Variable magnetic field electron spectrometer used to measure Resonance absorption electrons in the range of 20-500 keV

Resonance absorption (RA) occurs when a p-polarized electromagnetic (EM) wave obliquely incident on a plasma density gradient tunnels past its turning point and resonantly excites an electron plasma wave at the critical surface. This phenomenon is important in several plasma physics experiments, for instance in inertial confinement fusion (ICF) where it has a deleterious effect. Direct measurement of these RA electrons in the energy range of few tens to a few hundred kev's is a complicated and challenging task, among other reasons due to the relatively low magnetic field needed. The solution described here is a magnetic electron spectrometer (ESPEC) with variable magnetic field, instead of the constant field that is usually applied in ESPEC designs. Applying a continually changing magnetic field, much lower magnetic field at the entrance of the ESPEC and a stronger one towards the end enables to measure a wide spectral range of electrons, between 20 keV to 500 keV. The current design was tested first in an EM- simulation and then by integrating the diagnostic device. The ESPEC energy scale was calibrated by placing a series of Aluminum filters in front of the image plate detector. The electron spectra were acquired from plasmas generated by irradiating Aluminum targets with the combination of: ~ 300 ps pulses followed by 2 ps duration pulse comprised of a series of 50-200 fs lasers pulses from the ALEPH laser at CSU.



Work conducted with the supported of DOE Vannevar Bush Faculty Fellowship ONR award N000142012842, using facilities supported by LaserNet US grant DE-SC0019076. BA’s contribution was funded by the DOE Joint program on HEDLP via grant # SC0022169