Characterisation and Modelling of Ultrashort Laser-Driven Electromagnetic Pulses

Abstract

Recent advances on laser technology have enabled the generation of ultrashort (fs) high power (PW) laser systems. For such large scale laser facilities there is an imperative demand for high repetition rate operation in symbiosis with beamlines or end-stations. In such extreme conditions the generation of electromagnetic pulses (EMP) during high intense laser target interaction experiments can tip the scale for the good outcome of the campaign. The EMP effects are several including interference with diagnostic devices and actuators as well as damage of electrical components. The EMP issue is quite known in the picosecond (ps) pulse laser experiments but no systematic study on EMP issues at multi-Joule fs-class lasers has been conducted thus far. In this paper we report the first experimental campaign for EMP-measurements performed at the 200 TW laser system (VEGA 2) at CLPU laser center. EMP pulse energy has been measured as a function of the laser intensity and energy together with other relevant quantities such as (i) the charge of the laser-driven protons and their maximum energy, as well as (ii) the X-ray K_α emission coming from electron interaction inside the target. Analysis of experimental results demonstrate (and confirm) a direct correlation between the measured EMP pulse energy and the laser parameters such as laser intensity and laser energy in the ultrashort pulse duration regime. Numerical FEM (Finite Element Method) simulations of the EMP generated by the target holder system have been performed and the simulations results are shown to be in good agreement with the experimental ones.

Figure 1

(a) Conceptual scheme of laser-solid target interaction with EMP generation. (b) Typical EMP signal detected at 30 cm from the target. Inset: normalized spectrum averaged over 23 shots.

Figure 2

The EMP energy density as function of the laser intensity by: (left) changing the laser energy and (right) changing the laser pulse duration. The curves present the total charge deposited on target as function of the intensity obtained by computer simulations. The default input parameters of the simulation are given to be: E_las = 2.5J, T_las = 40 fs, Material Thickness e_tar = 10 μm, laser absorption C_abs = 0.4 and focal spot diameter d_tart = 14 μm.

Figure 3

Plot of the proton cut-off energy shown by the dashed curves with green diamonds, total measured charge on detector shown by dotted curve with blue triangles, electron charge shown by dash-dot curve with red circles and the EMP Energy, shown by the black curve, with squares as function of the laser pulse intensity.

Figure 4

Octave wavelet analysis of the EMP from 0 to 80 ns (left side) and from 70 to 120 ns (right side). The x-axis the time is given at the y-axis the frequency. Dark blue colors correspond to high intensity frequency modes.

Figure 5

(a) Reflecting coefficient as function of the frequency. Inset: target impedance matching with reflection coefficient S₁₁ as function of target impedance for f = 148 MHz. (b) Typical induced current into stepper motor phase line (shot 29 at 8/02/2018). Inset: relative energy of frequency modes injected into motor.

Figure 6

(a) Obtained frequency modes corresponding to the minima of the scattering parameter as shown in Fig. 5. The target holder is visible by the white rectangle at center while the target chamber is modelled as a cylinder. (b) One of the eigenmodes corresponding to eigenfrequency 818 MHz obtained by FEM simulation. The figure shows the electrical field strength at the surfaces. The dark rectangle in the center represents the target holder. Red colors correspond to high field strengths.

Figure 7

(a) Experimental setup: the red beam shows the laser path towards the focusing parabola and laser target. Two Moebius probes (Moebius 1 and Moebius 2) are inserted at 2.5 cm and 30 cm from the TCC respectively. The ToF detector and the Kalfa imaging system were placed outside the target chamber at 40 and 10 degree respectively with respect to the target normal. (b) The pinhole matrix of Al foil targets.