Effects of Resonant Absorption in Direct-Drive Target Designs on OMEGA
48th Annual Meeting of the American Physical Society Division of Plasma Physics Philadelphia, PA 30 October–3 November 2006 I. V. Igumenshchev
University of Rochester
Laboratory for Laser Energetics
8DN
8DN
5JNF OT
-BTFS
QVMTF 24 24
*OUFOTJUZ SFMBUJWFVOJUT 21 m24 24 2 8211
Resonant absorption is important during the rapid increase of laser power in direct-drive target designs on OMEGA
Summary
• Resonant absorption on OMEGA is determined by linear effects.
• Resonant absorption is important during laser pickets or at the beginning of long laser pulses, when the density length scale near the critical
surface is relatively small, L < 2 nm.
• In spherical implosions, resonant absorption can enhance the earlier- time laser absorption up to 20%.
• Planar OMEGA experiments will validate theoretical predictions with the use of inclined s- and p-polarized laser beams.
Collaborators
V. N. Goncharov W. Seka
D. H. Edgell V. A. Smalyuk
T. R. Boehly J. A. Delettrez
A simplified model of resonant absorption predicts a very large electric field at the resonance peak
An oblique ray incident onto a cold inhomogeneous plasma slab z > 0
• Mechanisms limiting the field:
– electron–ion collisions – thermal convection
– nonlinear wave breaking
• Ponderomotive force ~ E8 ^ hd 2B can be important in the
resonance region.
- [
Z
-
[ Y
~QF [ ~
f TJOi f
2BCT
∇OF
i f= -1 ~2pe] gz /~2
E 1 z E : z
d 2
=- f 2f Ez 2
• The linearized electron-momentum equation
u
t me u
m n P
E 1
e
e e
e e e
2
2 =- -o - d
combined with Maxwell’s equations,1
The effect of Langmuir waves has been included in the calculation of laser absorption in the 1-D code LILAC
TC7514
,
v . j
j
c i
i i i
i
E E E
E E
4 0
4 4
3
w Te
2 2
2 2
em pe
:
: d d
d d
- + + =
= + -
+
~ ~r
r ~ o
~
r ~ o
D ^ b
_ _
^
h l
i i
h
1-D planar geometry
1Forslund et al., Phys. Rev. 11, 679 (1975).
• Solutions for s- and p-polarized light are independent
• Laser absorption in LILAC: Q= j :E.
Laser absorption can be split into two components
oem = (collisional damping)
ow = (collisional damping) + (Landau damping) v E
Q E
8
3
w 8
w 2 2 Te
2 2
2 2
2 2
em em
pe d:
=
+ +
o +
~ o
~
r o
~ o r
^ h
EM component Wave component
Thermal electrons Hot electrons
Generation of Langmuir waves is the dominant mechanism that limits the amplitude of resonant fields
TC7516
• Under typical conditions on OMEGA,
cm
>> ,
P CH targets- ,
; I
E
1 10 10 8
to
e e
2 13 2
2 las
fs
# :
m .
r
, . ,
L.1 to 2nm T .0 5 to keV1
] g W
the convection of the Langmuir waves reduces the amplitude of the resonance field below the wave-breaking limit.
• Landau damping of Langmuir waves produces hot electrons with Th ≈ 5 to 10 keV.
• The ponderomotive force
does not exceed the pressure- gradient force and has a small
dynamic effect.
I = 5 × 1014 W/cm2 i = 23.2°
Te = 960 eV
; nN
O FO DSJU
& Z & [ SFMBUJWFVOJUT
8JUIPVU
∇1FUFSN
Generation of Langmuir waves is the dominant mechanism that limits the amplitude of resonant fields
• Under typical conditions on OMEGA,
cm
>> ,
P CH targets- ,
; I
E
1 10 10 8
to
e e
2 13 2
2 las
fs
# :
m .
r
, . ,
L.1 to 2nm T .0 5 to keV1
] g W
the convection of the Langmuir waves reduces the amplitude of the resonance field below the wave-breaking limit.
• Landau damping of Langmuir waves produces hot electrons with Th ≈ 5 to 10 keV.
• The ponderomotive force
does not exceed the pressure- gradient force and has a small
dynamic effect.
I = 5 × 1014 W/cm2 i = 23.2°
Te = 960 eV
8BWFCSFBLJOHMJNJU
; nN
O FO DSJU
& Z & [ SFMBUJWFVOJUT
8JUIPVU
∇1FUFSN
oX
∇1F &r
Generation of Langmuir waves is the dominant mechanism that limits the amplitude of resonant fields
TC7516b
• Under typical conditions on OMEGA,
cm
>> ,
P CH targets- ,
; I
E
1 10 10 8
to
e e
2 13 2
2 las
fs
# :
m .
r
, . ,
L.1 to 2nm T .0 5 to keV1
] g W
the convection of the Langmuir waves reduces the amplitude of the resonance field below the wave-breaking limit.
• Landau damping of Langmuir waves produces hot electrons with Th ≈ 5 to 10 keV.
• The ponderomotive force
does not exceed the pressure- gradient force and has a small
dynamic effect.
I = 5 × 1014 W/cm2 i = 23.2°
Te = 960 eV
8BWFCSFBLJOHMJNJU
; nN
O FO DSJU
& Z & [ SFMBUJWFVOJUT
8JUIPVU
∇1FUFSN
oX
∇1F &r
Reflected laser-light measurements in the planar
OMEGA experiments will validate theoretical predictions
Measurement of reflection
from a probe beam Several laser beams can be used simultaneously on OMEGA
i = 23°, 48°, and 62°
I = 1014 to 1015 W/cm2 Picket or square laser pulses
• Five laser beams at 23.2°.
• Background beams have a mixed s and p polarization.
• Diagnostics will also include shock breakout and x-ray
TPSQQPMBSJ[FE
QSPCFCFBN
1SPCF CFBN
#BDLHSPVOE CFBNT
%FUFDUPS
i
Simulation of 23 °, 1-ns laser beams predict different absorption of s- and p-polarized light
TC7518
Simulations with a single 23° laser beam
8DN
8DN
5JNF OT
-BTFS
QVMTF 24
24
*OUFOTJUZ SFMBUJWFVOJUT 21 m24 24 2 8211
• The effect of resonant absorption is smaller for beams with larger incident angles.
241m2424
241m241
5JNF OT
-BTFS QVMTF
24
*OUFOTJUZ 8DN
8
i
3 nN S
O nN S nN
241 m24 24 241 m241 8
Planar simulations with angle-dependent laser intensity estimate absorption in spherical implosions
Illumination of a
spherical target Simulated absorption for a 25-kJ implosion
I(r) & I(i), i = 0...r/2
• Angle-dependent intensity in planar geometry
I]rg =I0e-_r r0in
Resonant absorption is important during the rapid increase of laser power in direct-drive target designs on OMEGA
TC7512
Summary/Conclusions
• Resonant absorption on OMEGA is determined by linear effects.
• Resonant absorption is important during laser pickets or at the beginning of long laser pulses, when the density length scale near the critical
surface is relatively small, L < 2 nm.
• In spherical implosions, resonant absorption can enhance the earlier- time laser absorption up to 20%.
• Planar OMEGA experiments will validate theoretical predictions with the use of inclined s- and p-polarized laser beams.
