Intense laser (> 10{sup 21} W/cm{sup 2}) driven hard x-ray sources offer a new alternative to conventional electron accelerator bremsstrahlung sources. These laser driven sources offer considerable simplicity in design and cost advantage for multiple axis views and have the potential for much higher spatial and temporal resolution than is achievable with accelerator sources We have begun a series of experiments using the Petawatt Laser system at LLNL to determine the potential of these sources for radiography applications Absolutely calibrated spectra extending to 20 MeV and high resolution radiographs through a {rho}r{>=}150 gm/cm{sup 2} have been obtained The physics of these sources and the scaling relationships and laser technology required to provide the dose levels necessary for radiography applications will be discussed Diagnostics of the laser produced electrons and photons will be addresse

Sefcik, J. A.

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UCRL-JC-130942 PREPRINT Hard X-ray Production from High Intensity Laser Solid Interactions M. D. Perry, J. A. Sefcik, T. Cowan, S. Hatchett, A. Hunt, M. Moran, D. Pennington, R. Snavely, S. C. Wilks This paper was prepared for submittal to the 12th Topical Conference on High Temperature Plasma Diagnostics Princeton, NJ June 7-11,199s June 3,199s permission of the DISCLAIMERThis document was prepared as an account of work sponsored by an agency ofthe United States Government.  Neither the United States Government nor theUniversity of California nor any of their employees, makes any warranty, expressor implied, or assumes any legal liability or responsibility for the accuracy,completeness, or usefulness of any information, apparatus, product, or processdisclosed, or represents that its use would not infringe privately owned rights.Reference herein to any specific commercial product, process, or service by tradename, trademark, manufacturer, or otherwise, does not necessarily constitute orimply its endorsement, recommendation, or favoring by the United StatesGovernment or the University of California.  The views and opinions of authorsexpressed herein do not necessarily state or reflect those of the United StatesGovernment or the University of California, and shall not be used for advertisingor product endorsement purposes.Hard x-ray production from high intensity laser solid interactions M.D. Perry, J A Sefcik, T Cowan, S. Hatchett, A Hunt, M Moran? D Pennington, R Snavely, and S C Wilks Lawrence Livemore National Laboratory, Livermore, California 94550 Intense laser (> 1 02’ W/cm2) driven hard x-ray sources offer a new alternative to conventional electron accelerator bremsstrahlung sources. These laser driven sources offer considerable simplicity in design and cost advantage for multiple axis views and have the potential for much higher spatial and temporal resolution than is achievable with accelerator sources We have begun a series of experiments using the Petawatt Laser system at LLNL to detennine the potential of these sources for radiography applications Absolutely calibrated spectra extending to 20 MeV and high resolution radiographs through a pr 2 150 gmlcm2 have been obtained The physics of these sources and the scaling relationships and laser technology required to provide the dose levels necessary for radiography applications will be discussed Diagnostics of the laser produced electrons and photons will be addressed I. INTRODUCTION Electron beam accelerators are commonly used in ballistics research to assess the dynamic behavior of explosively driven systems The Lawrence Livennore National Laboratory uses the FXR facility at Site 300 to produce flash x-rays from an electron beam source The Dual-Axis Radiographic Hydrotest Facility under construction at Los Alamos National Laboratory will provide a two-axis view of fast moving objects with some ability to image with several exposures The use of petawatt class lasers for the production of copious amounts of high energy x- rays has been discussed for over a decade [ 1,2] In fact, intermediate energy (0.1-l MeV) s-ray’ production with table-top size terawatt class lasers is now a sub-field within the strong field interaction community Numerous groups have observed electrons or bremsstrahlung spectra extending beyond 1 MeV [3,4,5,6] Recent experiments on the Petawatt laser at the NOVA laser facility at LLNL have demonstrated the short (cl psec) laser pulses, when properly focussed on high-z targets, can produce hard, intense x-ray spectra that can be used for radiography Scaling arguments suggest that a laser driven radiographic source could have a performance that rivals that of electron beam driven radiographic devices II. RECENT EXPERIMENTS Petawatt shots occurred during the week of September 5-10, 1997 Basic laser data was a pulse duration of 460 + 40 fsec and a compressor throughput of 84% Typical laser energy measured before compression was 450 J in a 46 3 cm diameter beam Due to the hole in the paraboloid, 7 4% of the incident energy is lost. The beam was focused using a