⇦Chinese Physics Letters, 2018, Vol. 35, No. 9, Article code 092901 Generating Proton Beams Exceeding 10 MeV Using High Contrast 60TW Laser * Yi-Xing Geng(耿易星)1, Qing-Liao(廖庆)1, Yin-Ren Shou(寿寅任)1, Jun-Gao Zhu(朱军高)1, Xiao-Han Xu(徐筱菡)1, Min-Jian Wu(吴旻剑)1, Peng-Jie Wang(王鹏杰)1, Dong-Yu Li(李东彧)1, Tong-Yang(杨童)1, Rong-Hao Hu(胡荣豪)1, Da-Hui Wang(王大辉)1, Yan-Ying Zhao(赵研英)1, Wen Jun Ma(马文君)1, Hai-Yang Lu(卢海洋)1**, Zhong-Xi Yuan(袁忠喜)1, Chen-Lin(林晨)1**, Xue-Qing Yan(颜学庆)1,2 Affiliations 1State Key Laboratory of Nuclear Physics and Technology and CAPT, Peking University, Beijing 100871 2Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006 Received 15 May 2018, online 29 August 2018 *Supported by the National Basic Research Program of China under Grant No 2013CBA01502, the National Natural Science Foundation of China under Grant Nos 11475010, 11575011 and 11535001, and the National Grand Instrument Project under Grant No 2012YQ030142.
**Corresponding author. Email: hyluforever@gmail.com; lc0812@pku.edu.cn Citation Text: Geng Y X, Liao Q, Shou Y R, Zhu J G and Xu X H et al 2018 Chin. Phys. Lett. 35 092901 Abstract A prototype of a laser driven proton accelerator is built at Peking University. Protons exceeding 10 MeV are accelerated from micrometer-thick aluminum targets irradiated by tightly focused laser pulse with 1.8 J energy and 30 fs duration. The beam energy spectrum and charge distribution are measured by a Thomson parabola spectrometer and radiochromic film stacks. The sensitivity of proton cut-off energy to the focusing of the laser beam, the pulse duration, and the foil thickness are systematically investigated in the experiments. Stable proton beams have been produced with an optimized parameter set, providing a cornerstone for the future applications of laser accelerated protons. DOI:10.1088/0256-307X/35/9/092901 PACS:29.27.-a, 41.75.Jv, 42.55.Rz, 52.38.Kd © 2018 Chinese Physics Society Article Text The rapid development of ultra-intense, ultra-short laser technology permits laser focusing intensities exceeding ${10}^{18}$ W$\cdot$cm$^{-2}$, making the acceleration of energetic proton beams possible with lab-top laser systems. Laser driven proton beams exhibit unique features: low emittance,[1] small source size,[2] short bunch duration, and kA peak current. Since the energy deposition of high-energy protons in matter is localized due to the Bragg peak, such ion sources are promising tools in warm dense matter generation,[3] ion-driven fast ignition,[4] injection source[5] of conventional accelerators, radiographic imaging,[6] and cancer therapy.[7] Several laser proton acceleration schemes have been proposed to optimize different laser and target parameters. Two major ion acceleration mechanisms are target normal sheath acceleration (TNSA)[8-12] and radiation pressure acceleration (RPA).[13-16] In TNSA, hot electrons are generated by the laser pulse near the laser-irradiated surface, which then penetrate to the rear side of the target. These energetic electrons establish a high gradient electrostatic sheath field which can accelerate ions to relativistic velocity within tens of micrometers. The maximum energy of proton beam produced by the TNSA mechanism is 85 MeV.[17] In contrast to TNSA, RPA realizes volume acceleration of the ions and electrons synchronously, relying on the equilibrium between the electrostatic and the radiation pressure. Recently, a 93 MeV proton beam has been generated, using nanometer-thick foils and the ultra-high contrast laser system.[18] The RPA regime requires demanding laser pulse parameters, for example, ultra-high laser intensity, sharp rising front, and ultra-high intensity contrast, which are not easy to meet at the same time for most laboratories. Up to date, TNSA is still the dominate mechanism in most experiments. In this Letter, we report the recent experimental results on laser driven ion acceleration based on TNSA, utilizing a newly built compact laser plasma accelerator (CLAPA) at Peking University. CLAPA[19,20] aims to generate MeV energy high peak current stable proton beams at the 1 Hz operation rate, for the application of biological cell irradiation, plasma diagnostics, space radiation environment simulation, etc. The sensitivity of accelerated proton cut-off energy on the intensity of the laser beams, the pulse duration, and the foil thickness have been systematically investigated in the experiments. Under the