Characterization of wave-front corrected 100 TW, 10 Hz laser pulses with peak intensities greater...

REVIEW OF SCIENTIFIC INSTRUMENTS 77, 023102 �2006�

Characterization of wave-front corrected 100 TW, 10 Hz laser pulseswith peak intensities greater than 1020 W/cm2

Yutaka Akahane, Jinglong Ma, Yuji Fukuda, Makoto Aoyoma, and Hiromitsu KiriyamaAdvanced Photon Research Center, KANSAI Research Establishment,Japan Atomic Energy Research Institute, 8-1 Umemidai, Kizu, Kyoto 619-0215, Japan

Julia V. Sheldakova and Alexis V. KudryashovAdaptive Optics for Industrial and Medical Applications Group, Adopt Limited,Sudostroitelnaya Strasse 18, Building 5, Moscow 115407, Russia

Koichi YamakawaAdvanced Photon Research Center, KANSAI Research Establishment,Japan Atomic Energy Research Institute, 8-1 Umemidai, Kizu, Kyoto 619-0215, Japan

�Received 20 May 2005; accepted 19 December 2005; published online 9 February 2006�

An improvement of laser-focused peak intensity has been achieved in a JAERI 100 TW Ti:sapphirechirped-pulse amplifier chain with a feedback controlled adaptive optics system operating at a 10 Hzrepetition rate. The Strehl ratio was enhanced to 0.8 by means of a Bimorph deformable mirror witha Shack-Hartmann wave-front sensor. Measurements of optical parameters of the laser pulse and anexperimental tunneling ionization yield of helium have practically confirmed focusing toultrarelativistic intensities of over 1020 W/cm2 within 16% accuracy. © 2006 American Institute of
Physics. �DOI: 10.1063/1.2166669�
I. INTRODUCTION

During the past decade, there has been great progress inthe chirped-pulse amplification �CPA� technologies for thegeneration of ultrafast high-peak power laser pulses. Mean-while 100 TW class CPA laser systems are routinely operatedat high repetition rates in modest-size laboratories all overthe world.1–3 With these systems, fully relativistic peak in-tensities greater than 1019 W/cm2 have been reached andthey are applied to various high-field experiments, such asmonoenergetic electron generation with energies as high as170 MeV from rare-gas targets,4–6 neutron generation7 andion acceleration over MeV energies from solid targets,8,9 andintense keV x-ray line emission from atomic cluster targets.10

From the point of view of high intensity laser applica-tions, focusability of the laser beam is one of the most im-portant features of the high-peak power CPA system. On thegeneral assumption that temporal and spatial shapes of thelaser focus are sech2 and Gaussian, respectively, the peakintensity is determined as follows:

IPeak =EL

�

2

�w2 , �1�

where EL, �, and w are energy of the laser pulse, full width athalf maximum �FWHM� pulse duration, and 1/e2 radius ofthe focus, respectively. When the 100 TW class laser pulse isfocused by an f /3 parabola ideally, the maximum focusedpeak intensity, in the absence of any distortions, would beexpected to reach the 1020 W/cm2 level.11 This is difficult torealize in typical high-peak power CPA laser systems for thefollowing reasons: thermal lensing due to the high averagepower operation, relatively low optical quality of the large
diameter Ti:sapphire crystals used in the amplifiers, and low
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optical fidelity of the compressor gratings that can distort thewave front of compressed laser pulses. Wave-front distortionresults in focal plane aberrations when the laser pulse is fo-cused, which appears in both the enlargement of the focalspot size and the lowering of the energy content in the spot.The focused peak intensity achieved relative to the ideal caseis the Strehl ratio and can be decreased to values below 0.2.Consequently, such intensities are limited to the 1019 W/cm2

level in typical repetition-rated CPA laser systems.Recently, the adaptive optics has been applied to several

