INTRODUCTION
The generation of beams of multi-MeV ions in the interaction of high power laser pulses with thin foil targets continues to attract considerable international interest, due to the uniquely short pulse and low emittance properties of the beam, and the potentially compact nature of the source (Borghesi et al., Reference Borghesi, Fuchs, Bulanov, Mackinnon, Patel and Roth2006; Badziak, Reference Badziak2007; Flippo et al., Reference Flippo, Hegelich, Albright, Yin, Gautier, Letzring, Schollmeier, Schreiber, Schulze and Fernandez2007; Schollmeier et al., Reference Schollmeier, Roth, Blazevic, Brambrink, Cobble, Fernandez, Flippo, Gautier, Habs, Harres, Hegelich, Hessling, Hoffmann, Letzring, Nürnberg, Schaumann, Schreiber and Witte2007; Nickles et al., Reference Nickles, Ter-Avetisyan, Schnuerer, Sokollik, Sandner, Schreiber, Hilscher, Jahnke, Andreev and Tikhonchuk2007; Faenov et al., Reference Faenov, Magunov, Pikuz, Skobelev, Gasilov, Stagira, Calegari, Nisoli, De Silvestri, Poletto, Villoresi and Andreev2007; Strangio et al., Reference Strangio, Caruso, Neely, Andreoli, Anzalone, Clarke, Cristofari, Del Prete, Di Giorgio, Murphy, Ricci, Stevens and Tolley2007; McKenna et al., Reference McKenna, Lindau, Lundh, Carroll, Clarke, Ledingham, McCanny, Neely, Robinson, Robson, Simpson, Wahlström and Zepf2007). At the laser intensities available at present, the main ion acceleration occurs at the rear of the target and is driven by relativistic electrons accelerated at the front, irradiated surface, that propagate through the target and produce an electrostatic sheath at the rear surface (Wilks et al., Reference Wilks, Langdon, Cowan, Roth, Singh, Hatchett, Key, Pennington, Mackinnon and Snavely2001). The main ion species accelerated is the proton, due to its high charge-to-mass ratio and the presence of hydrogenated contamination layers on the target surfaces (Hegelich et al., Reference Hegelich, Karsch, Pretzler, Habs, Witte, Guenther, Allen, Blazevic, Fuchs, Gauthier, Geissel, Audebert, Cowan and Roth2002; McKenna et al., Reference McKenna, Ledingham, Yang, Robson, McCanny, Shimizu, Clarke, Neely, Spohr, Chapman, Singhal, Krushelnick, Wei and Norreys2004). The challenge, for potential applications of this unique source, is to enhance and control the source and beam properties.
Preplasma, which is typically produced by prepulses and amplified spontaneous emission (ASE) at the leading edge of a high power laser pulse, can play a significant role in defining the properties of the ion beam. It has been shown that prepulse induced heating and expansion of the target rear surface reduces the maximum proton energy (Roth et al., Reference Roth, Blazevic, Geissel, Schlegel, Cowan, Allen, Gauthier, Audebert, Fuchs, Meyer-Ter-Vehn, Hegelich, Karsch and Pukhov2002; Fuchs et al., Reference Fuchs, Cecchetti, Borghesi, Grismayer, D'humieres, Antici, Atzeni, Mora, Pipahl, Romagnani, Schiavi, Sentoku, Toncian, Audebert and Willi2007). Furthermore, Kaluza et al. (Reference Kaluza, Schreiber, Santala, Tsakiris, Eidmann, Meyer-Ter-Vehn and Witte2004) report that the optimum target thickness for proton acceleration strongly depends on prepulse duration and Lindau et al. (Reference Lindau, Lundh, Persson, McKenna, Osvay, Batani and Wahlström2005) report proton beam steering due to ASE-driven target rear surface deformation. However, even for conditions for which the target rear surface is unperturbed, plasma expansion at the front surface can significantly affect rear surface ion acceleration. Recent, theoretical studies have shown that by controlling the scale length of the front surface preplasma, laser absorption efficiency, and therefore proton acceleration, can be enhanced (Sentoku et al., Reference Sentoku, Bychenkov, Flippo, Maksimchuk, Mima, Mourou, Sheng and Umstadter2002; Seo et al., Reference Seo, Yoo and Hahn2007; Lee et al., Reference Lee, Pae, Suk and Hahn2004; Andreev et al., Reference Andreev, Sonobel, Kawatal, Miyazaki, Sakai, Miyauchi, Kikuchi, Platonov and Nemoto2006). Proton energy enhancement at laser intensities between 1018 and 1019 W/cm2, where the effect is predicted to be most pronounced (Sentoku et al., Reference Sentoku, Bychenkov, Flippo, Maksimchuk, Mima, Mourou, Sheng and Umstadter2002), has been observed using ultrashort (tens of femtosecond) laser pulses (Maksimchuk et al., Reference Maksimchuk, Gu, Flippo and Umstadter2000; Yogo et al., Reference Yogo, Daido, Fukuki, Li, Ogura, Sagisaka, Pirozhkov, Nakamura, Iwashita, Shirai, Noda, Oishi, Nayuki, Fuji, Nemoto, Choi, Sung, Ko, Lee, Kaneda and Itoh2007; Glinec et al., Reference Glinec, Genoud, Lundh, Persson and Wahlström2008).