condition of optimum parameters, proton beams with energy up to 10 MeV have been obtained with 2.5-μm-thick aluminum foils irradiated by 60 TW laser. In the meantime, the proton cut-off energy, charge, and divergence angle are carefully analyzed at different target thicknesses while keeping the laser parameters as constants. These significant scaling laws of laser and targets parameters reveal the laser plasma interaction mechanism and provide a research basis for the wide potential applications of the laser accelerated beams in the future. A 200 TW double chirped-pulse amplification (CPA) Ti:sapphire laser system has been installed as the driven source for the ion acceleration. The schematic structure of the laser system is shown in Fig. 1(a). The system delivers laser pulses with a central wavelength of 800 nm and a 45 nm bandwidth, which can be compressed down to 25 fs duration at full width at half maximum (FWHM). Under full power operation, the output energy is 5 J. Temporal contrast is a key issue for laser ion acceleration. In the case of a poor contrast, the micrometer to nanometer thick solid target could be evaporated and ionized before the arrival of the main laser pulse.[21,22] To suppress the pedestal or the pre-pulse level, a cross-polarized wave (XPW) filter[23] has been installed between the regenerative amplifier and the PW stretcher. Measurements of the ps time scale using the third order scanning autocorrelator with a high dynamic range show that the intensity contrast is ${10}^{-10}$ at 40 ps before the main pulse, as shown in Fig. 1(b). To reduce the damage on the optics and maintain the stability of the laser system, in the primary experiments of CLAPA, the laser system was only operated at half of its full power. Combing the laser compressor and transmission efficiency, the actual pulse energy on the target was 1.8 J with a duration of 30 fs. Figure 2(a) is the sketch of the experimental setup. The p-polarized laser beam was focused by an $f$/3.5 off-axis parabola onto free standing metal foil targets at ${30}^{\circ}$ incident angle with respect to the target normal direction. A deformable mirror with an aperture of 110 mm coupled to a wavefront sensor (Phasis SID4) was mounted before the compressor to correct aberrations that originate in the laser system and the experimental system. The optimized laser focal spot diameter (FWHM) was 5 μm, with 25% of laser energy in it (Fig. 2(b)), corresponding to an intensity of $8.3\times {10}^{19}$ W$\cdot$cm$^{2}$.
cpl-35-9-092901-fig1.png
Fig. 1. (a) The layout of the CLAPA laser system. (b) Measurement of picosecond time scale contrast of the laser pulse using a third order scanning autocorrelator.
A Thomson parabola spectrometer (TPS) was placed 140 mm behind the target.[24] The TPS was shifted $3^{\circ}$ from the target normal direction towards the laser axis to effectively accept high energy protons.[25,26] The acceptance angle was $1.6\times {10}^{-6}$ sr with an entrance pinhole of 200 μm diameter. The parabolic ion traces were recorded using a multi-channel plate (MCP) coupled with a phosphor screen,imaged to a 16-bit CCD to provide real-time analysis of the obtained ion energy spectra. A 30-μm-thick aluminum foil was attached onto the front surface of the MCP to block scatter-laser light and low energy heavy ions. Spatial distribution and energy spectra of the accelerated proton beams were also detected by stacks of radiochromic film (RCF) placed 40 mm behind the target. Since the pronounced energy deposition of ions is at the end of their stopping range, the color change of each RCF layer was mainly caused by the protons within a certain narrow band of energy. Therefore, the ion energy spectrum can be retrieved by the stack, using the Monte Carlo ion transport code, SRIM[27] and deconvolution method.[28] The stack consisted of one HD RCF and two MD RCFs to separately detect the low energy and high energy protons for their different sensitivities in dose. They were both calibrated at the 2$\times$6 MV tandem accelerator at Peking University.
cpl-35-9-092901-fig2.png
Fig. 2. (a) Schematic of the experimental setup. The main laser pulse was incident at 30$^{\circ}\!$ with respect to the target normal direction, and the accelerated proton beam was recorded by a TPS coupled with an MCP and RCF stack behind the target. (b) The inset is the focal spot of the laser pulse with an FWHM radius of 5 μm.
