Inertial Confinement Fusion [quarterly rpt Oct-Dec 1992]

73 Pages·1993·7.302 MB·English

Centimeter
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 mm
1 2 3 4 5
lltll"°°°....+.
Inches
• ',2,.L:
tLllt_
IUlINIIIII_lllil_
+o_'+ ___
BY IMR_E, INC. _

Lawrence
Livermore
National
Laboratory
-"- _ _li Wli,
ICF Quarterly Report "...:...:....::.'._.,.,t,,__,,,_,,,_,,),_,,,,,_,,,,_,,,,,_,,__,,,_{,,,,_
2-D Silltttlati(,its ()[
Ultra-lltteltse, Short-l'ltls('
Laser l)l_sllt_ lltteracti()lts
The ICF Quarterly Report ispublished
four times each fiscal year by the Inertial Confinement
Fusion Program at the Lawrence Livermore National
Laboratory. The journal reports selected current research
within the ICF Program. Major areas of investigation
presented here include fusion target theory and design,
target fabrication, target experiments, and laser and opti-
cal science and technology. Questions and comments
relating to the technical content of the journal should be
addressed to the ICF Program Office, Lawrence
Livermore National Laboratory, P.O. Box 5508,
Livermore, CA 94551.
The Cover:
The ions associated with a plasma
that has been irradiated by an ultra-intense laser pulse
are shown in two directions. The rippling of the criti-
cal surface is due to a Rayleigh-Taylor instability that
develops as the laser (acting as the light fluid) pushes
against the plasma at the critical surface (the heavy
fluid). The bottom plot shows that eventually, since
the light pressure associated with the central bubble
is so large, a plasma, void forms in the overdense
plasma. This plasma-laser interaction can now be
modeled and predicted by particle in cell (PIC) simu-
lations. See the article, "Two-Dimensional Computer
Simulations of Ultra-Intense, Shc,rt-Pulse Plasma
Interactions," page 28.
UCRL-LR-105821-93-1
Distribution Category UC-712 Thisdocument was prepared asanaccountofwork sponsored byan
October - December 1992 agency oftheUnitedStatesGovernment. Neither theUnited States
Government northeUniversity ofCalifornianoranyoftheiremploy-
eesmakesanywarranty, express orimplied, orassumes anylegallia-
bilityor responsibility fortheaccuracy,completeness, orusefulness
ofanyinformation,apparatus, product, or processdisclosed, orrep-
resents thatitsusewould notinfringeprivately ownedrights.
Referenceherein toanyspecificcommercial products,process, or ser-
vicebytradename,trademark, manufacturer,orotherwise, does not
necessarily constitute orimply itsendorsement, recommendation, or
favoring bytheUnitedStatesGovernment or theUniversity of
California. Theviews andopinions ofauthors expressed hereindo
notnecessarily stateorreflect thoseof theUnitedStatesGovernment
Printed intheUnitedStatesofAmerica or theUniversity ofCaliforniaand shallnotbeused foradvertising
National TecAhvnaicilaalbIlnefforrommation Service or productendorsement purposes.
U.S.Department ofCommerce
5285Port Royal Road Workperformed under theauspices oftheU.S.Department of
Springfield, Virginia22161 EnergybyLawrence Livermore National Laboratory under Contract
Pricecodes:printed copyA03,microfiche A01. W-7405-Eng-48.
_TL_-_::_-2_2__Z-2:_.7__--=_. "...... ' " . _tm_._u_J!Rmm_',_._.,-=::.27T._::.T_-2:::_7 z: :_7::::_:TLL_:::_.C__L_Z_:Z::772:_.Z_.:L:LTL::ZZ:_2Z
N I;'TiAL (70 N F1N EM ENT
FUSION
Quarterly Report octol,.,.D- ece,,,b/9e9r2
Vohtllw 3, Nllnlber 1
In this issue:
Foreword iii
The Beamlet Front End: Prototype of a New Pulse Generation System 1
\\:t' duvuh_pud ,lied h'slud ,_n,i,.]\','_ncl,d tr_,Yd_'llds\ ,qeln I_l Nd:_,lass last,r_, Ihat tm_duces 3-ns output
pub.cs with t,ncr_',\ ul_ h_ ]tl.-_ Iand a tl,ll-l_*ptx'd _ - '_cm ['l(hllll pr, ffih'.
