ICF-related research at Strathclyde

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Nov 15, 2013 (3 years and 10 months ago)

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ICF
-
related research at Strathclyde

Paul McKenna

University of Strathclyde

EPSRC grant:
EP/E048668/1

Key physics for ICF diagnosed by ion emission

1.
Fast electron generation and transport in dense
plasma

2.
Shock propagation physics


3.
Laser
-
ion source development (ion fast ignition)



(Nuclear diagnostics of laser
-
plasma)

Fast electron generation and transport:

Diagnostic

Energy
range

Issues

K


emission

10s keV

Wavelength shift with temperature

CTR / OTR
emission

MeV

Limited to thin targets due to electron
bunch dephasing

“Escaped” electron
spectrometry

MeV

Target charges to MV potentials

Notoriously difficult to measure fast electrons in solid targets. Each
diagnostic has limitations due to assumptions, model dependences etc.

Examples:

Our approach: Ion emission as a diagnostic

Laser pulse

Target

RCF stack

Sample RCF

Beam sampling
for analysis

Protons

Structured
sheath



Maximum proton energy


electron density (MeV energies)



I
ntensity distribution


electron transport filamentation



Proton divergence with energy


electron sheath profile



Proton spectrum


electron temperature (model)





Thick solid density targets can be investigated (>mm)


0
100
200
15
20
25
30
35
40
Scale length,
L
O
(

m)
Maximum proton energy (MeV)
0
100
200
0
1
2
3
4
5
6
7
Scale length,
L
O
(

m)
Energy conversion efficiency (%)
I
abl


t

0
100
200
15
20
25
30
35
40
Scale length,
L
O
(

m)
Maximum proton energy (MeV)
0
100
200
0
1
2
3
4
5
6
7
Scale length,
L
O
(

m)
Energy conversion efficiency (%)
0
100
200
15
20
25
30
35
40
Scale length,
L
O
(

m)
Maximum proton energy (MeV)
0
100
200
0
1
2
3
4
5
6
7
Scale length,
L
O
(

m)
Energy conversion efficiency (%)
I
abl


t

0
100
200
15
20
25
30
35
40
Scale length,
L
O
(

m)
Maximum proton energy (MeV)
0
100
200
0
1
2
3
4
5
6
7
Scale length,
L
O
(

m)
Energy conversion efficiency (%)
0
100
200
15
20
25
30
35
40
Scale length,
L
O
(

m)
Maximum proton energy (MeV)
0
100
200
0
1
2
3
4
5
6
7
Scale length,
L
O
(

m)
Energy conversion efficiency (%)

10
11
10
12
10
13
0
5
10
15
20
25
30
35
40
Intensity (W/cm2)
Shock front position (um)
Cu 0.5 ns
Cu 1.5 ns
Cu 3.5 ns
Au 0.5 ns
Au 1.5 ns
Au 3.5 ns
-60
-40
-20
0
20
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
X-position (microns)
Density (g/cm3)
0.5 ns
1.5 ns
3.5 ns

10
11
10
12
10
13
0
5
10
15
20
25
30
35
40
Intensity (W/cm2)
Shock front position (um)
Cu 0.5 ns
Cu 1.5 ns
Cu 3.5 ns
Au 0.5 ns
Au 1.5 ns
Au 3.5 ns
-60
-40
-20
0
20
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
X-position (microns)
Density (g/cm3)
0.5 ns
1.5 ns
3.5 ns


Proton measurements show that controlled preplasma
expansion leads to enhanced energy coupling to fast electrons

1: Laser propagation and energy absorption

P. McKenna et al, LPB
26

591
-
596 (2008)


D.C. Carroll et al, CRP
10

188
-
196 (2009)

OSIRIS Simulations

-200
-100
0
10
18
10
20
10
22
10
24
X (

m)
Electron density (cm
-3
)
-10
0
10
20
10
23
10
24
10
25
X (

m)
n
e
(cm
-3
)
Pollux 0.5 ns
Pollux 3.5 ns
Expt. 0.5 ns
Expt. 3.6 ns


Preplasma expansion enhances electron
energy spectrum



Self focusing and beam break
-
up observed



changes to the electron injection angle

0
20
40
10
3
10
4
10
5
Electron energy (MeV)
Number of electron (arb. units)
Sharp gradient
Pollux 0.5 ns
Pollux 3.5 ns
Sharp density
gradient

0.5ns Pollux
density profile

3.5ns Pollux
density profile

Simulations by Roger Evans



Evidence of collimation of fast electrons in solid targets by self
-
generated B
-
field observed using proton emission

2. Collimation of fast electron transport

Yuan…McKenna., submitted (2009)