In the experiment, the aluminum foils with various thicknesses (0.65 μm–6 μm) were applied. Figure 3 shows the results of the accelerated proton beam using 2.5-μm-thick Al foils. Typical RCF stack images are displayed in Figs. 3(a)–3(c) with the corresponding energy marked on the top right corner of each layer. The calibrated proton charge from low energy to high energy is in turn 150 pC, 25 pC and 5 pC. The dark red dots are due to the damage of the RCF, and are excluded during analysis. In theory, protons originally from surface contamination layers of the Al target should be accelerated by the sheath field mainly along the target normal direction. However, in Figs. 3(a)–3(c), one can see that the proton beam clearly deviates from the target normal direction, which are presented by the dashed white lines. Both the divergence and the deviation angle versus different proton energies are analyzed and plotted in Fig. 3(e). The deflecting angle of proton beams increases from ${5.2}^{\circ}$ to ${8.5}^{\circ}$ with an increase of energies from 4.6 MeV to 7.9 MeV. The previous research shows that the deflection of proton beams could be induced by several factors, such as Coulomb potential change[26] and prepulse before the main laser pulse.[29-31] One dimension (1D) MULTI[32] hydrodynamic code shows that the scale length of pre-plasma is 0.45 μm and the rear surface of the target remains sharp. From Fig. 3(e) we can also see that the divergence of the accelerated proton beams decreases from $\pm {5.2}^{\circ}$ to $\pm {3.7}^{\circ}$ as the energy increases. This tendency is similar to Ref. [33], while with significantly reduced values. It is due to our better laser contrast, under which the electrons heating is dominated by the $J\times B$ mechanism, leading to a higher electron density at the rear surface, driving a rapid phase of proton acceleration before significant deformation of the target, and resulting in a more collimated proton beam.[33,34]
cpl-35-9-092901-fig3.png
Fig. 3. The results of the accelerated proton beam using 2.5-μm-thick Al foils. (a)–(c) The RCF stack trace is shown as a false color image. (b) The typically proton spectrum recorded with the magnetic spectrometer with a cut-off energy of 10.2 MeV in black line. The 2D simulation result using the EPOCH is displayed in the blue line. (e) The deflecting angle and divergence of different proton beams.
The exponential proton spectrum recorded by the TPS is shown by the black line in Fig. 3(d) with a maximum proton energy of 10.2 MeV. It is coinciding with the scale law reported in Ref. [33]. To interpret the acceleration process, a two-dimensional (2D) particle-in-cell simulation using the EPOCH code[35] is performed. The simulation box is $100\times 80$ μm$^{2}$ in size with a spatial grid of $10000\times 1600$, and open boundary condition for particles and field. Each cell is filled with 50 particles. The target is categorized into two parts based on 1D MULTI simulation: (1) the pre-plasma on the front surface with an exponential increase profile and a scale length of 0.45 μm, (2) the over dense plasma zone with a thickness of 2.9 μm and a uniform solid density of 400$n_{\rm c}$, where $n_{\rm c}$ is the critical plasma density. A thin proton layer of 50 nm is attached to the back of the solid plasma. Both the spatial and the temporal envelopes of the p-polarized laser pulse are set to be Gaussian, with a focal spot diameter of 5 μm (FWHM) and a pulse duration of 30 fs (FWHM). The dimensionless laser peak amplitude $a=\frac{eE_{\rm L}}{m_{\rm e}\omega_{\rm L}c}=6.5$ (corresponding to the laser intensity $I\sim 8.3\times {10}^{19}$ W/cm$^{2}$ at $\lambda=0.8$ μm), where $\omega_{\rm L}$ is the laser frequency, $\lambda$ is the laser wavelength, $c$ is the speed of light in a vacuum, and $m_{\rm e}$ and $e$ represent the rest electron mass and charge, respectively. The simulated proton spectrum is shown in Fig. 3(d) in the red line with a cut-off energy of 14.2 MeV. There are several reasons for the difference in the proton cutoff energy. As we know, a pre-plasma with proper density distribution can enhance the laser self-focusing and the hot electron generation, benefiting the proton acceleration. Three-dimensional thermal expansion may cause a smaller density gradient at the rear surface of the target,[36] which may weaken the charge separation field. Secondly, as mentioned above, the high energy protons were deflected from the target normal direction due to the effect of laser prepulse, which may exceed the acceptance angle of the TPS, thus the proton energy sensitively relies on the detecting direction in experiments. The real maximum proton energy has probably not been detected in our experiments.
