NIF and JLF User Group Meeting 2014
Four graduate students from the High Energy Density Physics (HEDP) Research Group gave posters at the National Ignition Facility (NIF) and Jupiter Laser Facility (JLF) User Group Meeting in February.
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 7Li(d,n)8Be. This process suffers from 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 metal frame of variable shape and size. Since the frame is not perturbed from a laser shot, this target removes common issues associated with solid target positioning, ensuring that alignment is not the limiting factor for shot rate. In addition, this apparatus provides control of flow rate and target thickness, and allows for the high repetition rates required to generate desired neutron fluxes with a portable laser-based system. The repetition rates possible with this setup will be useful for many other applications in high energy density physics. The apparatus design will be presented and early progress will be described, including demonstration of thin film rejuvenation at a 10 Hz repetition rate.
This work was performed with support from DARPA.
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.
This work was performed with support from ASOFR, DARPA, and allocations of computing time from the Ohio Supercomputing Center.
Creating Near Uniform Temperature Solid Density Plasmas with Intense Femtosecond Lasers
The isochoric heating of reduced mass targets was investigated with the SCARLET laser using 10J, < 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.
This work is supported by DOE office of fusion science under grant # DE-SC0008730.
Laser Diagnostics for Ohio State's Petawatt-Class Scarlet Laser
At Ohio State’s petawatt-class Scarlet laser, we have recently completed implementation of techniques to characterize laser properties relevant to the field of High Energy Density Physics. Laser parameters that sensitively affect experiments can now be tracked on a shot-to-shot basis. This is achieved through a dedicated suite of on-shot diagnostics which measure energy, spectrum, focal spot quality and pulse duration simultaneous with experiments. These include a focal spot camera, spectrometer, energy meter, and autocorrelator, each of which has been calibrated for differences between the diagnostic location and the experimental target location. A glass slide diagnostic exploits the plasma mirror effect to verify laser pre-pulse levels on the nanosecond scale, similar to a water cell diagnostic. A scanning third-order cross-correlator, developed in-house, measures pre-pulse and post-pulse features on the picosecond scale. A wavefront sensor detects laser wavefront distortions, yielding focal spot quality and permitting improvement using a future deformable mirror. The Scarlet laser leverages crossed-polarized wave generation (XPW) to improve contrast, and simple checks of this apparatus have been implemented. Working in concert, these diagnostic tools provide an understanding of the laser necessary to explore the widest range of cutting-edge High Energy Density Physics at the Scarlet Laser Facility.
This work was supported by the US DOE, NNSA, and Fusion Science Center.