0
300
600
900
1200
1500
0
10
20
30
40
Target thickness (

m)
Maximum proton energy (MeV)


ballistic model (27
o
)
RCF
TP-Spec
0
300
600
900
1200
0
100
200
300
400
500
Target thickness (

m)
Sheath diameter (

m)


ballistic model (27
o
)
Inferred from expt.
Simulations with 2
-
D hybrid LEDA code

Electron refluxing within thin targets perturbs B
-
field structure

Ne no B field

Ne with B field

Simulations by Alex Robinson (RAL)

3: Effects of target material on beam filamentation


CH

SiO
2

glass

Bk7 glass

Li

Al

Au

Laser
pulse

Target

RCF
stack

Sample RCF

Protons

Target

Effective Z

Resistivity

[
Ω.m]

Al

13

10
-
8

C
3
H
6

5.4

10
13

Li

3

10
-
7

SiO
2

11.6

10
14

ZEPHROS hybrid
-
PIC simulations

Simulations by Alex Robinson (RAL);

Li curve calculation by Mike Desjarlais (Sandia)

4. Shock propagation physics



Exception sphericity of implosion required for ICF



Non
-
uniformities in illumination or target roughness amplified by Richtmeyer
-
Meshkov and Rayleigh
-
Taylor instabilities



Uniform drive pressure can result in non
-
uniform shock propagation depending on
grain alignment in the material



e.g. Be is naturally polycrystalline with different shock velocities along different
crystal axes


grain size is ~10

m

D Swift et al.,

Shock uniformity measurements using proton emission

Our approach


use proton emission imaging to measure perturbations of
the initial shock breakout

CPA illumination timed to coincide with shock breakout thus imprinting the
rear surface geometry on the ion emission.

Proof
-
of
-
principle tests in January 2010

Sub
-
micron structure
on target surface

Reproduced in
proton beam

M. Roth
et al
., PR
-
STAB
5
, 061301 (2002)

Lindau
et al.
, PRL 95, 175002 (2005)

Proton emission is sensitive to shock
breakout

Laser pulse energy (J)
0
100
200
300
400
Conversion efficiency (%) to protons
with energy greater than 4 MeV
0
1
2
3
4
5
6
7
8
10 micron
25 micron
Robson
et al,

Nat Phys 2007



E
L

10
µm Al,

~5 µm

25
µm Al,

~5 µm

Controlling the front surface density
gradient gives a factor of 2 increase in
conversion efficiency



2
µm Al,

~80 µm, ~10
19

Wcm
-
2


Thin targets and defocused
laser spot gives even higher
conversion efficiency



DT fuel at 300g/cc



35

洠mgni瑩n獰t

Curved proton rich target

5: Laser
-
ion source development



Proton energy scaling with ps pulse



Spectral shaping with dual CPA pulses



Techniques to enhance conversion efficiency

1.
Proton emission applied as a diagnostic of fast electron generation
and transport

Examples:


Electron generation as a function of plasma scale length


Collimating effect of self
-
generated magnetic fields


Electron transport filamentation


Electron transport in compressed targets (HiPER, LULI)


2.
Shock propagation physics


Ion diagnostic technique to be trialled in January 2010


3.
Laser
-
ion source development (ion fast ignition)


Spectral control and enhancement of conversion efficiency


4.
Nuclear diagnostics of laser
-
plasmas

Summary of ICF
-
related physics at Strathclyde

Collaboration:

P. McKenna et al

SUPA, Department of Physics, University of Strathclyde


D. Neely,
A.P.L. Robinson

et al


STFC, Rutherford Appleton Laboratory


R G
Evans

Imperial College London


M. Borghesi, M. Zepf et al

School of Mathematics and Physics, Queen’s University Belfast
.


J. Fuchs et al


LULI Ecole Polytechnique, France


M. P. Desjarlais

Sandia National Laboratories, New Mexico



6: Nuclear activation

1


Development of laser
-
plasma nuclear diagnostics



choice of activation reactions with well
-
known
cross sections;



spectral, spatial and yield measurements of n,

,

ions;



significant development work required for in
-
situ
measurements in noisy plasma environment, using
radiation hardened detectors;

2


Innovative nuclear diagnostics

Examples may include:



fusion reaction history measurements using gamma detectors (NIF)
(D + T




+
5
He)
;



charged particle detection to measure yield of neutronless reactions
(e.g. D +
3
He


p (15 MeV) +
4
He
);



Higher nuclear yields expected; observation of lower cross section and higher
threshold energy reactions;

Effect of angle change with energy

Magnetic field generation during electron propagation is described by combining
Ohm’s law with Faraday’s law:

f
j
E



Ohm’s law

= resistivity

= fast electron current density


f
j
Generates a magnetic field that
pushes electrons towards regions of
higher current density