Imaging Biological Objects with X-Ray Lasers 10
Lsine, the _utpul ,d,l ni_ kel-likt, _dlN_mallv pumlwd \-_a\' l,>,er at 44.S3 eN,we _d_tained images _U
ral sperm Inwlci with - :_-q}-:\ -,t_atial rcs_,lutit,n. 1h_,_,c n',qllt_, itlu_,trat; ,the capability _t using x-ra\' lasers
t_ xicxx _n'gani_,n> Iwdral_'d in phv,-i_,h_vic,_lly n_nmal en\'ironm,,nls, lhu_, revealing their natural structure.
Coherent XUV Generation Via High-Order
ttarmonic Generation in Rare Gases 15
_\'t' I't'ptH'[ t_llt'Xl_Ul'illlt'lllS lO.,-clwrat_' hi,_h-brighll_ess, __dv_,i't,nt, Nk:\' radiati_m by high-order
harm_ufi_ g_'llt'l',lii_.H) ill l'dl't' 'Z,,ISt'>. [],ll'l//tqli_. _.H'dt'l'S '_.'i{]l [)hilt(III t'lli'l'_i,L'S tlp [(_Sl)t'\' arc t_bsc'rved.
Theory of High-Order Harmonic Generation 21
\Ve pre,,,nl a thctucti_al de_.cripti_m _*thigh--t,rdcr harm_mic gem'ratiol_ in rare e,asc,,, l)etaih_,d
c,_]c_ll,_ti_n>, Jlli'ltlttillU_ lilt' :",Jll'.;It' ,l[(Hll I't'StlI'U/SI' ,/lid Illd'_l'a.}ScI)pjt }':'}l,lSU matching, alhm' us to
e_,lablish the pdl'dll_t'tt'l" Sl.'_dCl' t{H",_ptimum harm, mit c,mvcrsion.
Two-Dimensional Computer Simulations of
Ullra-lntense, Short-Pulse Laser-Plasma Interactions 28
\\'t' I'ulVe de\ t*l_pt'd d_'laih'd, 2-1), pariMe in ct'li (PIC) simulali_ms ,ff intense-laser plasma
inleracti_ms. W,.' can plt,_ti, lt,llicit,lat i,1,-,t.rlight al,_,t_rpti_m by the plasma and the production
t)] 111_2\" L']U'_ [I'()Ils.
Neutron Detectors for Measuring the Fusion Burn History of ICF Targets 35
_,\'t' have dc\'t'lol_cd hre'-. (--Ill(l() ps), Inedium- (--I:t(I ps), al/d l'_ig,h-re,4()hltioll (< 3(}ps) neutron detectors
Ior mt,,isuring ]('[ tar,Kct btlrn hiq_,'ius, ih_, 111edsurcnlen|s char,lt [eri/e the imph_si_m and ]l_,drodvnanlics
_1 the largpt alld nru a -cnMlixt, indicat¢_r ¢_1,_ur abilit\' t_,,'_cctll'atel\, illodtq lqlUl-_,}:[l',-inspol'[ belween
lhc laser and the target.
The Recirculator 41
lhe dc_,ign and c_,>l c_u>iderali_m,, l_r a nlv_aiouh' c]dss hcavv i¢_11lusion driver art, discussed.
Scientific Editor These recin ulatilLe, illt]ll,.[i(lll ,I¢.'C¢']t'l'dh_l'_ I'_IOV ]U,lt] h) stlsl.,lllli,1] CI)sl rt'dtlcli_}ns.
Shamasundar N. Dixit
Lasnex Evolves to Exploit Computer Industry Advances 5{)
Editorial Staff \Vr ha\u nl_u.h.,llli:,:t,d thu l.,*_sm,\ _ ¢1¢,,_h} make theln n_.tn't' powerful and portable among
Robert Condouris diltt,lt, lll II'hlt]lil'_t'>. ( }Ill"I{d'*',isilIIt'I'I,lcC -\'_,it'lll t'l_,l_'_]_."-t,l.'-,Ul'St_._pro e,am lnsne\, manipulate
Marie Kotowski \'arhfl_h,s ,111d5,Hl_l_lltint,s, dlld dt,v,.,l/,p ]'_,1_kd}'.,us tor ',pt't'i,l] pl'O]UCls.