In this paper, we report on the effects of controlled front surface plasma expansion on proton acceleration at the rear of thin metallic foils in the ultraintense picosecond laser pulse regime. It is in this regime of intensity and pulse duration that the highest energy protons are presently produced (Snavely et al., Reference Snavely, Key, Hatchett, Cowan, Roth, Phillips, Stoyer, Henry, Sangster, Singh, Wilks, Mackinnon, Offenberger, Pennington, Yasuike, Langdon, Lasinski, Johnson, Perry and Campbell2000; Robson et al., Reference Robson, Simpson, Clarke, Ledingham, Lindau, Lundh, McCanny, Mora, Neely, Wahlström, Zepf and McKenna2007). This regime is also directly relevant to the fast ignition approach to fusion energy (Tabak et al., Reference Tabak, Hammer, Glinsky, Kruer, Wilks, Woodworth, Campbell, Perry and Mason1994; Sakagami et al., Reference Sakagami, Johzaki, Nagatomo and Mima2006). That scheme involves a laser pulse, with similar parameters, that propagates through coronal plasma and delivers energy, usually in the form of fast electrons, inside a precompressed deuterium-tritium pellet.
EXPERIMENTAL
The experiment is performed using the Vulcan petawatt laser at the Rutherford Appleton Laboratory (Danson et al., Reference Danson, Brummitt, Clarke, Collier, Fell, Frackiewicz, Hawkes, Hernandez-Gomez, Holligan, Hutchinson, Kidd, Lester, Musgrave, Neely, Neville, Norreys, Pepler, Reason, Shaikh, Winstone, Wyatt and Wyborn2005). Chirped pulse amplified (CPA) 1054 nm wavelength (λ) light, in pulses with duration equal to 1 ps (full width at half maximum (FWHM)) is used to accelerate protons. A plasma mirror (Dromey et al., Reference Dromey, Kar, Zepf and Foster2004), positioned in the focusing beam, suppresses the intensity of the ASE pedestal to ~1011 W/cm2. The mirror is operated at 20° angle of incidence, in a p-polarized geometry, with a measured reflectivity of 32%, providing a maximum energy on target of 115 J. The CPA pulses are incident on target at 10° and focused to a spot size of 5 µm (FWHM), to achieve a calculated peak intensity, I CPA, equal to 3 × 1020 W/cm2.
The expanding preplasma is produced using low energy “ablation” laser pulses of 6 ns duration (with a fast rise time of 0.2 ns and slow decay to about 50% of the peak intensity at 6 ns) and wavelength equal to 1053 nm. The pulses are focused, using an f/10 lens, to an approximately flat-top intensity distribution of 450 µm diameter on the target front surface, centered at the same position as the CPA focus. The pulse energy is controlled to vary the intensity, I abl, in the range 0.5 to 5 TW/cm2. The ablation pulse arrives on target first, and the time delay of the arrival of the CPA pulse, Δt, is varied in the range 0.5 to 3.6 ns, with 0.2 ns precision.
The targets are 25 µm-thick planar Cu foils or 25 µm-thick Au foils with a periodic groove structure (lines) on the rear surface. The lines have a period of 10 µm and a peak-to-valley depth of 1.1 µm, with a sinusoidal profile.
The spatial and energy distributions of the beam of accelerated protons are measured using a passive stack of dosimetry film (Gafchromic© film, HD-810 and MD-V2-55). The plasma expansion is characterized by application of interferometry (Benattar et al., Reference Benattar, Popovics and Sigel1979) on a 527 nm wavelength probe beam directed across the target. Interferograms are recorded with a 16 bit CCD camera, 5 ps after the arrival of the CPA pulse, and with a spatial resolution of 5 µm.