high-peak power CPA laser systems for wave-frontcorrection.12–14 With these devices in their multipass ampli-fiers and after compressors, near diffraction-limited focusinghas been obtained with Strehl ratios as high as 0.8. Morerecently, a generation of 1020 W/cm2 intensity was reportedand used for laser nuclear reaction within a tightly focused5 �m2 area operating at 10 Hz.15 A highest peak intensity of0.7�1022 W/cm2 has also been generated by focusing 0.1Hz intermittent Ti:sapphire laser pulses with an f /0.6parabola,16 though, �f /1 tight-focusing geometry, e.g., a fo-cal length of �3 cm, is practically limited for laser applica-tion. Furthermore the focusing and evaluation conditions ofthe laser beam in the former laser system were not reportedin detail, and the latter system would benefit from a higherrepetition rate that is beneficial for many applications. Basedupon these considerations, we have implemented the adap-tive optics system and multipurpose f /3 off-axis parabolainto our JAERI 100 TW laser system in order to generate anultrarelativistic intensity of over 1020 W/cm2 at a 10 Hz rep-etition rate. In this research, we have also utilized the mea-
surement of photoionization yield of helium atoms to evalu-
© 2006 American Institute of Physics2-1

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http://dx.doi.org/10.1063/1.2166669

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023102-2 Akahane et al. Rev. Sci. Instrum. 77, 023102 �2006�

ate the improved laser-focused peak intensity, as well as themeasurements of optical parameters of the high-peak powerlaser pulse.

In the meantime the laser-focused peak intensity is con-ventionally calculated by independently measuring opticalparameters, such as a pulse energy, duration, spot size, andenergy content in the spot. In the practical high-field experi-ments, however, the one that is calculated from the results ofindependent measurements of optical properties �called anintensity calculated by “optical measurements” in followingsentences� was known to be uncertain because the opticalmeasurements were not coincident. In order to obtain thesame laser-focused peak intensity as calculated from the op-tical measurements at the focal point, all the optical param-eters should be satisfied simultaneously at the focal point inthe experimental chamber. It is, however, difficult to confirmthis satisfaction in nature only from optical measurements.On the other hand, the focused peak intensity is evaluated bythe atom-field interaction. Optical-field ionization �OFI� ofatoms is one of the most fundamental ones, and its ionizationratio is utilized as a reference of the laser-peak intensity.Fittinghoff and co-workers have investigated the OFI of he-lium atom in detail, including the polarization state of thelaser pulse, and have been first to utilize the OFI yield as“direct intensity metering” tool.17–19 Especially the tunnel-ionization yield of He+ ion was precisely measured and com-pared with the results calculated by the Ammosov-Delone-Krainov �ADK� model,20 based on a quasiclassical tunnelingtheory within ±15% accuracy.21 Consequently, there are sev-eral researches in which they have used the OFI yield of He+

as a reference for their laser intensity calibration. Larochelleet al. have used this technique in their photoionization ex-periment of xenon at the laser intensities from 1013 to1016 W/cm2.22 More recently, Chowdhury et al. have ob-served the highly charged argon ions in the fully relativisticintensity of over 1019 W/cm2 using multiterawatt 25 fs laserpulses with an intensity calibration by OFI of helium.23 Theyhave utilized the intensity calibration with the energy scalingof the laser pulse. Although the laser-focused peak intensitywas estimated to be 6�1019 W/cm2 in their optical mea-surements, the peak intensity calibrated by OFI of heliumwas reported to be smaller by a factor of 3. They assumedthat the cumulative optical errors in their convoluted opticalmeasurements were responsible for the degradation of thefocused peak intensity. However, the roots of these opticalerrors are not clear quantitatively. McKenna et al. have alsocalibrated the focused intensity of 3.0±1.8�1019 W/cm2 bymeasuring the OFI yields of argon ions instead of the He+

ion with a factor of 1.5 uncertainty.24 Therefore more carefulexamination and improvements of the experimental appara-tus and procedure may be necessary to evaluate the laserintensities in the ultrarelativistic regime with high accuracy.