Peter W. Murphy
JoyI'erez Facilities Report, October-December 1992
D_sig.Staff Publications
Ellen L. Baldwin
Art Staff
TID Ari Division
. ,
" ":_'_" . ,,_ ..,t :. _ .._lo _, .. r_,,_ ?d _ _.:b. '

Foreword
This issue of The ICF Quarterly contains eight articles reporting on the progress on vari-
ous activities within the Inertial Confinement Fusion Program.
The leading article, "The Beamlet Front End: Prototype of a New Pulse Generation System,"
discusses the system design and performance ofthe front end of the Beamlet Demonstration
Project laser, which isbeing constructed as a testbed for future ICF laser systems. This laser
system incorporates new technologies for spatial and temporal pulse shaping, bandwidth gen-
eration and amplification. The present performance of the front end has exceeded the require-
ments for achieving the full system milestone.
The second article, "Imaging Biological Objects with X-Ray Lasers," reports on the progress
indeveloping x-ray lasers of ever shorter wavelengths and the advances inx-ray optics devel-
opment for imaging ofbiological objects. The authors demonstrate imaging rat-sperm nuclei
with resolution of about 500 A with an x-ray laser at about 45fi_.The resolution islimited
mainly by the limitations in the x-ray optics. Future improvements inx-ray optical technology
should further improve this limit on resolution.
The next two articles, "Coherent XUV Generation via High-Order Harmonic Generation
in Rare Gases" and "Theory of high-Order Harmonic Generation" present, respectively, the
recent experimental and theoretical advances in the nonperturbative interaction of atoms
with intense laser fields. Experimental results illustrate that high energy photons can be
efficiently generated by very high-order harmonic emission by an atom subjected to an
intense laser field. This process could have practical applications in developing sources of
coherent XUV radiation. The theoretical article confirms the experimental observations and
demonstrates how detailed, first-principle calculations can be used to determine optimal
conditions for efficient harmonic generation.
The article "Two-dimensional Computer Simulations of Ultra-Intense, Short-Pulse Laser-
Plasma Interactions," continues along the theme of theoretical investigations of interactions
of intense laser with matter--in this case a preformed plasma, lt is shown that the laser
absorption can be very high and that MeV electrons can be produced because of the
tremendous ponderomotive acceleration.
Recent progress in developing high-resolution, time-of-flight neutron detectors isreported
in the article "Neutron Detectors for Measuring the Fusion Burn History of ICFTargets." Since
neutrons escape the target without encountering collisions, a time resolved detection of neu-
trons provides an accurate "movie" of the target implosion. The 20-ps resolution, scintillator
detector should prove tobe a powerful diagnostic tool for characterizing target implosions.
The next article, "The Recirculator," provides an overview of a conceptual design and
cost analysis for a heavy ion fusion reactor. The authors show that the use of recirculating
induction accelerators to boost the energy of the heavy ions can lead to substantial cost sav-
ings. Several engineering and physics issues associated with this design are also discussed.
The last article, "Lasnex Evolves to Exploit Computer Industry Advances," briefly outlines
the improvements made to the Lasnex codes to make them more powerful, user friendly,
portable from machine tornachine, and to conform toindustry standards. The ability to do
production runs on workstations, rather than on Cray supercomputers, has lead tosubstantial
cost savings.
The issue concludes with the Nova facility report and a list of publications from within
the ICF program.
Shamasundar N. Dixit
Scientific Editor
iii
J_r--Z--_|_____.L.' LL ILL.EILii..I2.i_iJi Ii_iiii".__.-i-.1211J.i.l.J.i.i.i.. _-_ Jl II IIIIL_-_L--I_I _. 'i ......_.-_[[_JLI[-II_-_.II_i i-. i .IC _i.....i;._i-iii-i ....