cpl-35-9-092901-fig4.png
Fig. 4. (a) Cut-off energy for accelerated proton of 2.5 μm Al foils as a function of target position while keeping laser energy and pulse duration as a constant, and the dashed line shows the variation of $a_{0}$ in different defocusing distances. (b) Cut-off energy for accelerated proton 2.5 μm Al foils versus quadratic spectral phase while keeping fixed laser energy and target position. (c)–(g) The spot profile of laser beam in different defocusing distances, the marked number is the defocusing distance, the positive sign means that the target is positioned after the focusing position, and the negative one is located in the opposite direction.
In the experiment, the scan of the laser focus position (laser-target coupling position), as well as the laser duration (the relative distance between the compressor grating pairs) was employed to find the optimization matching of laser and target parameters. Figure 4(a) is the result of the laser focus scan with fixed laser energy and pulse duration. Position 0 is the focal position measured with low energy laser pulses and a high magnification imaging system. We can see that the optimum position for proton acceleration is actually $-$75 μm. It is mainly due to the change of focus position for low energy and high energy laser pulses, caused by their different wavefronts and divergence angles. A similar phenomenon has been reported in Refs. [37,38], explained as that the defocusing of the laser pulse could depress the effect of amplified spontaneous emission (ASE) on the target and increase the spot size to induce more hot electrons.[37,38] In Ref. [38], the contrast of the ASE pedestal is ${10}^{-6}$, while the laser contrast in this experiment is improved to ${10}^{-9}$, thus the above reasons become minor. However, Figs. 4(c)–4(g) are the images of laser focus spot at different positions taken by a 20X image system coupled with a 12-bit camera. The asymmetry in laser focusing and defocusing may be due to the phase distribution of our laser pulse, and need further investigation. The asymmetry of the laser spot also results in an asymmetry of proton cutoff energy duration and the focusing scale, as shown in Fig. 4(a). The corresponding $a_{0}$ is plotted as the dashed line. It is roughly consistent with the TNSA scaling law $E_{\max}\sim a_{0}$,[39] and the deviation is due to unconsidered experimental factors, such as preplasma evolution due to laser prepulse, and the spot size in various focal offsets. The laser pulse duration was varied by scanning the compressor grating separation. The influence of quadratic spectral phase on proton cut-off energy with fixed energy and target position is shown in Fig. 4(b). The 0 point means the optimum compressor position for laser pulse, as detected to be 30 fs. The increase of quadratic spectral phase of 1000 fs$^{2}$ results in the broadening of pulse duration down to 100 fs, and the maximum proton cut-off energy reduced by 28%. Therefore, the shortest pulse duration is the optimum.
cpl-35-9-092901-fig5.png
Fig. 5. Cut-off energy for accelerated proton versus target thickness under the best acceleration condition measured by TPS.