In this article we report on the generation and evaluationof the focused peak intensity of 1.2�1020 W/cm2 operatingat 10 Hz which was confirmed not only by optical measure-ments but also by measuring the OFI yield of helium withimproved experimental apparatus and procedure. The re-mainder of this article is organized as follows. In Sec. II, we
describe the JAERI 100 TW laser system including an adap-

tive optics system and a focusing chamber. Then in Sec. III,a detailed setup of the adaptive optics system and the resultsobtained with this system are presented. Finally, in Sec. IV,we show the results of a laser-focused peak-intensity calibra-tion by measuring the He+ OFI yield with improved experi-mental apparatus and procedure to overcome the disagree-ment between optical measurement and OFI experiment. Byso doing, peak intensities calculated from independent mea-surements of optical properties and those determined fromoptical-field ionization of helium agree within 16%. Thisagreement is assumed in scaling to higher, ultrarelativisticintensities.

II. JAERI 100 TW Ti:SAPPHIRE LASER SYSTEM

A four-stage Ti:sapphire CPA system1 was used for allthe experiments described in this article. This laser system iscapable of generating 100 TW, 20 fs laser pulses at a 10 Hzrepetition rate. The schematic of the laser system is shown inFig. 1. Seed pulses with 10 fs duration and at the centerwavelength of 800 nm were generated from a mirror-dispersion-controlled, self-mode-locked Ti:sapphire oscilla-tor. The seed pulses were then temporally expanded to 1 ns�FWHM� by the stretcher before amplification and then am-plified in regenerative and two multipass amplifiers up to�2 J in this study. After amplification, the laser pulse wasintroduced into a beam attenuation line for energy adjust-ment. The beam attenuation line consisted of wedges, half-wave plates, and polarizers with high optical qualities of� /10, where � is the center wavelength of the laser pulse of800 nm, the combination of which exhibits a net variableextinction from 1 to 10−6. These optics were placed after thelast amplifier and before the pulse compressor in order toprevent accumulation of B integral and additional high-orderdispersions after compression. Since the laser-pulse energywas attenuated after amplifier stages under full power opera-tion, the attenuation had no effect on the beam qualities, suchas spatial phase and focal spot size. The laser pulse from thebeam attenuation line was then up collimated to 5 cm indiameter by a Galilean telescope and compressed in agrating-pair pulse compressor located in an evacuated cham-ber ��10−4 Torr�. The second grating in the pulse compres-sor was mounted on a stepping-motor-driven linear stagewith �10 �m position resolution for optimization of thegrating separation in vacuum. The FWHM duration of the

FIG. 1. Schematic of the 100 TW, 10 Hz Ti:sapphire laser system.

compressed laser pulse was measured to be 23±1 fs using a


023102-3 Characterization laser pulses Rev. Sci. Instrum. 77, 023102 �2006�

single-shot second-order autocorrelator. For the wave-frontcorrection, the high-peak power laser pulse after pulse com-pression was reflected by the deformable mirror, and subse-quently reflected from a beam splitter with 98% reflectanceand high optical quality of � /10. The main part of laser pulsewas introduced into an ultrahigh-vacuum �UHV, 6�10−9 Torr� chamber passing through a 5 �m thickness pel-licle �both sides AR coated, National Photo Color PLC-3.0-700-900-SSTL96, optical quality of � /6� that separatedUHV from vacuum in the compressor chamber. In the UHVchamber, the laser pulse was guided by a pair of gold-coatedplane mirrors �Sigma TFG-100C15-10, optical quality of� /10� and focused by the gold-coated f /3.2 �effective fnumber� off-axis parabola �OAP, Optical Surface OAP4-152BG, nominal f number of f /1.6, and optical quality of� /6�. The maximum laser-pulse energy at the focal point wasmeasured to be 600±30 mJ by using an energy meter with athermal surface absorber head �Ophir L40�150�A-SH�.