THE BEAMLET FRONT END:
PROTOTYPE OF A NEW PULSE GENERATION SYSTEM
B. M. Valz Wonterghem
D. R. Speck
M. J. Norman
V.KarpeTzko
R. B. Wilcox
Introduction
Here we describe the system components and per-
In conceptual designs I for new ICF facilities, multi- formance, based upon over 100 full front end shots
megajoule Nd:glass-based laser systems irradiate being completed since we achieved the assembly mile-
targets. To achieve the required cost efficiency, extensive stone inSeptember 1992.
use ismade ofvery high density mulfisegment ampli-
System Architecture
tiers in a multipass configuration with high beam fill
factors. Large aperture optical components in such facil-
ities will experience very high fluences and require suf- Figure I shows a schematic diagram of the front end
ficient bandwidth to suppress the damage and energy and identifies five main subsystems. The master oscilla-
loss caused by stimulated Brillouin scattering. 2,3In tor is a diode pumped Nd:YLF microchip laser operat-
addition, various target irradiating schemes require ing in a single longitudinal mode. Its output ischopped,
large bandwidth and flexible pulse shaping. With the amplified and fed into a single mode optical fiber. The
technology employed in the Nova front end, itwould be resulting 1-_tspulses are coupled into an optical inte-
exceedingly expensive tobuild a pulse generation and grated circuit that produces shaping and bandwidth.
distribution system that would satisfy the requirements The temporally and spectrally shaped pulses are then
of multimegajoule lasers. A front-end system with more sent out to the preamplifier section in the laser bay
flexibility, better stability and reliability, and lower through a 60m long optical fiber. Figure 2 provides a
initial and operating cost isneeded, view of the arnplifier system mounted on a space frame.
The Beamlet Demonstration Project isbeing con- Aregenerative amplifier provides a large gain and feeds
structed as a testbed for the basic physical principles on a spatial beam shaper that produces a square beam with
which future ICF laser systems will be based. 4For this smooth, but steep edges. The beam ismagnified toits
project, we developed and finished the initial testing of final size and preamplified in a novel 4-pass rod ampli-
an advanced front end. This front end system makes fier. The output beam is relayed into the Beamlet cavity
extensive use of new technologies to provide reliable through two transport spatial filters.
operation and simultaneous flexible spatial beam shap-
Oscillator and
ing, temporal pulse shaping (200 ps to 10ns), band-
width generation (up to 30GHz), and output pulse
Pulse Forming System
energies exceeding 10J.
We produced 3-ns output pulses with energy up to The master oscillator is a diode pumped Nd:YLF
10.5 Jand a flat-topped 5 x5 cm beam profile. We also microchip laser operating in a single longitudinal mode
demonstrated amplification of shaped pulses with a at 1053 nm. A Pockels cell reduces the duty factor by
15:1 contrast and bandwidth exceeding 48 GHz. The chopping the CW output into 1-_s pulses at a 1-kHz
present performance exceeds the expected input rate. These pulses are launched into a single mode,
energy required for driving the Beamlet laser toits
milestone shot (5 kJat 3c0in a 3-ns pulse).
BI:AMI IT [-'RC)NI /-.'NI) ....:::::::±-::/I: ..................... .III. --7111-111 _ . ....... ::_:::.....UI......................... -:_]L :-.
polarization preserving optical fiber after being Asignificant operational advantage of this systern is
amplified by a diode laser pumped CW slab ampli- the provision ofhigh-qttality polarization preserving
tier. The peak power is about 250 mW at this point, connectors for coupling between fibers and devices.
Tile fiber couples the pulses into an advanced optical These allow for repeatable and alignment free reconfigu-
integrated circuit that produces shaping and band- ration of components and diagnostics. Fiber-optic split-
wid th. This LiNbO 3waveguide circuit has two ters and switches are being used toroute optical signals
amplitude modulators and a phase modulator, which tovarious diagnostics. Except for the master oscillator
are driven by low voltage external circuitry. The pre- breadboard, allhardware israck-mounted (Fig. 3).
sent device has an insertion loss of less than 5 dB, Using tile present master oscillator, the pulse deliv-
while the combined extinction ratio of the modula- ered to the high bay preamplifier has a peak power of
tors exceeds 50 dB. These are very important param- 5-8 mW, equivalent to an energy of 15-24 pJin a 3-ns
eters because they ultimately determine tile prepulse pulse. The 60-m length ofthe transport fiber to the high
contrast of the shaped pulse and the signal-to-noise bay does not exceed critical lengths for dispersion LD
ratio of the following preamplifier. In contrast to the and self-phase modulation Lnlfor normal operating
highly resonant discrete modulators being installed conditions. For a 10-mW peak power and a 100-ps rise
on Nova and the Optical Sciences Laser, the wave- time, we calculate LDto be 450 m and Lnl43km, assure-
guide electro-optic phase modttlator is a travelling ing typical values for fiber and material dispersion. (_
wave device with a wide bandwidth up to 17GHz.