The variation of proton cut-off energy versus the target thickness is shown in Fig. 5. At each target thickness, more than 5 shots were taken. The symbols represent the averaged energy and the error rms bars show the deviation between these shots. The optimum target thickness is around 2.5 μm. It is consistent with the previous research that the superposition of recirculating hot electrons in a thin target can significantly increase proton energy.[40] However, in the existence of a pre-pulse, the target with thickness less than 2 μm could be seriously deformed or even evaporated, which can suppress the acceleration process;[21,22] therefore, the maximum proton energy is decreased down to 7 MeV with the decrease of the target thickness. In conclusion, a prototype of a laser driven proton accelerator has been built at Peking University. Protons exceeding 10 MeV have been accelerated from 2.5-μm-thick Al targets irradiated by a 1.8 J, 30 fs, and tightly focused laser pulse. The beam energy spectrum and charge distribution are measured by TPS and RCF stacks. The sensitivity of proton cut-off energy to focusing of the laser beams and pulse duration has also been investigated. It is shown that a slightly defocusing laser results in higher proton energy. The influence of pulse duration on proton acceleration is significant and the increasing pulse duration up to 100 fs results in a reduction of 28% of the maximum energy. The proton energy also depends sensitively on the target thicknesses. With the optimized parameters, very stable proton beams exceeding 10 MeV have been achieved in CLAPA. These experiments are critical for the generation of stable protons, which are the cornerstone for the wide potential applications of the laser accelerated beams in the future. References Ultralow Emittance, Multi-MeV Proton Beams from a Laser Virtual-Cathode Plasma AcceleratorReview of laser-driven ion sources and their applicationsProposal for the Study of Thermophysical Properties of High-Energy-Density Matter Using Current and Future Heavy-Ion Accelerator Facilities at GSI DarmstadtProton acceleration experiments and warm dense matter research using high power lasersControlled Transport and Focusing of Laser-Accelerated Protons with Miniature Magnetic DevicesFeasibility of using laser ion accelerators in proton therapyUsing Target Ablation for Ion Beam Quality ImprovementIon acceleration enhanced by target ablationPlasma Expansion into a VacuumEnergetic proton generation in ultra-intense laser–solid interactionsIntense electron and proton beams from PetaWatt laser–matter interactionsSelf-Organizing GeV, Nanocoulomb, Collimated Proton Beam from Laser Foil Interaction at 7 × 10 21     W / cm 2 Generating High-Current Monoenergetic Proton Beams by a CircularlyPolarized Laser Pulse in the Phase-StableAcceleration RegimeHighly Efficient Relativistic-Ion Generation in the Laser-Piston RegimeMono-Energetic Proton Beam Acceleration in Laser Foil-Plasma InteractionsMaximum Proton Energy above 85 MeV from the Relativistic Interaction of Laser Pulses with Micrometer Thick CH 2 TargetsRadiation pressure acceleration of protons to 93 MeV with circularly polarized petawatt laser pulsesBeam Line Design of Compact Laser Plasma AcceleratorRecent progress of laser driven particle acceleration at Peking UniversityEffects of laser prepulses on laser-induced proton generationInfluence of shock waves on laser-driven proton acceleration10^?10 temporal contrast for femtosecond ultraintense lasers by cross-polarized wave generationTunable High-Energy Ion Source via Oblique Laser Pulse Incident on a Double-Layer TargetAngular Distributions of Fast Electrons, Ions, and Bremsstrahlung x/ γ -Rays in Intense Laser Interaction with Solid TargetsOn the use of SRIM for computing radiation damage exposureRadiochromic film imaging spectroscopy of laser-accelerated proton beamsInfluence of the Laser Prepulse on Proton Acceleration in Thin-Foil ExperimentsEffect of Plasma Scale Length on Multi-MeV Proton Production by Intense Laser PulsesDivergence of laser-driven relativistic electron beamsMULTI — A computer code for one-dimensional multigroup radiation hydrodynamicsThe scaling of proton energies in ultrashort pulse laser plasma accelerationEnhanced proton beam collimation in the ultra-intense short pulse regimeContemporary particle-in-cell approach to laser-plasma modellingEffects of nanosecond-scale prepulse on generation of high-energy protons in target normal sheath accelerationInfluence of laser irradiated spot size on energetic electron injection and proton acceleration in foil targetsEnhancement of ion generation in femtosecond ultraintense laser-foil interactions by defocusingLaser-driven proton scaling laws and new paths towards energy increaseEnhancement of Proton Acceleration by Hot-Electron Recirculation in Thin Foils Irradiated by Ultraintense Laser Pulses
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