III. WAVE-FRONT CORRECTION

A. Experimental setup of an adaptive optics system

Our adaptive optics system consists of a deformable mir-ror, a wave-front sensor, and a personal computer for closed-loop operation,25 which is shown in Fig. 2. The deformablemirror is based on a Bimorph structure, whose substrate con-sists of piezodisks, 31 electrodes, where the mirror is gluedon the top of the substrate. The dimension of electrodes areais 40�40 mm2, and arrangement of the electrodes is likemany sectors in three concentric circles of 9, 25.6, and 42.8mm diameters, respectively, which is shown in Fig. 2�b�. Asurface shape of each part of the mirror is deformed by ap-

FIG. 2. �a� Experimental setup. G, second compressor grating mounted on astepping-motor-driven linear stage; DM, deformable mirror; BS, 98% beamsplitter; P, 5 �m-thick pellicle; OAP, off-axis parabola; M, removable mir-ror; MO microscope objective �NA=0.4�; F, neutral density and interferencefilters; SM CCD, spot monitor CCD camera; W, wedge; L, f = +3.2 mplanoconvex lens; FF CCD, far-field monitor CCD; WFS, wave-front sen-sor; V, variable leak valve; PC, personal computer; HV, stack of high-voltage power suppliers for deformable mirror. �b� Electrodes arrangementof the Bimorph deformable mirror. A 40�40 mm2 area is divided by 30electrodes. There is another one electrode that is placed on the lower piezo-disk and covers all of the electrode area, which defines the fundamentalcurvature of the DM surface.

plying a high voltage to each corresponding electrode inde-


pendently. By controlling the high voltages applied on 31electrodes, the mirror surface can be modified arbitrarily. Theclear aperture and stroke of the deformation of the mirrorsurface were 55 mm and ±4 �m, respectively. The wave-front sensor is the Shack-Hartmann-type, which consists of a40�40 microlenslet array coupled with a charge-coupled de-vice �CCD� camera. A distortion of the wave front on eachsmall part of the laser beam is converted into a positionalvariation in the focal plane. The accuracy and maximum dy-namic phase range of the sensor are 0.1 and 10 �m, respec-tively. The personal computer calculates the wave front fromthe sensor CCD image, determines the shape of the mirrorsurface, and applies a high voltage to each electrode of thedeformable mirror which establishes feedback control in theadaptive optics system.

In order to design the adaptive optics system for correct-ing the wave front of the particular laser pulse, the advancewave-front measurements have been performed before andafter pulse compression in the laser chain. In these measure-ments, the critical wave-front aberration of the beam wasfound to be astigmatism �see Fig. 3�c�� on the pulse compres-sor gratings. A root-mean-square �rms� value of the wavefront in this case was 0.485�. As a result, the maximum peakintensity of the laser pulses focused by the f /3.2 off-axisparabola was only 2.6�1019 W/cm2 corresponding to a lowStrehl ratio of 0.20. To compensate for this aberration, thedeformable mirror was placed in the evacuated beam lineafter pulse compressor.

The setup of wave-front correction system is shown inFig. 2. The ultrashort laser pulse was reflected by the de-formable mirror first, and propagated to the 98% reflectancebeam splitter. The leakage of the beam splitter was split intotwo beams by a wedge that was placed out of vacuum, anddelivered to the Shack-Hartmann sensor for the wave-frontmeasurement and far-field monitor with long �f = +3.2 m�focal lens associated with a CCD camera, respectively. Ourclosed-loop adaptive optics system was capable to operatewith higher repetition rates than that of the laser system 10

FIG. 3. Results of the wave-front correction. ��a� and �b�� The far-fieldimages focused by f = +3.2 m lens, ��c� and �d�� the measured wave frontsbefore and after correction, respectively. The Stehl ratios of �a� 0.2 and �b�0.8, and rms wave front distortions of �c� 0.485� and �d� 0.102�, respec-tively. Astigmatism in distortion is calculated to be �c� 2.25� and �d� 0.211�,respectively.

Hz, and the wave front was corrected within a few seconds



after several tens of feedbacks once the system was started tooperate.

