Optical Thomson Scattering on MAGPIE Generator

1. Optical Thomson Scattering on MAGPIE Generator Cerberus Laser Project Sidxms January 24th 2012

2. Thomson Scattering • Scattering of electromagnetic radiation is a powerful diagnostic for plasma conditions. • Incident electromagnetic wave is scattered via interaction with a charged particle. • Ion contribution is negligible due to greater mass. • Information can be obtained about plasma temperature and velocity.

3. Scattering Regime • Scattering can occur under two regimes – incoherent or collective depending on the parameter α = 1/kλD . • Incoherent scattering occurs from random electrons and gives Doppler broadening. Can deduce Te • Coherent scattering occurs from electrons localised to Debye Length. Can work out Te and Ti. Both need density measurements from interferometry. • Assumptions - Maxwellian distribution for electrons and ions. Non Relativistic, No magnetic Fields, Ti=Te=10eV, 1017 cm-3

4. Measuring Velocity • Doppler shift of the scattered spectrum can be attributed to bulk motion • Frequency shift is given by • Component of velocity in the direction of k is measured. Δω=k.v k kout kin Flow direction v Scattering volume

5. Cylindrical Wire Array • Cylindrical implosion of fine wires ( Al (30um) or W (5 -10 μm) ,16 - 32 wires) • 3 main stages– ablation , implosion , stagnation • We measure flow velocities during the ablation phase. • Measurements needed to compare with simulation codes , improve X-ray yield Images from 2005 Plasma Phys. Control. Fusion 47 A91

6. Experimental Setup • Commercial YAG laser amplified in 25mm Silicate glass amplifier • 4J , 8ns FWHM , 532nm , 1 shot every 10 minutes. • Injection seeded for narrow bandwidth improves spectral resolution ( ~ 0.02nm line width c.f. 0.4nm for YAG )

7. Scattering Geometry • Scattering cross section is very low so need to use Brewster windows at entry + exit and baffles to suppress stray light. • Accounting for finite collection angles – roughly 10-12 of incident light is collected. Temporally gate 4ns of emission with Andor Imaging spectrometer. • 7 fibre array used to obtain spatial information (700um spatial resolution, 1.3mm spacing)

8. Velocity Measurements • Spatially resolved spectra show blue/red Doppler shifts from background wavelength. (32x10 μm W Array, 153ns) • Gaussian fits applied to line-outs (Low error – few percent). The broadening width gives Ti

9. Velocity Profiles for Al and W Arrays • Ablated plasma accelerated towards axis reaching peak velocity of 1.2-1.3 x 107 cm/s • As the plasma reaches the axis, it decelerates due to collisions with other plasma streams and the precursor. • 3D MHD simulations agree well with Al data but not quite so well with W data close to precursor. • Ion temperature profile shows increases on axis corresponding to ‘thermalisation of the kinetic energy’. Later times – low temperature due to radiation loss from narrow precursor.

10. Precursor Measurements W W • Early time (~ 100ns ) measurements of ion temperature on axis for Al and W arrays. • Single peak indicates Ti > ZTe • High initial temperature (20keV) indicates higher flow velocities early in time ~ 1.8 x 107 cm/s. Later temperature agrees well with the measured flow velocity at that time. • For Al arrays, the measured temperature agrees well with the measured velocity. Theoretical fit gives range of plasma parameters. Decrease in ion temperature due to radiative cooling and energy transfer to electrons. Al Al

11. Electron Temperature • Old measurement from ‘Plasma Gun’ used to set up experiment with better rep rate. Central Peak from stray light Gaussian base is the actual signal • Te can be deduced from Maxwellian fit ~ 2eV

12. Not Really Relevant! • Scattering at density gradients can be experimental hazard! Shadowgraph of a radial wire load

13. Conclusion • A Thomson scattering diagnostic was used to measure velocity and temperature profiles of ablation flow in cylindrical wire arrays. • The ablation flow undergoes acceleration towards the axis reaching velocities of 1.2-1.3 x 107 cm/s in Al arrays and 1x107 cm/s in W arrays. • Good agreement with MHD codes for Al arrays but collisionality not modelled correctly for W arrays. • Precursor temperature is well matched with measured flow velocities.