Hicks, D.G.

Li, C.K.

Meyerhofer, D.D.

Petrasso, R.D.

Phillips, T.W.

Ram, A.K.

Roberts, S.

Sangster, T.C.

Schnittman, J.D.

Seguin, F.H.

Sorce, C.

Soures, J.M.

Stockl, C.

DSpace@MIT

Studies of MeV Fast Protons Produced in Laser Fusion ExperimentsD. G. Hicks, C. K. Li, F. H. Seguin, A. K. Ram, and R. D. PetrassoPlasma Science and Fusion Center, Massachusetts Institute of Technology, Massachusetts 02139J. M. Soures, D. D. Meyerhofer, S. Roberts, J. D. Schnittman, C. Sorce, and C. StocklLaboratory for Laser Energetics, University of Rochester, Rochester, New York 14623T. C. Sangster and T. W. PhillipsLawrence Livermore National Laboratory, Livermore, California 94550(November 24, 1999)Enhanced laser-induced acceleration of fast protons up to>1 MeV has been observed on the 60-beam, 30-kJ OMEGAlaser at intensities of 1015Wcm 2. These energies are morethan 5 times greater than previously observed on single-beamexperiments at equivalent intensities. The total energy inthese protons is  0:1 % of the laser energy and inferredhot electron temperatures are 10 { 20 keV. High-resolutionspectroscopy has detected proton spectra with intense, regularlines.A number of experiments have observed that ions atsuprathermal energies are produced during high-intensitylaser interactions with solids [1] { [6]. These experimentsshow that protons are always present in the ion signal,regardless of the type of target material. Such acceler-ated ions, or fast ions, are thought to be associated withhot electrons [7] { [9] which produce electrostatic eldsthrough charge separation.A distinctive feature of fast proton spectra is the pres-ence of a well-dened, maximum cuto energy. Forsingle-beam experiments, Tan et al. [1] observed the scal-ing of maximum proton energy with laser intensity to be:Emax= 3:51 10 3(I2)1=3(1)over the range I2= 1014  3 1018Wcm 2m2, whereEmaxis the maximum proton energy in keV, I is thelaser intensity in Wcm 2, and  is the laser wavelengthin m. More recently, this relationship has been veri-ed at high intensities with substantially dierent laserconditions [2,6]. At least 3 dierent detector types wereused in all these studies. From this scaling, it would beexpected that, at I2= 1014Wcm 2m2correspond-ing to conditions on laser fusion experiments, Emax160 keV. Instead, our studies on the 60-beam OMEGAlaser have measured proton energies greater than 1 MeV.In addition to observing these unexpectedly high maxi-mum energies, we have also performed high-resolutionmeasurements of the proton spectra. Previous studies ofthe proton spectrum using low-resolution time-of-ighttechniques often showed exponential-like velocity spec-tra consistent with the isothermal, self-similar plasma-FIG. 1. Fast proton spectra for a single shot measured con-currently by the two spectrometers, CPS-1 (at 235 cm) andCPS-2 (at 100 cm). The gaps are due to detector dead space.Total particle yield per MeV is inferred by assuming isotropicemission over all solid angles. Although the spectral shapesdier, the endpoint energies are approximately equal.expansion model [1,11] or its modications which includetwo electron temperatures [8] or multi-ion species [4,5].Our high-resolution studies reveal for the rst time thepresence of intense, regular spectral lines.The experiments were performed on the OMEGA lasersystem at the Laboratory for Laser Energetics, Univer-sity of Rochester. OMEGA is a 60-beam, neodymium-doped phosphate glass laser capable of delivering 30 kJ offrequency-tripled, 0.35 m light [12]. Irradiation unifor-mity is accomplished using distributed phase plates andsmoothing by spectral dispersion (SSD) [13]. For thesestudies, laser pulse shapes were mostly 1-ns square, witha few 400-ps square pulses. The laser, which was used to1FIG. 2. Scaling of maximum fast-proton energy with I2:open and solid circles are for glass targets with 1 ns and 0.4 nspulses respectively, while open and solid triangles are for plas-tic targets with 1 ns and 0.4 ns pulses respectively. Error bars,at less than 2 %, are smaller than the plotting symbols. Thedashed line is extrapolated from the scaling of single-beam ex-periments (Eq. 1), while the dotted line is from experimentsusing 8-beams [1]. There is no signicant dierence betweenshots with or without SSD.directly illuminate the target, delivered energies of 10 to30 kJ. Laser intensities varied from 1014to 1015Wcm 2,where the intensity is calculated by dividing total