RESULTS
The main findings of our investigation result from changes to the spectral and spatial intensity distributions of the proton beam measured at the rear of the target. The beam parameters are measured as a function of (1) I abl for fixed Δt = 0.5 ns, and (2) Δt for fixed I abl = 1 TW/cm2, for otherwise identical CPA pulse and target parameters. Figure 1a shows example proton energy spectra from a scan of I abl. The measured scale length of the electron density, along the target normal, in the underdense region of the expanding plasma at the target front surface is marked for each plot. Hereafter, we refer to this, the scale length of the outer part of the preplasma, as L O, and we refer to the density scale length in the inner region, near the critical density, as L I. L O is determined by fitting the relation n e(x) ∝ exp(−x/L O) (where n e is the electron density and x is the distance from the target surface) to the electron density profile extracted from the interferometric probe measurements. As shown in Figures 1b and 1c, as L O is increased from ~0 (no ablation pulse) to ~60 µm, the maximum proton energy, E max, increases by about 25% and the efficiency of conversion, η, of laser energy to protons (above a lower detection threshold of 2.4 MeV) increases by about 50%. The proton “temperature,” T, is also observed to increase, by about 40%, from 3.5 to 4.8 MeV over this range. As L O is increased beyond 100 µm, E max, T, and η all decrease below their respective values for the case of a sharp density gradient. The same dependency on L O is measured whether the preplasma scale length is varied by changing I abl or Δt, as clearly observed in Figures 1b and 1c.
Fig. 1. (a) Example proton energy spectra, measured using stacked dosimetry films, for different I abl and fixed Δt = 0.5 ns; (b) Maximum proton energy, and (c) Laser-to-proton energy conversion efficiency as a function of L O, obtained for different I abl up to 5.0 TW/cm2 for fixed Δt = 0.5 ns (triangles), and for different Δt up to 3.6 ns for fixed I abl = 1.0 TW/cm2 (circles).
Whereas there is an optimum preplasma expansion to enhance E max, T, and η, improvements in the spatial-intensity profile of the proton beam are observed for all cases in which a preplasma expansion is produced by the ablation pulse, with the most uniform proton beams measured for the longest preplasma scale lengths. In agreement with previous observations reported in Carroll et al. (Reference Carroll, McKenna, Lundh, Lindau, Wahlström, Bandyopadhyay, Pepler, Neely, Kar, Simpson, Markey, Zepf, Bellei, Evans, Redaelli and Batani2007), we measure significant improvements in the uniformity and circularity of the beam over the full proton energy range. For example, in a sample region corresponding to 50% of the beam area, in the center of the beam, the standard deviation of the proton dose at 10 MeV decreases from 36% of the mean value, in the absence of the ablation pulse, to 19% for L O = 145 µm. The deviation of the spatial profile of the proton beam from a perfect circle (circularity measured as 4π·Area/(Circumference)2) decreases from 4% to 1% for the same sample proton dose distributions. Using targets with a periodic groove structure on the rear surface, we determine, from the number of lines reproduced in the proton spatial intensity profiles, the proton source size decreases with increasing proton energy, from ~600 µm at 4 MeV to ~50 µm at 30 MeV in the absence of an ablation pulse. An increase of between 20% and 50% is measured, at all energies, for I abl = 0.5 TW/cm2 (L O = 38 µm), and as I abl is increased to 5.0 TW/cm2 (L O = 145 µm), the source size is reduced by a factor of two. We do not observe a significant change to the proton beam normalized transverse emittance, measured using the technique described by Cowan et al. (Reference Cowan, Fuchs, Ruhl, Kemp, Audebert, Roth, Stephens, Barton, Blazevic, Brambrink, Cobble, Fernandez, Gauthier, Geissel, Hegelich, Kaae, Karsch, Le Sage, Letzring, Manclossi, Meyroneinc, Newkirk, Pepin and Renard-Legalloudec2004). Typically, we obtain values of the order of 0.011 π mm mrad at example proton energy of 6 MeV.
We measure the density profile in the preformed plasma at the target front surface up to an electron density limit of ~4 × 1019 cm−3 due to refraction at the steep density gradients. In order to determine the full initial density profile, two-dimensional hydrodynamic simulations are performed using the Pollux code (Pert, Reference Pert1981). Cylindrical symmetry is used with a 300 µm by 300 µm grid with cell size equal to 1.5 µm. The target material is Cu. The laser wavelength and spot radius are set at 1.06 µm and 220 µm, respectively, and the pulse has a rise time of 0.2 ns and thereafter remains at a chosen intensity. Two distinct regions of preplasma expansion, with scale lengths L I and L O, are clearly observed in the example results in Figure 2a. These calculated density profiles are in good agreement with the corresponding measured profiles (also shown). The simulation results predict an increase in L O from 31 µm to 182 µm for an increase in Δt from 0.5 ns to 3.5 ns, for fixed I abl = 1 TW/cm2, and the corresponding measured values are 50 ± 10 µm and 200 ± 40 µm, respectively. From the simulation results we infer that L I, in the region of the relativistically corrected critical density (~11n c), increases from ~0.6 µm to ~0.9 µm for an increase in Δt from 0.5 to 3.5 ns. The small increase (4%) in the thickness of the overdense region of the target is expected to have negligible effect on the proton acceleration.
