Lecture 2: Symmetry issues

Oswald Willi

Institut für Laser- und Plasmaphysik

Vorlesung SS 2007User ID: LaserPasswort: Plasma

Tel. 0211 81 12157

oswald.willi@laserphy.uni-duesseldorf.de

Different ignition concepts

The Rayleigh-Taylor instability

RT instability in inertial confinement fusion

RT instability induced by laser imprint

RT instability induced by target modulations

The Rayleigh-Taylor instability occurs when a heavy fluid sits on top of a lighter fluid

The RT instability is studied due to its potential to degrade the implosion performance

Rayleigh-Taylor instability

Face on and side on radiographic techniques are used to measure RT growth

Temporal evolution of the Rayleigh-Taylor instability (Osaka)

A shaped, low adiabat x-ray drive is generated in a hohlraum. Foil diagnosis is through imaging in

back illumination with x-rays.

Rayleigh-Taylor instability

Ablative stabilization reduces the growth rate, results from material flowing through the RT

unstable region with a velocity va

RT growth rate scaling with wavelength

For ICF, a high aspect ratio target is required to amplify the ablation pressure, but Rayleigh-Taylor

instability limits the aspect ratio

Cylindrical Rayleigh-Taylor instability

A series of 2-D images showing RT growth in converging geometry

3-D hydrodynamic simulations of RT instability at fuel-pusher interface

The uniformity problem

Laser imprinting and the RT instability in inertial confinement fusion

The physical process of imprint is similar to that of the classical Richtmyer - Meshkov instability

Laser spatial non-uniformities give rise to perturbations in the driving pressure

Mechanism for non-uniform imprinting due to lack of thermal smoothing

Observations of laser imprinting and 3-D hydro-code simulations

Considerable growth is observed in the case of a smooth CH foil driven by a laser beam with a single

60µm mode perturbation

Model based on single-mode imprint scalings from simulations predicts multi-mode result

Single-mode imprint simulations provide scalings for time and amplitude of saturation

Saturation of imprint is predicted by single mode simulations for start-up conditions

Proposed solutions to the imprint problem

Multi-wavelength driveProblem: early-time imprint is still a problem, even at long

Low intensity start-upProblem: imprint still seen at the very lowest irradiances

Ultra-broad bandwidth lasersProblem: not developed

Beam smoothing techniquesProblem: expensive

Indirect-direct drive

Foam.buffered targets

Indirect drive

Problem: unable to control the blow-off plasma, leading to large instability generation. Also - strong shock wave launched into the solid - preheat

Supersonic x-ray preheating of a foam overcoat. Large initialDac can be created, with no blow-off problem and shock generation

Problem: conversation to x-rays

Schematic of the operation of random phase plate arrays

Random phase plates and equivalent focal planes

Arrangement of induced spatial incoherence system

Variation of ISI smoothing with bandwidth

Even ISI irradiation still show plasma non-uniformities

Equivalent plane images of raw and optically smoothed laser beams

Side-on XUV radiographs of premodulated CH foils driven by coherent, ISI/RPP smoothed laser drives

and soft x-rays to bare targets

Plasma smoothing

Thermal smoothing of non-uniform laser deposition in a preformed plasma

Schematic of hybrid x-ray optical drive schemes using foam overcoats

Diffusive electron conduction acts to thermally smooth the non-uniform energy deposition fusion

Soft x-ray imaging system with submicron spatial resolution

Transmission radiograph recorded on a foil target irradiated by an RPP-smoothed laser

Transmission radiograph recorded on an uncoated foam + foil target

Foam buffering mitigates the imprint problem

2D LASNEX simulations show the increased scalelength in the gold-coated foam target

Density contour plots of a plain plastic foil accelerated with coherent radiation

Density contour plots of a CH foil and pre-irradiated foam buffer layer accelerated with coherent

radiation

Simulation of laser imprint seeding and RT growth

Foam overcoatings have been shown to affect imprinting and target stability

Spitzer simulations of foam-buffered targets show higher ablation velocities and reduced RT growth

compared to bare targets