Student Seminar - February 2020

Joe Owen

Investigation into the performance of mid-Z hohlraum wall liners for producing X-ray drive

M-band transitions in Gold are responsible for producing a population of high energy X-rays (> 1.8 keV) in indirect drive inertial fusion [1] experiments. These X-rays can preheat the fuel, cause the ablator-fuel interface to become unstable to Rayleigh-Taylor instabilities [2], and introduce radiation non-uniformity to the X-ray drive [3]. The work presented here investigates the performance of mid-Z lined hohlraums for generating an X-ray drive with a spectrum absent of high energy X-rays. Using the radiation hydrodynamics code h2d the removal of the M-band transitions is observed in the Cu lined hohlraum resulting in a close to 3x reduction in the total hard X-ray energy. Total radiation energy in the Cu lined hohlraum is 83% of the energy in the Au hohlraum for this 1 ns pulse. Instantaneous soft X-ray drive energy for the lined hohlraum exceeds that of the Au hohlraum late in the pulse.

[1] Robey, H. F. et al. Experimental measurement of Au M-band flux in indirectly driven

double-shell implosions. Phys. Plasmas 12, 1–7 (2005).

[2] Betti, R., Goncharov, V. N., McCrory, R. L. & Verdon, C. P. Growth rates of the ablative

Rayleigh–Taylor instability in inertial confinement fusion. Phys. Plasmas 5, 1446 (1998).

[3] Y. Li, J. Gu, C. Wu, et al, Physics of Plasmas 23, 7, 072705 (2016)

Matthew Khan

Absolute Measurements of Hot Electrons in near NIF-Scale Shock Ignition Conditions

Shock Ignition is an advanced scheme for laser-driven inertial confinement fusion (ICF) that separates the compression and ignition phase of an implosion into two distinct parts. The fuel is first compressed at sub-ignition velocities, offering robustness to hydrodynamic instabilities that plague all forms of ICF, then late in time a rapid increase in laser intensity launches a strong shock that will ignite the assembled fuel. Difficulties arise in launching this shock as high intensity lasers must propagate through the plasma corona produced during the compression phase that will have high thermal temperatures and long density lengthscales, conditions that are highly susceptible to the generation of laser plasma instabilities (LPI). These LPI can drive plasma waves resulting in the production of super-thermal, so called ‘hot’, electrons that have the potential to degrade the ignition capabilities of the implosion.

A campaign on the Omega laser facility recreated the plasma corona conditions that would be found in a near-NIF scale shock ignition implosion to study the production of these hot-electrons, assess their characteristics, and determine the potential of shock ignition. Heuristic x-ray models and Monte-Carlo simulations of Bremsstrahlung cannons are used to find the hot-electron temperatures and the conversion efficiency of laser energy into hot-electron energy. The results presented provide an encouraging outlook for shock ignition.

Matthew KHAN 1, Luca ANTONELLI 1, Kevin GLIZE 2, Nigel WOOLSEY 1, Wolfgang
THEOBALD 3, Christian STOECKL 3, Mingsheng WEI 3, Riccardo BETTI 3, Stefano ATZENI 4,
Warren GARBETT 5, Chikang LI 6 and Robbie SCOTT 2

1) Department of Physics, University of York, York, YO10 5D, UK
2) Central Laser Facility, STFC Rutherford Appleton Laboratory, Didcot, OX11 0QX, UK
3) Laboratory for Laser Energetics, University of Rochester, Rochester NY, 14623, USA
4) Dipartimento SBAI, Università“La Sapienza”, Roma, Italy
5) AWE Aldermaston, Reading, Berkshire, RG7 4PR United Kingdom
6) Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge,
Massachusetts 02139 USA

This work was supported by the EPSRC [EP/L01663X/1] and [EP/P026796/1]

Michael Mo

Application of Energy Resolved Actinometry in Ar-O2 Inductively Coupled Plasmas

Real-time feedback control is a pinnacle aim for improving plasma processing efficiency, especially in the semiconductor manufacturing industry. Current advanced diagnostic techniques, such as Two-photon Absorption Laser Induced Fluorescence (TALIF), can give good information about plasma parameters. However, these are difficult to implement on existing industrial machines due to their experimental complexity. Therefore, a non-intrusive method that can bridge the gap between ease of measurement whilst retaining accurate results is desired.

Energy Resolved Actinometry (ERA) is an optical-based diagnostic that can simultaneously determine the mean electron energy and atomic oxygen density, and has been applied to oxygen-argon plasmas in an Inductively Coupled Plasma (ICP) source, but operated in the (capacitive) E-mode [1]. The technique has been benchmarked against TALIF with good agreement [2]. However, in this regime, high sheath potentials exist and forms energetic ions that can damage substrate surfaces.

On the other hand, the (inductive) H-mode has lower potentials, and also provides orders of magnitude higher plasma densities, thus improving etch rates. Consequently, ERA has been implemented to determine the plasma parameters in this latter regime. Currently, modifications to the analysis are being investigated to determine accurate plasma parameters since ERA does not provide reasonable results in the H-mode compared to the E-mode.

Michael KT Mo, Andrew Gibson, Chris Bowman, Timo Gans, Deborah O’Connell

[1] Tsutsumi et al., J. Appl. Phys. 121, 143301 (2017)

[2] Greb et al., Appl. Phys. Lett. 105, 234105 (2014)