Fig. 2. (a) Electron density profiles at the front surface of a Cu target for different expansion times (0.5 ns and ~3.5 ns) at fixed I abl = 1 TW/cm2: solid lines are calculated using the Pollux hydrodynamic code; dashed lines are experimental measurements obtained from the interferograms. X = 0 corresponds to the initial target front surface. The inset is an enlarged region to show the extent of shock propagation in the target; (b) Example interferometric probe image showing channelling of the CPA laser beam in relatively short scale length preplasma; (c) Example interferometric probe image showing filamentation of the CPA laser beam in long scale length preplasma. The laser pulses are incident from the left in (b) and (c), and self emission at the critical surface is observed.
Importantly, there is evidence from four independent sources that expansion of the target rear surface does not occur prior to the arrival of the CPA pulse on target. Calculation of the position of the shock front, launched by the ablation pulse, using the approach and parameters given in Lundh et al. (Reference Lundh, Lindau, Persson, Wahlström, McKenna and Batani2007), indicates that it does not reach the target rear surface for the parameter range experimentally investigated. This is confirmed by the hydrodynamic simulation results, as shown in the inset of Figure 2a. The absence of target rear surface expansion is confirmed experimentally in the interferograms, within the limit of the spatial resolution. Finally, the periodic structure on the rear surface of the Au targets is clearly observed in the proton beam spatial intensity profile, indicating that the structure is unperturbed prior to the arrival of the CPA pulse.
From the transverse optical probe measurements we conclude that the observed changes to the proton beam result from changes to the CPA laser pulse propagation in the expanding plasma. For some laser shots, for the conditions of optimum E max, T, and η, a single channel, with width smaller than the nominal focusing cone of the CPA beam, is observed, Figure 2b, and is evidence of self focusing of the beam. This results in a smaller focal spot, and hence a higher I CPA, compared to the case of a steep density gradient. By contrast, for very long scale length expansion (L O > 100 µm), typically the CPA beam is observed to filament, Figure 2c, reducing I CPA. The measured changes in E max and T are consistent with the inferred qualitative changes in I CPA. The measured changes in η suggest enhanced laser energy absorption for optimum plasma expansion conditions, at which channeling of the CPA laser pulse is observed.
The proton beam uniformity is sensitive to a number of parameters, including the profile of the CPA laser focus, the angular distribution of the fast electrons generated, and the extent of small-angle scattering of the fast electrons as they propagate through the target (Fuchs et al., Reference Fuchs, Cowan, Audebert, Ruhl, Gremillet, Kemp, Allen, Blazevic, Gauthier, Geissel, Hegelich, Karsch, Parks, Roth, Sentoku, Stephens and Campbell2003; Schollmeier et al., Reference Schollmeier, Harres, Nürnberg, Blazevic, Audebert, Brambrink, Fernandez, Flippo, Gautier, Geissel, Hegelich, Schreiber and Roth2008). The mechanism by which the spatial intensity distribution of the proton beam is influenced by preplasma expansion is a subject for further investigation.
SUMMARY
We have directly measured the effects of front surface plasma expansion on the coupling of laser energy to protons (via fast electrons) for laser pulse parameters directly relevant to the fast ignitor scheme. We have demonstrated that, compared to a sharp density gradient, a preplasma with an underdense scale length of ~30 to 60 µm enhances proton acceleration due to self-focusing of the laser beam. The measured ~25% enhancement in the maximum energy, for I CPA = 3 × 1020 W/cm2, can be compared with the 40% enhancement predicted by Sentoku et al. (Reference Sentoku, Bychenkov, Flippo, Maksimchuk, Mima, Mourou, Sheng and Umstadter2002) for a laser intensity of 2.4 × 1020 W/cm2 (for λ = 1.054 µm). As the preplasma scale length increases to >100 µm the propagating laser pulse filaments, reducing the laser intensity and giving rise to increased proton beam uniformity, but with reduced energy and flux. The results highlight that properties of the proton beam (and therefore the electron source) can be actively manipulated by optical control of the plasma expansion.
ACKNOWLEDGEMENTS
We acknowledge fruitful discussions with Dr. A.P.L. Robinson and the expert support of the staff at the Central Laser Facility. This work was supported by the UK Engineering and Physical Sciences Research Council (EPSRC Grant number EP/E048668/1), the EU COST P-14 Action and the virtual institute VI-VH-144 (VIPBUL), funded by the Helmholtz Association.