Cassegrain telescope The primary mirror was a paraboloid (f=lSO cm) overcoated with an HfO,/SiO, multilayer designed for high reflectivity at normal incidence The secondary mirror was a 5 cm diameter fused silica flat which was overcoated with a HfOz/SiOz multilayer high reflective coating for 1054 nm and placed 10 cm from the target The beam size on the secondary mirror is 2 6 cm corresponding to an irradiance of 1 67x10” W/cm’ for a nominal 400 J pulse incident on the mirror At this irradiance, the top SiO, layer is converted to a critical density plasma within the first 50 fsec of the pulse As a result, the bulk of the laser pulse is reflected not by the multilayer structure of the mirror but rather by the sharp gradient plasma surface Measurements of the reflectivity from this plasma surface suggest a reflectivity of greater than 85% for the Petawatt pulse Two full scale shots into calorimeters were performed to measure the reflectivity in situ but failed due to difficulties with the Nova data acquisition system A total of ten Petawatt shots were obtained during this week. Of these, eight were on target and two into calorimeters A typical target is shown in Fig 1, The laser beam strikes the surface of the gold target at near normal (5”) incidence producing a strong relativistic electron current with a complicated angular distribution into the target, Electrons greater than approximately 2 MeV can escape the large space potential created within the target and penetrate the aluminum and CD, backing material Electron spectrometers are placed in the plane of incidence 2 spectrometers are placed in the plane of incidence at 30” from the laser axis and 95” from the axis. These electron spectrometers consist of a permanent dipole magnet with emulsion as the detector. The emulsion tracks arc analyzed by the NASA/Marshall Space Flight Center group at Huntsville. Alabama. CU ?olyethylene,CD2 piam=6 mm, Length= 1 cm) (2mm) , Fig. 1. P&watt radiography target. A typxal electron spectrum from this shot is shown in Fig. 2. This shot used a gold target of 0.S mm thickness, Electrons were observed at energies extending above 90 MeV, however the bulk of the distribution was in the range of -2-15 MeV where the emission was found to be forward directed with about eight times more flus observed at 30”; with respect to the laser propagation direction, as compared to the flux at 9S’. Note that an integration of the electron spectrum convolved with the brcmsstrahlung cross section does not give an accurate estimate of the total photon yield or laser coupling efficiency since this electron spectrum is effected b> the large space charge potential in the target and the encrgl loss (dE/dx) through the target assembl) The cnerg! loss through the target for the electrons depicted in Fig 2 was approximately 2.5~0 S MeV Fig. 2. Electron spectra acquired at 30 and 95 degrees with respect to the laser axis. Aside from the suppressed region belov 4 McV, the electron spectrum can be well fit b! a “two temperature-’ distribution. The spectrum observed at 30 degrees to the laser is shown m Fig N(E) dE = C (E”‘/E,“‘) Eup[-E/E,] dE(Eq. 1) show the incident beam undergoing filamentation where C is the scaling parameter and, E, is the characteristic electron energy. Detailed particle in cell (PIC) simulations suggest that this characteristic energy is vrell approximated by the cycle averaged quiver energy of a free electron within the intense field. <E,> = m,c’ 11 + 2U,/m,c’I”’ (4. 2) where U, (eV) = 9.33~10.‘” I(W/cm’) h’(pm). The laser conditions for the spectrum depicted in Fig. 3 were such that 350% of the 300 J incident on the target were focused within a 28x40 km spot. This corresponds to an average irradiance of 8~10’~ W/cm’ for this shot. The average quiver energ! for this irradiance (from Eq. 2) is 2.95 MeV in remarkable agreement with the characteristic electron energy for the bulk of the distribution of Fig. 3. The incident laser spot also contained sub- structure which would have seeded self-focusing in the preformed plasma. This self-focusing is almost certainly responsible for the “hot tail” of the distribution characterized by a second Maxwellian with a characteristic electron energy of 1 I .5 MeV. This is consistent with PIC simulations which in the preformed plasma. Fig. 3. Electron Spectrum and calculated Maxwellian distributions (Eq. 2). The solid black curve is the sum of the two distributions. High-energy bremsstrahlung u-rays generated b) these electrons in the gold target will produce photonuclear reactions in almost all materials associated with the target assembly and vacuum chamber. The target itself was designed to measure the bremsstrahlung spectrum by measuring the time of flight ncutron cncrgy spectrum associated with photodisintegration of deuterium. D(y.n)H [Q=2.405 McV]. The hard x-ray yield was so large however that the time of flight neutron scintillator array was ox,envhelmed b?, both 4 scintillator array was ovcnvhclmcd b> both prompt photons and background neutrons produced in the chamber \\alls. Substantial radloactivit) uas mcasurablc in the target assembl\ following the laser shot This activit! was due to photonuclear reactions in both the gold and surrounding copper target-holder producmg transmutation to platmum and nickel daughter isotopes. The specific reactions observed mclude ““Au(r,n)““.4u. identified by the dcca! of the !