55th Annual Meeting of the APS Division of Plasma Physics

September 17, 2014

55th Annual Meeting of the APS Division of Plasma Physics


The High Energy Density Physics (HEDP) research group is represented at the 55th annual meeting of the American Physical Society Division of Plasma Physics (APS DPP) with three talks and two posters.




Kevin George
Creating Uniform Temperature Solid Density Plasmas with Intense Femtosecond Lasers
The isochoric heating of reduced mass targets was investigated with the SCARLET laser using 10J, \textless~100 fs pulses. Laser intensity (focal spot size) as well as target dimensions (transverse and longitudinal) were varied in an effort to generate plasmas of uniform temperature and density. XUV imaging at 68 eV was employed to infer spatially resolved temperature maps while K-alpha spectroscopy was used to determine bulk target temperature. For 100 micron diameter, 3 micron thick Al/Cu/Al disk targets a bulk temperature in excess of 50 eV was achieved at solid density over a spatial temperature scale length of approximately 70 microns. A reduction of the target thickness and thus total mass was observed to increase target temperature.

Sheng Jiang
Shaping the Spectrum of Hot Electrons using Structured Targets
Hot electron generation is a crucial aspect of the intense laser solid interaction. Proper energy and angular distributions of the fast electrons greatly benefit subsequent processes such as X ray/gamma ray production and ion acceleration. Fast electrons generated using simple flat targets are large in charge, but usually have high divergence, low energy and broad spectrum, which limit the efficacy of their applications. We have used 3D LSP PIC simulations to develop a way to generate high energy, low divergence electrons using structures (spikes or fins) on the target front surface. When an intense, ultra-fast laser pulse interacts with these structures, electrons at the tip are accelerated via direct laser acceleration to energies much higher than the ponderomotive energy. The electric and magnetic fields from these super-hot electrons and the return current inside the structures guide the electrons, leading to a small divergence angle. Varying the structure shape can further tune the electron spectrum.

Andrew Krygier 
On The Origin of Super-Hot Electrons from Intense Laser Interactions with Solid Targets having Moderate Scale Length Preformed Plasmas
The results of a numerical study investigating the acceleration mechanism for super-hot electrons by an intense laser interaction with moderate scale length preformed plasma are described. The particle-in-cell code LSP is used to model a 100J, 175fs, peak intensity 6x1020W/cm2 laser in 2D Cartesian geometry. The laser interacts with a solid density Al target with a L=3μm scale length preformed plasma. We find that a simple three-step mechanism that we call loop-injected direct acceleration (LIDA) is overwhelmingly dominant in the acceleration of the hottest electrons. LIDA involves only well-known physics and is numerically observed over a range of pre-plasma and laser conditions. In LIDA, the laser heats the plasma near the critical surface expelling electrons from the region. Some of the expelled electrons follow looping paths away from the target, guided by quasi-static magnetic fields, and are injected into the intense region of the laser pulse where they are laser-accelerated until they escape into the target with large energy.
This work is supported by DOE contracts DE-FC02-04ER54789 and DE-FG02-05ER54834 and allocations of computing time from the Ohio Supercomputer Center.

Matt McMahon
Modeling target normal sheath acceleration using handoffs between multiple simulations
We present a technique to model the target normal sheath acceleration (TNSA) process using full-scale LSP PIC simulations. The technique allows for a realistic laser, full size target and pre-plasma, and sufficient propagation length for the accelerated ions and electrons. A first simulation using a 2D Cartesian grid models the laser-plasma interaction (LPI) self-consistently and includes field ionization. Electrons accelerated by the laser are imported into a second simulation using a 2D cylindrical grid optimized for the initial TNSA process and incorporating an equation of state. Finally, all of the particles are imported to a third simulation optimized for the propagation of the accelerated ions and utilizing a static field solver for initialization. We also show use of 3D LPI simulations. Simulation results are compared to recent ion acceleration experiments using SCARLET laser at The Ohio State University.
This work was performed with support from ASOFR under contract FA9550-12-1-0341, DARPA, and allocations of computing time from the Ohio Supercomputing Center.

Patrick Poole 
Design of a liquid membrane target for high repetition rate neutron generation
Ultra-bright, pulsed, spatially-small sources of energetic neutrons have applications in radiography and non-destructive remote sensing. Neutrons can be generated by a process wherein ions accelerated from a laser-irradiated primary target subsequently bombard a converter material, causing neutron-producing nuclear reactions, such as 7\textit{Li(d,n)}8\textit{Be}. Deuterons from this process are suppressed by contamination that builds up on the rear of the solid primary target. To eliminate this issue we propose a self-replenishing liquid membrane target consisting of heavy water and deuterated surfactant, formed in-vacuum within a moveable wire frame. In addition to removing issues associated with solid target positioning and collateral damage, this apparatus provides flow rate and target thickness control, and allows for the high repetition rates required to generate desired neutron fluxes with a portable laser-based system. The apparatus design will be presented, as well as a novel interferometric method that measures the membrane thickness using tightly-focused light.