inci-dent laser energy by pulse length and target surface area.Targets were 0.9 { 1 mm diameter spherical microballonswith glass or paralene (CH) shell material ranging from2 to 20 m in thickness.Proton spectra were observed using a charged-particlespectrometer [14] consisting of a 7.6-kG permanent mag-net with CR-39 nuclear track-etch detectors. The in-strument can measure total particle yields between 107and 1016and proton energies from 0.1 to 40 MeV. Useof a high-eld magnet in conjunction with single-particlediscrimination from track detectors gives this instrumenthigh energy resolution: better than 1 % over the energyrange for fast ions, or<3 keV at 500 keV. Systematicuncertainties are < 2%. A rapid, automated scanningsystem was developed which can readily count 106tracksper shot. Simultaneous measurements are made by twovirtually identical spectrometers positioned 101Æapart,one outside the OMEGA chamber at 235 cm from thetarget (CPS-1), the other inside at 100 cm (CPS-2).Sample spectra obtained simultaneously by each spec-trometer are illustrated in Fig. 1, clearly showing thecharacteristic maximum cuto energy [15]. Despite dif-ferent spectral shapes, the endpoint energies observedfrom both views are approximately equal and generallyare within 50 keV of each other (where the systematicFIG. 3. Maximum proton energy versus the hot electrontemperature inferred from the slope of the ion velocity spec-trum. Uncertainties in the inferred temperature are causedby uncertainties in determining the spectral slope. The lin-ear relation is in agreement with that found for single-beamexperiments in Ref. [1].error between instruments is < 40 keV).The measurements of maximum proton energy areplotted versus I2in Fig. 2. The scaling found previouslyon single-beam and 8-beam experiments [1] are shown bythe dashed and dotted lines respectively. For intensitiesless than  1.61014Wcm 2, the proton energies are be-low the detectable limit of 100 keV, while at intensitiesof 1015Wcm 2energies up to 1.4 MeV were observed.Neither laser-pulse duration (at 0.4 or 1 ns) nor applica-tion of SSD appear to make any substantial dierence tothe maximum proton energies; however, as Fig. 2 shows,maximum energies are generally higher for CH targetsthan they they are for glass targets.As illustrated in Fig. 2, the measured proton energiesof  1 MeV are more than 5 times greater than thoseobserved previously at the same I2[1]. In order toplace the OMEGA measurements in context, it is usefulto examine more closely some of these previous studies.The relationship given by Eq. 1 (dashed line in Fig. 2)was determined by studies over 4 orders of magnitudein intensity using nanosecond pulses of 10.6 m light atenergies up to 1 kJ focused to spot sizes of  100 m [1].A similar scaling was found using picosecond pulses of1.05 m light at low energy (30 J) focused to spot sizesof  12 m [2]. Despite the substantially dierent laserconditions of these two studies, there was no indicationof any signicant deviation from the scaling given by Eq.1, at least for the range of conditions covered by thesesingle-beam experiments. On the other hand, for ex-periments using 8 beams, a stronger scaling with I22was observed (dotted line in Fig. 2) [1,10]. It is notablethat these multi-beam experiments found that maximumproton energies were dependent on the target materialwhereas the single-beam experiments observed no suchdependency. On the 60-beam OMEGA laser, we observean even stronger scaling with I2than observed on the8-beam studies and we nd that glass and CH targetslead to dierent maximum proton energies. This evi-dence would appear to suggest that the presence of manybeams on OMEGA might be the cause of these elevatedproton energies. However, it should be emphasized thatlaser conditions on OMEGA are unique in many otherways besides the presence of 60 beams and, without per-forming a systematic study, it is diÆcult to isolate anysingle factor that might be responsible for producing theelevated proton energies.Since fast ions are generally thought to be associatedwith hot electrons driving the plasma expansion into vac-uum [7{9], it is useful to infer an eective hot electrontemperature from the proton spectra. To do this re-quires that a model be used to describe the plasma expan-sion. One such description is the isothermal, self-similarmodel [7] which predicts that the slope of the velocityspectrum is a function of the hot