““A” to ‘“‘Pt emitting nuclear gamma-ra!‘s at 356 and 333 kcV. and o’Cu(y.n)‘LCu and “iCu(y,n)“‘Cu. which were tdentiticd b) their subsequent beta decal (positron emission) to “‘Ni and “Ni (Fig. 4). Positron emission was determined by observation of 511 kcV y-rays resulting from annihilation radiation in the target asscmbl> Positive identification of both copper radioisotopes was established bk fitting the deca! curve with a two component decay with half-lives of 9.7 min and 12.7 hr (Fig. 4b). The fit gives an Identification probability of better than 99% for “Cu and “‘Cu. respectivcl) The threshold gamma-ray energy for photo-activation of the gold is 8.06 MeV, “‘Cu is Y 91 MeV and IO.85 Me\’ for “Cu; indicating a large flu\- of high- energ! bremsstrahlung Fig. 4: a. Gamma spectrum of target aasemhl? acquired for ~30 minutes. p+ activity decay curve . , Time (set) Fig. 1: b. Decay curve of 511 keV annihilation peak with calculated decay curw for isotopic composition of natural copper. Multiple radiographs thru varyng thicknesses of lead wcrc acquired during the shot series. Two of these, acquired with the film pack placed 81 cm from the target and at 45’ to the laser axis, are shown in Figs. 5 and 6. The radiograph in Fig. 5 was acquired on shot 27090808 using tmh film. The depth designation on the figure corresponds to the amount of lead in front of the test object. For depth of 65, 80 and 10X mm, two additional features were machined into the object at 14 mm less depth of Pb Accounting for the difference in distance between the TLD (100 cm) and the film pack (81 cm), this radiograph would correspond to a dose of >l Me\: photons as measured on the TLD of 0.15 Rad. as measured fmm the TLDs at 100 cm from the target on these shots was: O.l<Ey<O.5 MeV = 1.6~10~ photons/cm’ (0.24 Us), O.j<Ey<O.R MeV = l.lxlOg photons/cm’ (0.03 Rads), 0.8<Ey<S MeV = 1.7x10* photons/cm’ (0.24 Rads) and 8<Ey<20 MeV = 5.5u106 photons/cm’ (=.02 Rads). The radiograph in Fig. 6 is instructive since on these last two shots, high quality data was obtained from all diagnostics. The film pack was I left in the target chamber for shots 27091004. 1006, and 1008. Shot 27091004 was a verl strange shot which yielded an inverted electron angular distribution, and low u-ray yield relative to the other shots. The contribution of x-ray output at 45” from this shot relative to 27091006 and 1008 can be neglected. Radiographs corresponding to a difference in film density of >0.2 can be observed through 133 mm of lead (pQ150 g/cm’). Due to the attenuation of the lead, onI> photons with energy greater than approximately 1 MeV contributed to this radiograph The integrated dose Fig. 5. Radiograph acquired using tmh film al 15” to the laser axis through vat-?-ing thick-•ess of lead attenuator. The film pack was placed 81 cm from the target (Shnt 2709OSOS). 6 Fig. 6. Radiograph acquired using hmr film at 45” to the laser axis through varying thickness of lead attenuator. The hard I- ra: yield from two shots 27091006 and 1008 wem summed together on this radiograph. High quality images of the far-field laser beam distribution were acquired on these shots (Fig 7). The m~ages show substantial beam distortion resulting from both pump-induced and thermal dIstortion in the No\a disk amplifiers This distortion grows throughout the day as the amplifiers heat. OnI> 9 4 and IO’% of the energy is contained within the central 25 pm spot. respectivcl> for the two shots. The pulse cncrg! ws ~280 and 290 J for lhr shots As a result. in the absence of self-focusing. the highest irradiance present on target was =2x10” W/cm’. Only the central spot would have had sufficient irradlance to produce the high energ) electrons ncccssary to produce the hard (>I MeV) x-ral photons. Hcncz. both the radiograph of Fig. 6 and the TLD spectrum were produced by =56 J (sum of energy within the central spot for 27091006 and 1008) focused to a” irradiance of = 2x10” W/cm’ Fig. 7. a) Far-tield distribution from shot 27091006 (top) and, h) shot 27091008 (bottom). In additmn to serving as a testament to the need for the deformable mirror to correct this problem, the far-field data illustrates the need to account for the spatnl distribution of rradiance 111 inteyreting an! shot data The electron spectra are relativeI? insensitive to an!’ portion of the dlstnbution less than = IO”’ W/cm’ since ektrons bclo\\ =? MeV cannot escape the target <as dcscnbcd prc\iously This is not the case of the bremsstrahlung spectra \\hlch will include contributions from all hot electrons in the target The thm tar.