Regenerative a,mnlifier
The _-modulatioll voltage remains however weakly
frequency dependent. A detailed description of the
oscillator and pulse forming system has been pub- Bringing the energy from picojoule tojoule levels
lished previously. 5 requires amplification by 12orders of magnitude. The
FI(;URI_1.Schematic
diagram of the Beamle!
front end system, iden-
tifying the major sub- Rod
systems: master / amplifier 4-pass rod
oscillator, pulse form- amplifier stage
ing system, regenera- Spatial
tive amplifier, spatial filter
beam shaping, and a Regenerative
4-pass 5-cm-diam rod amplifier Regenerative
amplifier. Atr_l)-mopti- amplifier Otlt[.)tlt
cal fiber transports the diagnostics
pulses from the oscilla- ,, Faraday
tor room to the laser rotator
bay. The output trans-
port filter relays the
beam to the Beamlet
cavity injection system.
"x
Diagnostic
packages _ 4-pass output
are located at the out- \_,4;:_,.,_,,,:+_,..,:.
put of the regenerative Be,ma
amplifier, and the shaping
input and outpt, t ofthe 6tl-m optical fiber link 4-pass input
4-pass rod amplifier, fr()m master oscilla tor diagm Jstics
rot)na to high bay V Output
Optical fiber
link
Otasbcleillator
Rack mounted integrated modulators
(pulse sllaping ancl bandwidth)
amplification process must maintain pulse shape within ,hake one single round-trip through tile ring before
controlled limits, maintain the bandwidth while avoid- leaving through the exit polarizer. When the Pockels
ing phase-to-amplitude-modulation conversion, and cell isswitched to half-wave retardation, the pulse
add minimal noise to maintain the high contrast, remains in the cavity and undergoes amplification
Conventional front-end technology applies the shaping until the half-wave voltage is switched off. Leakage
and phase modulation to much stronger signals. The through the exit polarizers (< 0.2¢/,)is blocked by a
most convenient method to achieve high gain is toinject pulse slicer. The cavity round trip time amounts to
the pulse in a resonant cavitv with a moderate gain rod 13.4 ns, but tile 1-ns rise and fall time of the Pockels
amplifier (1.5 <G <6).7Tomaintain the temporal shape, cell limit the useful gain window to 10ns.
the amplifier must operate in its linear regime; the pulse We modified the ring cavity by introducing two
must leave tile cavitv before gain depletion effects lenses, each having a focal length L/4, where Lis the
become important. This amplification system isknown total circumference (about 4 m). The input aperture is
as a regenerative amplifier, s therefore image relayed onto itself after one round
We adapted the ring regenerative amplifier design trip, while the second aperture acts as a spatial filter
currently used in chirped pulse amplifier systems) _ in the focal plane. The output beam has a symmetric,
Figure 4 shows a diagram of the amplifier. Two Faraday nearly Gaussian profile. The thermal relaxation time
isolators prevent the return of anlplified spontaneous of the 4-mm-diam amplifier rods is 9.6 s. A 0.2-Hz rep-
emission pulses to tile modulators. The ring cavity is etition rate prevents undesirable build-up of thermal
made up using two thin fih'n polari;:ers and two high lensing and birefringence.
reflectors. A KD*P Pockels cell is used as switching ele- Two Nd:glass heads provide a combined small sig-
ment in combination with a z-cut quartz 90°rotator, nal gain up to6. The energy stored in the mode volume
With no voltage applied to the Pockels cell, the pulses amounts to_150 mJ. Tile amplifier operates in the linear
operating region by switching out the pulse after
approximately 14round trips; this avoids pulse shape
_q{_ __i!. els between 5and 7 mi, the PSD isless than 10%. When
--' I :__{_i_ operating in the linear gain regime, itisadvantageous to
i_'__:i, _!;[__!_/__'_i_;'i" _ distortion (PSD) by gain depletion. At output ene:gy lev-
ficient stored energy and minimizing passive losses in
__ ktheeepcatvhiety.gaTinilepuerserooufntdwotrdipiffleorwen,twahtihleermmaalintapihnoinspghatesuf-
rack mounted. Beam transport is accomplished using single mode
FIGURE 2. View of the front end preamplifier section in the laser optical fibers.
bay. lt consists of aregenerative amplifier, beam shaper, and 4-pass
rod amplifier.