High-power laser experiment forming a supercritical collisionless shock in a magnetized uniform plasma at rest

We present an experimental method to generate quasiperpendicular supercritical magnetized collisionless shocks. In our experiment, ambient nitrogen (N) plasma is at rest and well magnetized, and it has uniform mass density. The plasma is pushed by laser-driven ablation aluminum (Al) plasma. Streaked optical pyrometry and spatially resolved laser collective Thomson scattering clarify structures of plasma density and temperatures, which are compared with one-dimensional particle-in-cell simulations. It is indicated that just after the laser irradiation, the Al plasma is magnetized by a self-generated Biermann battery field, and the plasma slaps the incident N plasma. The compressed external field in the N plasma reflects N ions, leading to counterstreaming magnetized N flows. Namely, we identify the edge of the reflected N ions. Such interacting plasmas form a magnetized collisionless shock.

Figure 1

Schematic view of our experiment (a) before and (b) after the shot. Solid arrows represent an external magnetic field. Al plasma expands (white arrows) and pushes magnetized N plasma to generate a collisionless shock (dotted curve).

Figure 2

(a) Bird's eye view of experimental setup. (b) The side view of the setup. The target normal ($X^{\prime }$ axis) is in the $x-z$ plane. (c) The top view of the setup. The probe laser (${\vec{k}}_i$: $p$ axis) for measurements of TS ion feature (IAW) focuses at TCC, and the scattered lights are measured from two different directions IAW-1 (${\vec{k}}_{s,1}$) and IAW-2 (${\vec{k}}_{s,2}$). (d) The measurement wave numbers ${\vec{k}}_{\mathrm{IAW},1}={\vec{k}}_{s,1}-{\vec{k}}_i$ and ${\vec{k}}_{\mathrm{IAW},2}={\vec{k}}_{s,1}-{\vec{k}}_i$ are roughly longitudinal and transverse to the flow, respectively.

Figure 3

SOP images for cases with (a) no ambient gas ($P_N=0$) and no external magnetic field ($B_0=0$), (b) $P_N=5$ Torr and $B_0=0$, and (c) $P_N=5$ Torr and $B_0=3.6$ T. The dashed line P0 in panel (a) shows a constant velocity of 800 km ${\mathrm{s}}^{-1}$. The curve of constant deceleration R1 in panel (b) is described by parameters $v_0=1600$ km ${\mathrm{s}}^{-1}$ and $t_0=48$ ns, and curves P1 and P2 are represented by $v_0=590$ km ${\mathrm{s}}^{-1}$ and $t_0=148$ ns. TCC is located at $X^{\prime }=1.4$ cm ($p=0$), and epochs of TS measurements shown in Fig. 6 are shown by white circles with error bars meaning gate width. Assuming a plane wave with normal vector along the $X^{\prime }$ direction, we also put white squares representing positions and times of TS measurements in Fig. 7 ($p=1$ mm), and white triangles where we estimate upstream plasma parameters ($p=-2$ mm).

Figure 4

Time evolution of self-emission intensity (the intensity as a function of time) at fixed positions $X^{\prime }$, for cases with (a) no ambient gas ($P_N=0$) and no external magnetic field ($B_0=0$), (b) $P_N=5$ Torr and $B_0=0$, and (c) $P_N=5$ Torr and $B_0=3.6$ T. In panel (a), the black-dashed, black-dotted, and red-solid curves show temporal evolutions at $X^{\prime }=0.8$, 1.1, and 1.4 cm (TCC), respectively. Blue circles in panel (a) on each curve represent the time of passage at each position of a trajectory P0 in the $X^{\prime }\text{−}t$ plane [Fig. 3] represented by $X^{\prime }=v_{\mathrm{Al},0}t$ with a constant velocity $v_{\mathrm{Al},0}=800$ km ${\mathrm{s}}^{-1}$. In panels (b) and (c), black-dashed, red-solid, and black-dotted lines are for $X^{\prime }=1.3$, 1.4 (TCC), and 1.5 cm, respectively. Blue circles in panels (b) and (c) represent the same as panel (a) but for R1 and R2, respectively, whose functional form is given by Eq. (1). Similarly, green triangles in panels (b) and (c) represent the time of passage of P1 and P2, respectively.

Figure 5

Spatial distribution of self-emission for the case of $P_N=5$ Torr and $B_0=3.6$ T at $t=22$ ns (black-dashed line) and 23 ns (red-solid line), that is, one-dimensional slices of Fig. 3 at these epochs. In the gray-shaded regions, the detector gain becomes smaller due to pixel damage. Blue circles on each curve represent the position of passage at each epoch of a trajectory R2 represented by Eq. (1) with constants $v_0=1000$ km ${\mathrm{s}}^{-1}$ and $t_0=64$ ns.

