Optical Thomson Scattering on

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Optical Thomson Scattering on
MAGPIE Generator

Sidxms


Cerberus Laser Project

January 24
th

2012

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.

Scattering Regime



Scattering can occur under two regimes


incoherent or collective depending on
the parameter
α

= 1/k
λ
D

.


Incoherent scattering occurs from random electrons and gives Doppler
broadening. Can deduce T
e


Coherent scattering occurs from electrons localised to Debye Length. Can work
out T
e

and T
i
. Both need density measurements from
interferometry
.



Assumptions
-

Maxwellian

distribution for electrons and ions. Non Relativistic,
No magnetic Fields,

T
i
=T
e
=10eV, 10
17

cm
-
3

Measuring Velocity

Flow
direction v

k
in

k
out

k

Scattering volume



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

Cylindrical Wire Array

Images from
2005 Plasma Phys. Control. Fusion 47 A91



Cylindrical implosion of fine wires ( Al (30um) or W (5
-
10
μ
m) ,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

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 )


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)

Velocity Measurements



Spatially resolved spectra show blue/red Doppler shifts from background
wavelength. (
32x10
μ
m W Array, 153ns)




Gaussian fits applied to line
-
outs (Low error


few percent). The broadening
width gives T
i

Velocity Profiles for Al and W Arrays



Ablated plasma accelerated
towards axis reaching peak
velocity of 1.2
-
1.3 x 10
7

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.

Precursor Measurements


Early time (~ 100ns ) measurements of ion
temperature on axis for Al and W arrays.



Single peak indicates T
i

>
ZT
e




High initial temperature (20keV) indicates
higher flow velocities early in time ~ 1.8 x
10
7

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.


W

Al

Al

W

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



T
e

can be deduced from
Maxwellian

fit ~ 2eV

Not Really Relevant!



Scattering at density gradients can be experimental hazard!

Shadowgraph of a radial wire load

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 10
7

cm/s in Al arrays and 1x10
7

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.