B. Experimental results of wave-front correction

Figure 3 shows the measured wave fronts and far-fieldimages of the laser pulse before and after the wave-frontcorrection. The Strehl ratio of the laser pulse was signifi-cantly improved from 0.20 to 0.80 by correction, which ap-pears clearly in the reduction of the large pedestal around thefocal spot between the far-field images �Figs. 3�a� and 3�b��.The rms of wave-front distortion was reduced from 0.485�to 0.102�, that can be seen in Figs. 3�c� and 3�d�, and astig-matism measured from wave-front data was significantly re-duced to one-tenth by correction. From these results, it isconcluded that the adaptive optics successfully corrected thewave front of the laser pulse, and four times improvement ofthe focusability was expected at focus in the UHV chamber.

Then we measured the focal spot with the OAP in thechamber. As seen in Fig. 2, the spot image of the attenuatedlaser pulse was analyzed by a microscope objective lens�NewPort M-20X, numerical aperture �NA�=0.4� associatedwith a 12 bit CCD camera �DALSA SMD-1�. On the focalspot measurement a sliding mirror was inserted just beforethe focal point to form a vertical image point and to imagethe spot to the monitor. To eliminate the chromatic aberrationon the spot images, an interference filter �CVI F1.5-800-4-2.00, �center=800 nm with 1.5 nm bandwidth� was insertedinto the imaging path just before the CCD camera. To pre-vent air breakdown and optical damage of the lens and CCDcamera by the high intensity focused beam, the beam attenu-ation line was used. Since the laser-pulse energy was attenu-ated after amplifier stages under full power operation, beamqualities, i.e., beam profile and wave-front distortion, wereconserved. Therefore the attenuation itself had no effect onthe qualities of the focal plane, such as temporal profile,focal spot profile, spot size, and its energy content.

The measured focal spot image after wave-front correc-tion was shown in Fig. 4. The 1/e2 size of the focal spot was6.33±0.11 �m, which is very close to the diffraction-limited

FIG. 4. Measured focal spot image after wave-front correction. The diam-eter and energy content of 1 /e2 focal spot were 6.33±0.11 �m and 60.3%,respectively.

one of 6.29 �m. The Strehl ratio was calculated to be 0.72 in


this case. Based on the separate measurements of the laserenergy and pulse duration, the laser-focused peak intensitywas calculated to be 1.19±0.19�1020 W/cm2 with thelaser-pulse energy of 600±30 mJ. This Strehl ratio is slightlysmaller than 0.80 of the previous far-field measurement. Thisdegradation is considered to originate from the quality of theoptics placed after the 98% beam splitter, such as the pel-licle, guiding mirrors, and OAP.

IV. IN SITU MEASUREMENT OF A LASER-PEAKINTENSITY

As mentioned in the previous section, we have shownfrom optical measurements that the laser intensity of over1020 W/cm2 is accessible by wave-front correction. In thepractical high-field experiments, however, the peak intensitycalculated by optical measurements was known to be uncer-tain. To realize the calculated peak intensity, the measuredoptical parameters should be fulfilled simultaneously at thefocal point in the experimental chamber. In principle, unfor-tunately, there is no way to confirm it by optical measure-ments in situ. Therefore the OFI yield of helium as a functionof laser intensity was measured in order to calibrate and con-firm the focused peak intensity in situ, as discussed in Sec. I.

Regarding the OFI experiments for intensity calibration,there are at least two critical issues that must be addressed toachieve a result close to that of optical measurements. Oneproblem with laser focusing systems is an effect of surround-ing large focal volume in the laser focus. The interactionvolume in the OFI studies of gaseous targets is an “ex-tended” volume �in contrast to a solid target for example� asdetermined by the peak laser intensity but it is also a “dy-namic” volume as determined by the temporal envelope ofthe single laser pulse.18 On the laser axis, there is a relativelylong focal region before and after a best focal point that mayhave sufficient field strength to ionize atoms. The interactionvolume of the extended focal region is larger than that in thevicinity of the focal plane and ions generated in this ex-tended focal region can contribute significantly to the totalion yield. To alleviate this effect, the limitation of the detec-tion area by inserting a slit26 between the focal region anddetector is necessary in order to measure the ion yield as afunction of laser intensity accurately.

