Magnetized laser-produced plasmas are central to many novel laboratory astrophysics and inertial confinement fusion studies, as well as in industrial applications. Here we provide the first complete description of the three-dimensional dynamics of a laser-driven plasma plume expanding in a 20 T transverse magnetic field. The plasma is collimated by the magnetic field into a slender, rapidly elongating slab, whose plasma-vacuum interface is unstable to the growth of the "classical", fluid-like magnetized Rayleigh-Taylor instability.The combination of high-power lasers with externally applied high-strength magnetic fields of up to kT [1,2] has been seminal in the development of many recent applications in laboratory astrophysics [3][4][5][6][7], in novel concepts in laser-[8-10] and magnetically-driven [11,12] inertial confinement fusion physics, and in industrial applications [13,14]. Beside understanding the dynamics of the plasma expansion across a magnetic field, of particular importance is to grasp the nature of rapidly growing instabilities which may develop and profoundly modify the morphology and characteristics of these plasmas. Indeed, the presence of striations and flutes have often been associated with the development of instabilities and in particular with the lower hybrid drift instability (LHDI) or one of its variants [15][16][17]. In addition, anomalous resitivity driven by the LHDI [18,19] can also affect the plasma microscopically, with potentially important consequences on magnetic field diffusion and the growth of other instabilities. Among those, the magnetic Rayleigh-Taylor instability (MRTI) [20,21] is known to play a key role on the dynamics of laboratory [22], as well as astrophysical plasmas [23,24]. So far however, it has not been isolated in laser-produced high energy density plasmas.A major parameter affecting the stability and dynamics of these plasmas is the relative direction of the applied magnetic field with respect to the plasma expansion axis. While for an aligned magnetic field the plasma is collimated into an axisymmetric, stable jet-like flow [4,5], for a transverse magnetic field both stable[25] and unstable flows [26] were observed and a clear understanding of the plasma evolution is still missing.Here, we provide the first complete description of the three-dimensional dynamics of a laser-driven plasma plume in a transverse 20 T magnetic field. We show that the plasma is collimated into a slender, rapidly expanding slab, and demonstrate that under these conditions, the growth of flute-like, interchange modes at the plasma-vacuum interface that extend in the form of spikes into the vacuum is due to the classical, fluid-like, magnetic Rayleigh-Taylor instability (MRTI). Interestingly, we find that to recover quantitatively in the simulations the penetration of these spikes into the vacuum, a subgrid-scale model of anomalous resistivity needs to be included. This anomalous resistivity could be induced by the micro-turbulence generated by the LHDI, which for our plasma condition...

This manuscript is distributed under a Creative Commons Attribution-NonCommercial-NoDerivs License (https://creativecommons.org/licenses/by-nc-nd/4.0/), which permits distribution and reproduction for non-commercial purposes, provided the author and source are cited. General rightsCopyright for the publications made accessible via the Queen's University Belfast Research Portal is retained by the author(s) and / or other copyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associated with these rights. AbstractThe collimation of astrophysically-relevant plasma ejecta in the form of narrow jets via a poloidal magnetic field is studied experimentally by irradiating a target situated in a 20 T axial magnetic field with a 40 J, 0.6 ns, 0.7 mm diameter, high-power laser. The dynamics of the plasma shaping by the magnetic field are studied over 70 ns and up to 20 mm from the source by diagnosing the electron density, temperature and optical self-emission. These show that the initial expansion of the plasma is highly magnetized, which leads to the formation of a cavity structure when the kinetic plasma pressure compresses the magnetic field resulting in an oblique shock [A. Ciardi et al., Phys. Rev. Lett. 110, 025002 (2013)]. The resulting poloidal magnetic nozzle generates a standing conical shock that collimates the plasma into a narrow jet [B. Albertazzi et al., Science 346, 325 (2014).]. At distances far from the target, the jet is only marginally magnetized and maintains a high aspect ratio due to its high Mach-number (M ∼ 20) and not due to external magnetic pressure. The formation of the jet is evaluated over a range of laser intensities (10 12 -10 13 W/cm 2 ), target materials and orientations of the magnetic field. Plasma cavity formation is observed in all cases and the viability of long-range jet formation is found to be dependent on the orientation of the magnetic field.

We have investigated proton acceleration in the forward direction from a near-critical density hydrogen gas jet target irradiated by a high intensity (1018 W/cm2), short-pulse (5 ps) laser with wavelength of 1.054 μm. We observed the signature of the Collisionless Shock Acceleration mechanism, namely quasi-monoenergetic proton beams with small divergence in addition to the more commonly observed electron-sheath driven proton acceleration. The proton energies we obtained were modest (~MeV), but prospects for improvement are offered through further tailoring the gas jet density profile. Also, we observed that this mechanism is very robust in producing those beams and thus can be considered as a future candidate in laser-driven ion sources driven by the upcoming next generation of multi-PW near-infrared lasers.

High-intensity lasers interacting with solid foils produce copious numbers of relativistic electrons, which in turn create strong sheath electric fields around the target. The proton beams accelerated in such fields have remarkable properties, enabling ultrafast radiography of plasma phenomena or isochoric heating of dense materials. In view of longer-term multidisciplinary purposes (e.g., spallation neutron sources or cancer therapy), the current challenge is to achieve proton energies well in excess of 100 MeV, which is commonly thought to be possible by raising the on-target laser intensity. Here we present experimental and numerical results demonstrating that magnetostatic fields self-generated on the target surface may pose a fundamental limit to sheath-driven ion acceleration for high enough laser intensities. Those fields can be strong enough (~10 5 T at laser intensities ~10 21 W cm –2 ) to magnetize the sheath electrons and deflect protons off the accelerating region, hence degrading the maximum energy the latter can acquire.

Using a gas-jet flow, via the interaction between an ultrashort high-intensity laser pulse and plasma in the presence of a perpendicular external dc magnetic field, the short pulse radiation from a magnetized plasma wakefield has been observed. Different nozzles are used in order to generate different densities and gas profiles. The neutral density of the gas-jet flow measured with a Mach-Zehnder interferometer is found to be proportional to back pressure of the gas jet in the range of 1 to 8 atm. Strength of the applied dc magnetic field varies from 0 to 8 kG at the interaction region. The frequency of the emitted radiation with the pulse width of 200 ps (detection limit) is in the millimeter wave range. Polarization and spatial distributions of the experimental data are measured to be in good agreement with the theory based on the V(p)xB radiation scheme, where V(p) is the phase velocity of the electron plasma wave and B is the steady magnetic field intensity. Characteristics of the radiation are extensively studied as a function of plasma density and magnetic field strength. These experiments should contribute to the development of a new kind of millimeter wavelength radiation source that is tunable in frequency, pulse duration, and intensity.