Suppression of Electromagnetic Pulses from Laser-Target Interactions by Strong Magnetic Fields

Abstract

Laser-target interactions generate intense electromagnetic pulses (EMP) that can interfere with measurements and damage equipment. In this paper we show that applying a magnetic field to nanosecond pulse laser-target interactions decreases the magnitude of EMP. We demonstrate this effect in two experiments with different geometries (spherical vs. planar), laser intensities (${\sim}10^{13}$ vs. ${\sim} 10^{15}$~W/cm$^2$) and applied field strength (12~T vs. 0.1~T) that both observed suppression of EMP in the ${\sim} 1$~GHz band (by factors of $0.65\times$ and $0.32\times$ respectively). We then observe the opposite effect at high intensities with a picosecond pulse: for planar experiments with laser intensities ${\sim}10^{19}$~W/cm$^2$ and magnetic fields of 6--10~T, the magnitude of EMP is increased by a factor of $1.75\times$. These results provide a benchmark for models of EMP generation, but suggest that magnetic fields are not a viable solution for mitigating EMP on the high intensity laser facilities where it is most damaging.

Figures (9)

• Figure 1: a) 3-D model of the MagSDD platform showing two MIFEDS coils in a Helmholtz configuration around a capsule illuminated by 60 beams. b) Representative examples of the two laser pulse shapes used in the experiments.
• Figure 2: a) Raw B-dot (blue, orange) and hard x-ray diode (green) traces from an unmagnetized shot and a magnetized shot with the MagImp platform. The shaded region shows the time region that was included in the analysis. The x-ray data shown is from the unmagnetized shot: the magnetized signal is indistinguishable on this scale. b) The spectral power density of the B-dot signal within the selected time range from the same two shots.
• Figure 3: a) Total EMP (summed spectral power of the signal) and b) total summed HXRD channel 3 ($>60$ keV) signal for each shot, each normalized to the mean of the directly comparable unmagnetized shots.
• Figure 4: a) A photo of the setup with the magnetic field orientation, target position, and beam path. b) An average of the laser pulse throughout a run, as measured by a pick-off photodiode.
• Figure 5: a) A raw single shot antenna trace from the unmagnetized run. b) Median power spectra of the antenna signal across the magnetized and unmagnetized runs.