The other problem is the temporal broadening of laserpulse at the focal point. The duration of the compressedpulse is usually measured by the autocorrelator located in anair environment. Although the autocorrelator is used to mini-mize its pulse duration, the process includes balancing dis-persions of air, vacuum window, and optics in the autocorr-elator, such as beam splitters and a second-harmonic-generation �SHG� nonlinear crystal that are not included inthe main beam line. It means that the pulse duration is actu-ally broadened from its minimum at the focal point and thefocused peak intensity is decreased accordingly. To over-come this, we have minimized the pulse duration by adjust-ing the separation of the grating pair in the compressor tomaximize the OFI yields of various ions.

Based upon these considerations, several OFI experi-ments were carried out. A time-of-flight mass spectrometry
�TOF-MS� was used to measure the ion yield. A pair of ion-


extracting electrodes was set across the focal point, as shownin Fig. 5. In these experiments, rarefied gas pressures ofnoble gasses in the UHV chamber were controlled to�10−7 Torr by a variable leak valve. The ions generated atthe laser-atom interaction region were extracted by a staticelectric field and accelerated up to 2.2 keV. The length of thefield-free region of the TOF-MS was set to be 982 mm. Inthese experiments, a mobile slit with a width of 300 �m wasplaced on the central axis of the TOF-MS tube 7 mm downfrom the last electrode plate with a ground potential. This slitmade it possible to detect ions only from the tight-focused,high intensity region of the laser focus. It should be notedthat the slit was too wide to affect the ion yield until the laserintensity increased far above the saturation intensity of eachcharged state of rare-gas ion. Just above the mobile slit, asseen in Fig. 5, two pairs of ion-deflecting electrodes wereplaced to adjust the flight path of the ions toward the detec-tor. To detect the ions, a two-stage microchannel plate �MCP,Hamamatsu: F4655-12� was used, which has a rise time of400 ps. The TOF mass spectra were recorded by a digitaloscilloscope �Recroy: Waverunner LT-30� with 500 lasershots accumulation for sufficiently good signal-to-noise ra-tio. The laser-pulse energy was monitored by measuring ei-ther the 2% leakage through the 98% reflectance beam split-ter or the energy transmitted through the focal plane with anattenuated pulse. It should be noted the attenuated laser pulsewas focused with the identical condition as described inFig. 4.

At first, we verified the effectiveness of the slit on theOFI experiment. Figures 6�a� and 6�b� show the typical TOFmass spectra of argon ions without and with using the slit inthe TOF tube, respectively. Both spectra have been normal-ized for comparison. Peaks of the multiple-charged argonions over Ar3+ were clearly visible in these figures. For the

+ 2+

FIG. 5. Setup of the time-of-flight mass spectrometry �TOF-MS� for optical-field ionization �OFI� experiments. OAP, off-axis parabola; S, 300 �mwidth mobile slit; D, two pairs of ion-deflecting electrodes; MCP, two-stagemicrochannel plate.

peaks of Ar and Ar , ions in the TOF spectra were broad-


ened by the saturation of the detector and there is no com-parison of these two ion peaks between the two spectra. InFig. 6�a�, the relative yields of multiply charged argon ionsover Ar6+ were small compared with lower charged argonions such as Ar3+. On the contrary, in Fig. 6�b�, there was alarge enhancement in the relative yields of Ar6+ and overwhen the slit was inserted. Moreover, Ar9+ was visible inFig. 6�b�. And it was also found that the slit eliminates themost of the unwanted background ion signals produced fromthe contaminations in the UHV chamber such as H2O, N2,and hydrocarbon molecules. As a result, by inserting the slitin the TOF tube, the detection of the feeble signals frommultiply charged ions becomes possible with a high signal-to-noise ratio.