electron temperature,Th. For our studies, the spectra do not always have asingle, well-dened slope; however their character is gen-erally exponential-like and by a least-squares t to thevelocity spectra (avoiding the steep endpoint region), theaverage slope and thus an eective temperature can bedetermined. In Fig. 3 it is shown that the maximum pro-ton energy is proportional to the inferred Th, with a bestt giving the relationship Emax=Th= 55. This valueis close to that found experimentally by Tan et al. [1]where Emax=Th= 66, though, to the best of our knowl-edge, there is no theoretical explanation for this constantratio. We thus infer that the elevated proton energies aredriven by eective hot electron temperatures of 10 { 20keV.Measurements of hard x-rays made in conjunction withthese experiments indicate the presence of superhot elec-tron distributions with temperatures between 50 { 250keV at intensities of  1015Wcm 2. The x-ray sig-nal is a strongly increasing function of laser intensity inthe range 5 { 91014Wcm 2and is slightly reduced byapplication of SSD. It appears that such superhot elec-trons are associated with the two-plasmon decay insta-bility [16]. This discrepancy between the proton-inferredand x-ray-inferred hot electron temperatures is not with-out precedence. Tan et al. [1] found that these two typesof measurement generally agreed for temperatures below10 keV but disagreed for higher temperatures, with thex-ray inferred values being a factor of approximately 3times greater. One possible reason for this discrepancyis that the hot-electron temperature is evolving over theduration of the pulse and the x-ray and proton methodsare sensitive to dierent periods of this time evolution.FIG. 4. Fraction of incident laser energy converted to fastprotons with energies greater than 0.2 MeV plotted as a func-tion of the endpoint energy. Plotting symbols are as in Fig.2. Vertical bars on some data points represent the dierencesbetween the two spectrometers.In particular, the ion acceleration is greatest early in thelaser pulse when the highest ion densities give rise to thehighest space-charge elds [7]. This implies that the pro-ton spectra would weight more heavily the hot electrontemperatures existing at early time periods. Understand-ing the origin of this temperature discrepancy may allowthe two types of measurement to be used in a comple-mentary fashion to understand the complex hot-electrondistribution.In Fig. 4, the fraction of laser energy converted to fastprotons with energies greater than 200 keV is plotted ver-sus spectral endpoint. At intensities of  1015Wcm 2,corresponding to an endpoint of  1 MeV, the total en-ergy carried by the the fast protons is  10 3of thelaser energy, similar to the total energy of the superhotelectrons inferred from x-ray data. Plastic targets, whichusually produce higher maximum proton energies thanglass targets irradiated at the same laser intensity, havecorrespondingly higher total ion energies. To within thescatter of the data, protons from the 0.4 ns shots have ap-proximately the same fractional energy as those from the1 ns shots. This correlation between the energy fractionand the maximum energy, regardless of target material orpulse length, indicates that the maximum proton energycan be used as a rough measure of the total energy in thefast protons. It is noteworthy that, on previous single-beam experiments [6] at higher intensities of I2 1017Wcm 2where the maximum proton energy was also closeto 1 MeV, the fraction of laser energy carried by the fastion was also  10 3.Also shown in Fig. 1 is a spectrum with oscillations,3FIG. 5. Occasionally, the proton spectra exhibit discrete,regularly-spaced lines whose spacing increases with energy.These lines are super-imposed upon the exponential-like back-ground.features that are not repeatable from one shot to thenext, and not necessarily observed on both spectrome-ters. Occasionally, the oscillations take the form of in-tense lines, as shown in Fig. 5, though it is unclear atthis stage what specic conditions give rise to them. Thespacing between adjacent lines increases towards higherenergies, a characteristic that is common to all such linespectra. For a given laser intensity, spectra with or with-out lines do not show any signicant dierence in maxi-mum energy.Such strong spectral lines have not been observed be-fore and, to the best of our knowledge, are not predictedby existing