Figure 6

Background-subtracted TS spectra at TCC ($p=0$) obtained at (a) $t=10$ ns (b) 15 ns, (c) 23 ns, and (d) 30 ns, for the case of $P_N=5$ Torr and $B_0=0$. Shaded area around the incident wavelength of 532.0 nm is affected by stray light. Black curves are data of IAW-1, while the red curve in panel (b) is of IAW-2. Blue solid lines in panels (b) and (d) show the best-fit results, while the dashed line in panel (a) explains only the lower-temperature component (see the Appendix pp2-s1 for details). Blue dashed line (dot-dashed line) in panel (c) is theoretically expected spectrum from N plasma in equilibrium state with parameters $T_i=1$ keV, $Z{T}_e=0.73$ keV, $Z{n}_e=2.6\times {10}^{19}{\mathrm{cm}}^{-3}$, and $V_{X^{\prime }}=450$ km ${\mathrm{s}}^{-1}$ ($T_i=1.5$ keV, $Z{T}_e=1.4$ keV, $Z{n}_e=4.1\times {10}^{19}{\mathrm{cm}}^{-3}$, and $V_{X^{\prime }}=400$ km ${\mathrm{s}}^{-1}$).

Figure 7

(a) Measured TS spectrum at $t=23$ ns for $B_0=3.6$ T along the probe laser axis $p$, which was obtained by IAW-1. (b) Comparison of TS spectra at $p=1$ mm [dashed line in panel (a)] for $B_0=3.6$ T (red) and 0 T (black) cases. [(c)–(h)] Results of 1D PIC simulation with $B_0=3.5$ T at $t=23$ ns, in which the horizontal axis is the distance from the target $X^{\prime }$. (c) Electron phase-space plot. [(d), (e)] Ion phase-space plots. In these panels, blue and yellow points represent Al and N plasmas, respectively, and green curves show the electron bulk velocity. Bottom three panels show spatial profiles of transverse magnetic field strength (f), electron density $n_e$ (g), electron temperature $T_e$ [blue line in panel (h)], and emissivity of the plasma self-emission $n_e^2/\sqrt{T_e}$ normalized by far upstream value [red line in panel (h)].

Figure 8

Background-subtracted IAW-1 spectra at $t=10$ ns in the case of $P_N=5$ Torr and $B_0=0$ for positions $p=-2$ mm (red dotted line), $-1$ mm (green dashed line), 0 (black solid line), and 1 mm (blue dot-dashed line). The intensity in the gray-shaded region around the incident wavelength of 532.0 nm is diminished by a filter in the spectroscopic optics system to cut the stray light. TCC corresponds to $p=0$, so that the black solid line is identical to the data of Fig. 6.

Figure 9

Background-subtracted IAW-1 spectra at $t=23$ ns in the case of $P_N=5$ Torr and $B_0=3.6$ T for various positions $p$. The upper panel is for cases of $p=-2.0, -1.5, -1.0, -0.5$, 0.0, 0.5, and 1.0 mm, while the lower panel for cases of $p=1.0$, 1.5, and 2.0 mm. TCC corresponds to $p=0$. The gray-shaded region around the incident wavelength of 532.0 nm is the same as that of Fig. 8.

Figure 10

Enlarged view of Fig. 9 around 532 nm for $p=1$ mm. The solid black line is background-subtracted observed data, which is the same as red solid lines in Fig. 9. The gray-shaded region around the incident wavelength of 532.0 nm is the same as those of Figs. 8 and 9. The red-dashed line in this figure represents the best-fitted result (see text for details). The blue-dotted line is expected in the case of the same parameters as $t=10$ ns, $P_N=5$ Torr, $B_0=0$, and $p=-2$ mm (i.e., $T_i=T_e=5.9$ eV, $n_e=1.2\times {10}^{18}{\mathrm{cm}}^{-3}$, and $Z=2.8$).

Figure 11

Results of 1D PIC simulation with $B_0=0$ T at $t\approx 10$ ns (left) and $t\approx 23$ ns (right). Format is the same as Figs. 7–7. The horizontal axis is the distance from the target $X^{\prime }$. From top to bottom, panels of the first row are electron phase-space plots, and those of the second and third rows are ion phase-space plots. In these panels, blue and yellow points represent Al and N plasmas, respectively, and green curves show the electron bulk velocity. Panels of the fourth and fifth rows show the transverse magnetic field strength $B_y$ and electron density $n_e$, respectively. The bottom panels represent electron temperature $T_e$ (blue line) and emissivity of the plasma self-emission $n_e^2/\sqrt{T_e}$ normalized by far upstream value (red line). Note that the electron density $n_e$ is displayed with a logarithmic scale. The bottom panel represents electron temperature $T_e$ (blue line) and emissivity of the plasma self-emission $n_e^2/\sqrt{T_e}$ normalized by far upstream value (red line). Note that to enlarge the small variation around a structure “R1,” we take the range of the plasma self-emission $n_e^2/\sqrt{T_e}$ up to 20 (left panel) or 10 (right panel), which are different from plots in Figs. 7 and Fig. 12.

Figure 12

Results of 1D PIC simulation with $B_0=3.5$ T at $t\approx 5$ ns (left), $t\approx 10$ ns (center), and $t\approx 15$ ns (right). Format is the same as Figs. 7–7. The horizontal axis is the distance from the target $X^{\prime }$. From top to bottom, panels of the first row are electron phase-space plots, and those of the second and third rows are ion phase-space plots. In these panels, blue and yellow points represent Al and N plasmas, respectively, and green curves show the electron bulk velocity. Panels of the fourth and fifth rows show the transverse magnetic field strength $B_y$ and electron density $n_e$, respectively. The bottom panels represent electron temperature $T_e$ (blue line) and emissivity of the plasma self-emission $n_e^2/\sqrt{T_e}$ normalized by far upstream value (red line). Note that the electron density $n_e$ is displayed with a logarithmic scale.