Next the grating separation in the compressor was opti-mized at the focal point with monitoring the OFI ion yields.Figure 7 shows the typical ion yields of O3+–O5+ and H+ asa function of the separation of two gratings. These ions weregenerated from residual impurities by OFI in the UHV cham-ber, which were conveniently used for minimizing the pulseduration. The calculated FWHM pulse duration as a functionof grating separation was also shown in this figure. It isfound that the yields of all the ions were maximized whenthe grating separation was set to be 1164.78 mm, which is adifference of +270 �m from the original optimum separationof 1164.51 mm that was established using the SHG autocor-relation detection. The difference of grating separationmeans pulse broadening from 23 to 160 fs that is responsiblefor the degradation of peak intensity in the OFI experiment.Accordingly the grating separation was set to the value of1164.78 mm, and the minimization of the pulse duration atthe focal point was done. Note that the variation of the ionyields can be easily checked on the oscilloscope in real time,and this procedure is available in daily operation without any

FIG. 6. Ar ion TOF mass spectra. �a� Without slit. �b� 300 �m slit inserted.

additional setup.



After addressing the two critical issues, we have finallymeasured the laser energy dependence of the helium ionyield at the best experimental condition for intensity calibra-tion. Figure 8 shows the He+ yields, with the ion sourcevolume limited by the slit, as functions of both laser energyand least-squares-fitted peak intensity. The factor of intensitycalibration obtained from Fig. 8 was 2.02±0.30�1020 �W/cm2� ”J. From the energy scaling of the laserpulse, the maximum peak intensity was calibrated to be1.21±0.18�1020 W/cm2 at the laser-pulse energy of 600mJ, which well agrees with the calculated intensity from theoptical parameters with an accuracy of 16%. This is the firstresult, to our knowledge, that a calculated intensity based onindependent optical measurements has been confirmed by adirect OFI intensity calibration of helium at the 1016 W/cm2

level for ultrashort pulse durations ��20 fs�. Because wehave addressed the detailed pulse duration and transverse

FIG. 7. Dependences of the ion yields of O3+–O5+ and H+ as a function ofcompressor grating separation. The duration of the laser pulse as a functionof grating separation was also plotted in the figure, that was calculated fromthe minimum pulse duration in the SHG-autocorrelation measurement. Theoptimized grating separation in the OFI a experiment was 270 �m differentfrom that in the SHG autocorrelation �1164.51 mm, superimposed verticalline�.

FIG. 8. Intensity calibration by OFI of helium as a function of laser inten-sity. The ion source volume was limited by the 300 �m mobile slit to im-prove the signal-to-noise ratio. The filled and open circles are the experi-mental ion yields of He+ and He2+, respectively. The solid and dotted linesare the ion yields calculated from the ADK theory. The factor of intensity

20 2
calibration is 2.02±0.30�10 �W/cm � / J.

profile issues in the focal plane we can assume that this im-pressive argument persists up to the ultrarelativistic,1020 W/cm2 level. While all of our OFI intensity calibrationwas performed using linear-polarized laser pulse, it is pos-sible to perform the calibration with circular-polarized one.In the circular-polarized laser field, the nonsequential doubleionization of helium atom was reported to be suppressed,19

which could be beneficial for another calibration reference ina higher laser intensity region using He2+ yield.