theories. One possible interpretation of thesefeatures is in terms of ion acoustic waves. A simple anal-ysis shows that the increase in line spacing is character-istic of the behavior of ion acoustic waves in an expand-ing plasma. To see this, consider a plasma moving withbackground velocity V . An ion acoustic wave travelingin the same direction as V has a velocity u + V , whereu is the wave velocity in the reference frame of the mov-ing plasma. For an expanding plasma, V increases withdistance in the direction of the expansion which meansthat the velocity of successive density peaks in the ionwave will also increase with distance in the expansiondirection, producing peaks at dierent velocities. Thesepeaks will be super-imposed on the exponential back-ground spectrum as observed in Fig. 5. Using once againthe isothermal, self-similar expansion model, where theplasma velocity at a given time is proportional to po-sition, it can be shown that the dierence in velocity,V , between successive peaks is given approximately byV = 2(u+ V )=!t where ! is the wave frequency, andt is the expansion time. Should !t be constant over thisvelocity range, V would be proportional to V . For thelines in Fig. 5, a plot of V versus V , shown in Fig. 6,is well approximated by a straight line. Further stud-ies will be necessary to understand the origin of theseFIG. 6. A plot of the velocity dierence, V , between ad-jacent lines in Fig. 5 versus their average velocity, V . Theobserved linear relationship is expected if the lines are treatedas the density peaks of an ion acoustic wave in the expandingplasma.waves. Perhaps they are produced by stimulated Bril-louin scattering, known to occur only at low levels onOMEGA [17], or by vibrations in the near-solid plasmabefore expansion.In summary, fast protons with>1 MeV energies areproduced on targets irradiated by the OMEGA, 60-beamlaser system { energies more than 5 times greater thanpreviously observed on single-beam experiments at thesame I2. The total energy in fast protons above 200keV is  10 3of the laser energy. Hot electron tempera-tures inferred from the proton spectra are approximately10 { 20 keV, giving a ratio of maximum proton energyto hot electron temperature close to that observed pre-viously. X-ray measurements indicate the presence of a50  250 keV electron distribution also carrying  10 3of the laser energy. The dierence between the protonand x-ray inferred hot electron temperatures may indi-cate that these techniques are complementary ways of ex-amining the hot electron distribution. Particularly strik-ing is the new observation of intense spectral lines whichmay be associated with ion acoustic waves in the expand-ing plasma. The enhanced ion energies and appearanceof discrete, spectral lines may be of interest for futureion-acceleration schemes using high-intensity lasers.We are indebted to the OMEGA operations groupfor their expert help. This work was supported inpart by the U.S. Dept. of Energy Contract Num-ber DE-FG03-99SF21782, LLE sub-contract numberPO410025G, LLNL sub-contract number B313975, andthe U.S. Dept. of Energy OÆce of Inertial ConnementFusion under Cooperative Agreement No. DE-FC03-92SF19460.4[1] T. H. Tan, G. H. McCall, A. H. Williams, Phys. Fluids27, 296 (1984)[2] F. N. Beg et al., Phys. Plasmas 4, 447 (1997)[3] S. J. Gitomer et al., Phys. Fluids 29, 2679 (1986)[4] F. Begay, D. W. Forslund, Phys. Fluids 25, 1675 (1982)[5] R. Decoste, B. H. Ripin, Phys. Rev. Lett. 40, 34 (1978)[6] A. P. Fews et al., Phys. Rev. Lett. 73, 1801 (1994)[7] J. E. Crow, P. L. Auer, J. E. Allen, J. Plasma Phys. 14,65 (1975)[8] L. M. Wickens, J. E. Allen, P. T. Rumsby, Phys. Rev.Lett. 41, 243 (1978)[9] Y. Kishimoto et al., Phys. Fluids 26, 2308 (1983)[10] W. Priedhorsky et al., Phys. Rev. Lett. 47, 1661 (1981)[11] C. Chan et al., Phys. Fluids 27 266 (1984)[12] T. R. Boehly et al., Opt. Comm., 133, 495 (1997)[13] S. Skupsky, R. S. Craxton, Phys. Plasmas 6, 2157 (1999)[14] D. G. Hicks et al., Rev. Sci. Instrum. 68, 589 (1997)[15] D. G. Hicks, Ph.D. Thesis, Massachusetts Institute ofTechnology (1999)[16] C. Stockl, private communication, Nov. 1999[17] S. P. Regan et al., Phys. Plasmas 6, 2072 (1999)5

Studies of MeV Fast Protons Produced in Laser Fusion Experiments

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Studies of MeV Fast Protons Produced in Laser Fusion Experiments

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