Mode-Selective Laser Propagation and Absorption in Strongly Magnetized Inhomogeneous Plasma

Abstract

We systematically investigate the field-aligned propagation and collisional absorption of normally incident laser light in a strongly magnetized inhomogeneous plasma. Analytical expressions for electric fields in both vacuum and plasma are derived. Using analytical modelling and particle-in-cell simulations, we establish the cutoff conditions, absorption efficiencies, and scaling laws for the right-hand (R) and left-hand (L) circularly polarized waves. The dependence of collisional absorption coefficient on magnetic field strength, plasma scale length and laser intensity are quantified. In particular, L waves reflect at cutoff density, with absorption strongly enhanced as the magnetic field increases. For the R-waves, the absorption decreases with increasing magnetic field when the normalized electron cyclotron frequency is less than unity. However, when it exceeds unity, the R-waves propagate as whistler modes without a cutoff, allowing penetration into overdense plasma. This enables deep energy deposition inside overdense plasma. These results provide a framework for understanding laser-plasma energy coupling through collisional absorption in strongly magnetized inhomogeneous plasma.

Figures (3)

• Figure 1: PIC simulation of short-pulse laser ($\lambda=1~\mu\text{m}$, pulse duration $30$ fs, $I_L=10^{14}~W/cm^{2}$) normally incident on a highly magnetized collisional plasma ($B=2$, $n_e=n_c \cdot z_0/5\mu\text{m}$, $T=100$ eV). ($a_1 \sim a_3$) LCP laser: $t=10$, 30 and 50 $T_0$ (left to right, same as below); ($b_1 \sim b_3$) RCP laser; ($c_1 \sim c_3$) LP laser.
• Figure 2: Relative electric field $E(z)/E_0$ from standing wave model ($\lambda=1~\mu\text{m}$, $n_e/n_c=z_0/L_0$ with $L_0=5~\mu\text{m}$, $\nu_c=0.001$). (a) LCP with B=0.2: $E_{s,v}(z<0)=E_{i,v}+E_{r,v}$ (black) is the superposition of incident $E_{i,v}$ (red) and reflected laser $E_{r,v}$ (blue), $E_{s,p}(z \geq 0)$ (black) is the standing wave in plasma; (b) LCP, B=2: $E_{s,v}$ and $E_{s,p}$; (c) RCP, $B=0.2$, similar to (a); (d) RCP (whistler mode): $B_0=2*10^4$ T and $\lambda=1~\mu{m}$ (red, $B=2$), $B_0=6*10^3$ T and $\lambda=10~\mu{m}$ (black, $B=6$), and $B_0=2*10^4$ T and $\lambda=10~\mu{m}$ (blue, $B=20$), $L_0=20~\mu{m}$, $\nu_c=0.01$.
• Figure 3: Absorption coefficient $f_{abs}$ of R wave (solid curve) and L wave (dashed curve), obtained from standing wave model (a,b) and travelling wave model (c,d). (a) $f_{abs}$ v.s $L$: $B=0$ (black), $B=0.2$ (blue), $B=0.8$ (red) and $B=2$ (red), with $\nu_c=0.001$, $L_0=5\mu{m}$; (b) $f_{abs}$ v.s $B$: $L_0=5\mu{m}$ (black) and $L_0=50\mu{m}$ (blue), with $\nu_c=0.001$; (c) $f_{abs}$ v.s $I_L/I_L^*$: $B=0$ (black), $B=0.2$ (blue), $B=0.8$ (red) and $B=2$ (red); (d) $p_{abl}(a.u.)$ v.s $\lambda(\mu{m})$ for L wave: $B_0=0$, $I_L= 10^2 *I_{L0}^*$ (black); $B_0=6*10^3$ T, $I_L= 10^2 *I_{L0}^*$ (red); $B_0=6*10^3$ T, $I_L= 10^4 *I_{L0}^*$ (blue); $I_{L0}^*= 2.6\times 10^{9} Z ln \Lambda\cdot L_0$.