, normally not accessible to optical probing. With high laser intensity and concomitant relativistic electron mass correction, however, harmonic emission from much higher densities may be detected. By this means, the observation of sub-harmonic structure superimposed on emission from PW laser irradiated targets has been interpreted with a harmonic/cyclotron model to provide experimental evidence for large B-fields in over-dense plasma. Markedly different spectra are observed in 100 TW and PW laser experiments. Results for nominal focused intensities will be discussed, together with the emission model that indicate B-fields in excess of 100 MGauss have been measured for the first time.