Generates a magnetic field that pushes
electrons towards regions of higher
resistivity



f
f
j
j
B
E
B















t
t
Robinson and Sherlock, Phys.
Plasmas, 14, 083105 (2007)

Fast electrons

B field

Homogeneous
plasma

laser

Resistive generation of toroidal B
-
field. B
-
field pinches the fast electron beam

Magnetic collimation

Results: Maximum proton energy

Ballistic transport and P. Mora PRL 2003 plasma expansion model.

sheath
e
pp
N
pp
i
B
p
n
e
t
T
k
E






/
1
~
~
,
2
/
,
)]
1
[ln(
2
2
2




The scaling with target thickness is significantly different than expected from ballistic electron
transport

Yuan et al., Vulcan TAP

Fuchs et al., LULI (Nat. Phys. 2006)

Carroll
et al
, Phys. Rev. E
76
, 065401 (2007)

Beam divergence (degrees)

Proton energy (MeV)

Assume sheath profile

)
,
(
)
(
)
,
(
max
t
x
H
t
F
E
t
x
E

0
w
Ionizes hydrogen


proton

Ion front
profile

Sheath
profile

Beam divergence

Good fit?

0
w
No

Yes

Measured
divergence

0
w
modify

Initial sheath diameter

Sheath expansion is modelled

10um

Divergence

Source size

Model is benchmarked using grooved
target results

Scaling of the collimation effect

Bell
-
Kingham theory: In the limit of substantial heating, collimation parameter:

2
,
2
/
1
5
/
2
sec
5
/
2
2
/
1
511
,
10
/
3
511
,
5
/
1
5
/
2
5
/
2
5
/
3
23
)
2
(
ln
13
.
0








rad
p
m
f
f
TW
t
R
T
T
P
Z
n


Electron temperature from LEDA simulations

0
50
100
150
200
250
186
188
190
192
194
196
p / m
e
c
log(f
0
0
(p)) (at obs. cell)
0
50
100
150
200
250
186
188
190
192
194
196
p / m
e
c
log(f
0
0
(p)) (at obs. cell)


LEDA
fit at 13MeV
fit at 7MeV
fit at 8MeV
Temperature variation

Black line is electron spectrum at rear surface of a 400 micron Al target


Red is fit using the input electron distribution and temperature (9.2 MeV)

Same temperature!

Artificially increasing scattering:


Increase electron
-
ion scattering rate


Log
Λ = 2 → 10 → 100


Marginal effect on beam
smoothness

Field duration variation

P Mora PRL 2003 plasma expansion model

0
500
1000
1500
0
10
20
30
40
50
60
Target thickness (um)
Maximum proton energy (MeV)



p
2

p
4

p
-600
-400
-200
0
200
400
600
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
x 10
11
Position (um)
Electrical field (V/m)
E-field with space at peak field time
The temporal evolution is assumed to
be a combination of Gaussian increase
and exponential decrease.


This trend is supported by LEDA
simulation, as well as the previous
reports.

McKenna et al PRL 98, 145001 (2007)

Kar et al PRL 100, 105004 (2008)

Electrical field transverse distribution is
assumed to be parabolic function

Carroll et al PRE 76, 065401 (2007)

Brambrink et al PRL 96, 154801 (2006)

)
,
(
)
(
)
,
(
t
x
H
t
F
E
t
x
E
p

0
)
/
exp(
0
)
2
/
exp(
)
(
0
2
2






t
t
t
t
t
t
F

)
(
4
/
1
)
,
(
2
t
p
x
t
x
H


2
/
)
(
)
(
2
0
vt
w
t
p


-1
-0.5
0
0.5
1
1.5
2
2.5
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
-2
-1
0
1
2
3
0
200
400
600
800
1000
1200
1400
1600
Time (ps)
Sheath full width (um)
-800
-600
-400
-200
0
200
400
600
800
0
1
2
3
4
5
6
7
x 10
11
Position (um)
Sheath strength (V/m)
W0

Sheath size and source size with time

An example fit of beam divergence

Example sheath field profiles at different time

Transverse expansion velocity with time

From Patrizio Antici’s PhD thesis

E Brambrink et al PRL 96, 154801 (2006)

Both suggest an exponential decrease of expansion velocity with time

Comparing simulation and experiment results

Simulation densities used in plasma
expansion model

Sheath size as a function of target thickness

Reduced growth in sheath size for
thick targets

Lateral expansion of the fast
electrons is limited

Self
-
induced fields become more
important in thicker targets

title

I = 5 x 10
20
W/cm
2

Lancaster et al., PRL
98
, 125002 (2007)

Green et al., PRL
100
, 015003 (2008)

Target thicknesses ~100
µm

Diagnostics:



K


emission


XUV emission


Shadowgraphy

title

Fast electron

density

Magnetic Fields