On the basis of the peak-intensity calibration confirmedabove, an OFI experiment in the fully relativistic intensity of�1020 W/cm2 was performed. In this study, krypton and xe-non gases were selected, which have many ionic chargestates around the intensity regime. The setup of the experi-ment is identical to that used for the helium OFI. Figures9�a�–9�d� show the typical TOF mass spectra of krypton andxenon ions at the gas pressure of �10−7 Torr and at thelaser-peak intensity of 0.8�1020 W/cm2. These spectra werethe sum averages of 2000 laser shots. Our measurementswere concentrated on the highly charged, inner-shell ionsover Kr9+ or Xe9+, respectively, since the outer-shell ions�Kr+–Kr8+ or Xe+–Xe8+, respectively� are completely satu-rated at such high intensity. In the mass spectrum shown inFig. 9�b�, Kr21+ peaks were not visible due to the peak over-lap of the ionizing contaminations of O4+�C3+�. The maxi-mum observed krypton ion charge state was +24 �Fig. 9�b��,which is significantly higher than +19 observed in theprevious OFI study with the laser intensity of 2.6�1019 W/cm2.27 The maximum detected xenon charge statewas +26 �Fig. 9�d��, which is the same as that observedpreviously but the signal is clearly enhanced in this study.Since there is a large gap of ionization energy between Xe26+

and Xe27+ �857–1495 eV�, it is understood that Xe26+ wouldbe the maximum observed charge state. These results aretherefore solid evidence of the increase of laser-peak inten-sity by wave-front correction.

The peak laser intensity in the focal plane was deter-mined by OFI of helium atom. By limiting the intensity-dependent volume expansion with a slit mask and by settingthe compressor grating separation to maximize the OFI ionyield, we obtain good agreement between the results of di-rect intensity metering using OFI and those calculated basedon the independent measurement of laser-pulse properties. Inthe OFI experiment of krypton and xenon performed at thecalibrated laser intensity of 0.8�1020 W/cm2, the improve-ment of the focused peak intensity by wave-front correctionwas also confirmed.

In the future study, we plan to develop an in situ evalu-ation method of an ultrarelativistic laser field directly bymeasuring highly charged ion yields of heavy atoms as afunction of laser intensity in the 1020 W/cm2 region. Forlaser intensities above the relativistic threshold near1018 W/cm2, relativistic effects may alter intensity-dependent ion yields and the accuracy of simple ADKmodel. We continue to investigate this critical subject matter,and we believe that direct intensity metering via laser-induced OFI can be extended to ultrastrong laser fields ofultrashort duration using currently measured ion yields of

27,28
various rare-gas atoms as a base.


V. SUMMARY

We have corrected the wave front of the high-peakpower laser pulses using the closed-loop adaptive optics sys-tem in the JAERI 100 TW, 10 Hz laser chain for the im-provement of the focused peak intensity on targets. TheStrehl ratio of the laser pulse was significantly improvedfrom 0.20 to 0.80 by means of the Bimorph deformable mir-ror with the Shack-Hartmann wave-front sensor. The peakintensity was then enhanced to 1.2�1020 W/cm2 by focus-ing with a f /3.2 off-axis parabola, which was confirmed not

FIG. 9. TOF spectra of multiple-charged ions of ��a� and �b�� krypton, ��c�and �d�� xenon at the laser-peak intensity of 0.8�1020 W/cm2. An appear-ance of Ar8+ in �d� is due to the result of mixed gases in order to determinethe relative laser intensities among the gases.


only by optical measurements of the laser pulse but also byoptical-field ionization of helium with improvements of ex-perimental apparatus and procedure to limit the intensity-dependent volume expansion and overcome temporal broad-ening of the compressed pulse. A development of an in situevaluation method of the ultrarelativistic laser field directlyis currently underway by measuring highly charged ionyields of various rare-gas atoms as a function of laser inten-sity. The techniques described in this article will benefit thecomprehensive characterization of experimental parametersin the studies of laser-matter interactions in the relativisticintensity regime.

ACKNOWLEDGMENTS

The authors would like to thank N. Inoue, K. Tsuji, Y.Nakai, and Y. Yamamoto for their technical support in thisresearch. They also thank T. Kimura and T. Tajima for theirencouragement. This research was partly supported by aGrant-in-Aid for Specially Promoted Research �Contract No.15002013� of the Ministry of Education, Culture, Sports,Science and Technology of Japan.

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Characterization of wave-front corrected 100 TW, 10 Hz laser pulses with peak intensities greater...

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