@ brcmsstrahlung spectra from the electron distributions of Eq. I at 10’” W/cm’ (<E,>=l MeV). =lO’” W/cm (<EC>=3 MeV) and =lO” W/cm’ (<E,>=lO MeV) and a delta function distribution at E,=IO McV arc shonn in Fig. 8. This figure dramatlcall~ illustrates the impact of beam qualit! on the bremsstrahlung spectrum. A peromrr,, p1dse jbcmd f0 0 I2 pm dmmeter spt (IO’! W cm’,, will prod~r more than 100 times /he S-YLTJ dme irhovr 2 Mel’ m the SCI~E pzrise fOCll.Wd IO IO’” w cm2 In summan. the Scptembcr shot snics ws highly successful in providing us our first data on this nen type of source. High qualIt> radiographs were obtained through a <pr> >I50 g/cm’ along with electron spectra extending to 100 McV and photoactivation of the target assembl> The dose above =I MeV measured usmg thermoluminescent detectors on the last h\o shots at 45” and 100 cm from the target was over 0.24 rads. Accounting for the difference m angular distribution (also measured on these shots). this would hake 8 corresponded to 0.6+0 2 rads on axis. From the equivalent plane image, only 10% (56 J) of the incident laser energy was focused to a sufficient irradiance =2x10” W/cm’ to have contributed to the dose above 1 MeV. As a result of the rapid scaling of the bremsstrahlung output with electron energy and hence laser irradiance, an increase in hard photon yield approaching two orders of magnitude may be achievable by improving the beam quality to enable an irradiance approaching 10”’ W/cm” (800 3 in 450 fsec focused to a 15 pm diameter spot, 2x diffraction limited) These conclusions are supported by the large difference in the yield of high energy electrons from an early shot (27090507) with moderate beam quality (>I 6~10’ em/MeV-str) compared to that from a later shot with poor beam quality (27091006) 8 8x1 O7 e-/MeV-str) both at 3 0” and 12 5 MeV, a difference of 20. Finally, both in the electron and photon data, there is a strong high energy component which cannot be explained without filamentation of the beam in the preformed plasma resulting in small regions of locally high intensity or the presence of acceleration mechanisms III. FUTURE WORK In order to improve the quality of the laser beam, adaptive optics are being incorporated in the Petawatt design A deformable mirror will be placed in the master oscillator room and the beam will be conditioned to compensate for thermal aberrations in the amplifier section of Nova beamline 6 This will improve the intensity of the beam by approximately a factor of seven over the September shot series In addition, the energy of the beam is being increased from approximately 350 joules to 800 joules using the current diffraction gratings This should increase the radiation dose linearly In the long term, a set of compressor diffraction gratings will be sacrificed at a laser input power level of 1 5 kilojoules Grating development is underway to produce dielectric transmission gratings that will be capable of handling a five kilojoule pulse In addition, a demonstration will be performed on the Beamlet laser (the prototype laser for the National Ignition Facility (NIF)) to extract 2 sequenced pulses at 5 kilojoules each at 1~ within a few nanoseconds of each other The theoretical limit for pulse extraction from Beamlet is approximately 15 kilojoules If all the scaling works as predicted, the potential exists for the production of 50-200 rads at 1 meter or axis from 9 a BeamletlPetawatt laser device Further improvements using tailored target design might be possible Finally, because the pulse from a laser radiographic source is so short (<lo psec) compared to electron-beam driven pulses (-60-70 nsec), the use of time-gated imaging systems might allow for the production of a high- definition image because of the elimination of the scattered photon component ACKNOWLEDGEMENTS This work was performed under the auspices of the Department of Energy by the Lawrence Livermore National laboratory under Contract W- 7405-Eng-48 REFERENCES 1 W. Priedhorsky, D Lier, R Day, and D Gerke, “Hard s-ray measurements of 10 6 micron Laser Irradiated Targets,” Physical Review Letters, 47, 1661 (1981) 2 MD Perry; C Keane, and E M Campbell, “Ultrahigh Brightness Lasers and their Applications,” LLNL Internal document (1987) 3 J D Kmetec, C L Gordon, J.J. Macklin, B E Lemoff: and S.E. Harris, “MeV X-ray Generation with Femtosecond Lasers,” Physical Review Letters, 68, 1527 (1992) 4 G Malka and J L Miquel, Physical Review Letters, 77, 75 (1996). 5 G Malka, E Lefebvre, J L Miquel, and C Rousseaux, “Observation of Relativistic Electrons Produced in the Interaction of 400 Fsec Laser Pulses with Solid Targets,” Anomalous Absorption Conference: Fairbanks, Alaska, August 1996 6 P. Gibbon, “Efficient Production of Fast Electrons from Femtosecond Laser Interaction with Solid Targets,” Physical Review Letters, 73, 664 (1994) 10 Technical Information Department  • Lawrence Livermore National LaboratoryUniversity of California • Livermore, California  94551

Hard x-ray production from high intensity laser solid interactions

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Hard x-ray production from high intensity laser solid interactions

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