Laser-accelerated Proton Research Articles

Experimental demonstration of a laser proton accelerator with accurate beam control through image-relaying transport

A compact laser plasma accelerator (CLAPA) that can stably produce and transport proton ions with different energies less than 10 MeV, $l1%$ energy spread, several to tens of pC charge, is demonstrated. The high current proton beam with continuous energy spectrum and a large divergence angle is generated by using a high contrast laser and micron thickness targets, which later is collected, analyzed and refocused by an image-relaying beam line using a combination of quadrupole and bending electromagnets. It eliminates the inherent defects of the laser-driven beams, realizes precise manipulation of the proton beams with reliability, availability, maintainability and inspectability (RAMI), and takes the first step towards applications of this new generation of accelerator. With the development of high-rep rate Petawatt (PW) laser technology, we can now envision a new generation of accelerator for many applications in the near future soon.

Advances in Spectral Distribution Assessment of Laser Accelerated Protons using Multilayer CR-39 Detectors

We show that a spectral distribution of laser-accelerated protons can be extracted by analyzing the proton track diameters observed on the front side of a second CR-39 detector arranged in a stack. The correspondence between the proton track diameter and the incident energy on the second detector is established by knowing that protons with energies only higher than 10.5 MeV can fully deposit their energy in the second CR-39 detector. The correlation between the laser-accelerated proton track diameters observed on the front side of the second CR-39 detector and the proton incident energy on the detector stack is also presented. By calculating the proton number stopped in the CR-39 stack, we find out that its dependence on the proton energy in the 1–15 MeV range presents some discontinuities at energies higher than 9 MeV. Thus, we build a calibration curve of the track diameter as a function of the proton incident energy within the 1–9 MeV range, and we infer the associated analytical function as the calculations performed indicate best results for proton spectra within the 1–9 MeV range. The calibration curve is used as a tool to ascertain the pits identified on the surfaces of both CR-39 detectors to proton tracks. The proton tracks spatial distribution analyzed by optical and atomic force microscopy is correlated with the peculiarity of the used targets.

Laser-PIXE using laser-accelerated proton beams

Laser-driven proton acceleration is a field of growing interest, in particular for its numerous applications, including in the field of materials science. A benefit of these laser-based particle sources is their potential for a relative compactness in addition to some characteristics at the source that differ from those of conventional, radio-frequency based proton sources. These features include, e.g., a higher brilliance, a shorter duration, and a larger energy spread. Recently, the use of laser-accelerated protons has been proposed in the field of Cultural Heritage, as alternative source for the Particle Induced X-ray Emission diagnostic (“laser-PIXE”), a particular ion beam analysis (IBA) technique that allows to precisely analyse the chemical composition of the material bulk. In this paper we study the feasibility of the laser-PIXE using laser-accelerated proton beams. We focus on materials specifically of interest for the Cultural Heritage domain. Using Geant4 simulations, we show that the laser-PIXE allows analysing a larger volume than conventional PIXE, profiting from the large energy spread of laser-accelerated protons. Furthermore, for specific materials, the large energy spread allows investigating multilayer materials, providing an advantage compared to conventional PIXE technologies.

Absolute calibration of GafChromic film for very high flux laser driven ion beams.

We report on the calibration of GafChromic HD-v2 radiochromic film in the extremely high dose regime up to 100 kGy together with very high dose rates up to 7 × 1011 Gy/s. The absolute calibration was done with nanosecond ion bunches at the Neutralized Drift Compression Experiment II particle accelerator at Lawrence Berkeley National Laboratory (LBNL) and covers a broad dose dynamic range over three orders of magnitude. We then applied the resulting calibration curve to calibrate a laser driven ion experiment performed on the BELLA petawatt laser facility at LBNL. Here, we reconstructed the spatial and energy resolved distributions of the laser-accelerated proton beams. The resulting proton distribution is in fair agreement with the spectrum that was measured with a Thomson spectrometer in combination with a microchannel plate detector.

Application of nuclear emulsions for the identification of multi-MeV protons in laser ion acceleration experiments

Nuclear emulsions are applied to identify multi-MeV protons accelerated by an intense PW-class laser. Although nuclear emulsions, which record the three-dimensional trajectories of charged particles, feature high sensitivity and a sub-micron spatial resolution, they have never been used for the detection of laser-accelerated ions. This is because intense laser-matter interactions generate not only ions but also copious electrons, including secondary electrons induced by X-rays, which are recorded as background noise in a nuclear emulsion. Therefore, it is not a trivial task to identify ion tracks in a nuclear emulsion exposed to such extreme mixed radiation fields. Here, for the first time, we have succeeded in identifying the signals from laser-accelerated protons in nuclear emulsions by suppressing the background noise signals through the use of desensitized development conditions, i.e., a developer solution with a pH of 7.3 and a development time of 25 min. The energy spectrum obtained from a stack of nuclear emulsion films shows good agreement with that obtained from a stack of films of the conventional solid-state nuclear track detector material CR-39, which is sensitive only to ions. An additional benchmark experiment with 100-MeV protons from a conventional accelerator apparatus confirms that nuclear emulsions have the potential to measure 100-MeV-class laser-accelerated protons.

First demonstration of ARC-accelerated proton beams at the National Ignition Facility

New short-pulse kilojoule, Petawatt-class lasers, which have recently come online and are coupled to large-scale, many-beam long-pulse facilities, undoubtedly serve as very exciting tools to capture transformational science opportunities in high energy density physics. These short-pulse lasers also happen to reside in a unique laser regime: very high-energy (kilojoule), relatively long (multi-picosecond) pulse-lengths, and large (10s of micron) focal spots, where their use in driving energetic particle beams is largely unexplored. Proton acceleration via Target Normal Sheath Acceleration (TNSA) using the Advanced Radiographic Capability (ARC) short-pulse laser at the National Ignition Facility in the Lawrence Livermore National Laboratory is demonstrated for the first time, and protons of up to 18 MeV are measured using laser irradiation of >1 ps pulse-lengths and quasi-relativistic (∼1018 W/cm2) intensities. This is indicative of a super-ponderomotive electron acceleration mechanism that sustains acceleration over long (multi-picosecond) time-scales and allows for proton energies to be achieved far beyond what the well-established scalings of proton acceleration via TNSA would predict at these modest intensities. Furthermore, the characteristics of the ARC laser (large ∼100 μm diameter focal spot, flat spatial profile, multi-picosecond, relatively low prepulse) provide acceleration conditions that allow for the investigation of 1D-like particle acceleration. A high flux ∼ 50 J of laser-accelerated protons is experimentally demonstrated. A new capability in multi-picosecond particle-in-cell simulation is applied to model the data, corroborating the high proton energies and elucidating the physics of multi-picosecond particle acceleration.

DNA DSB Repair Dynamics following Irradiation with Laser-Driven Protons at Ultra-High Dose Rates

Protontherapy has emerged as more effective in the treatment of certain tumors than photon based therapies. However, significant capital and operational costs make protontherapy less accessible. This has stimulated interest in alternative proton delivery approaches, and in this context the use of laser-based technologies for the generation of ultra-high dose rate ion beams has been proposed as a prospective route. A better understanding of the radiobiological effects at ultra-high dose-rates is important for any future clinical adoption of this technology. In this study, we irradiated human skin fibroblasts-AG01522B cells with laser-accelerated protons at a dose rate of 109 Gy/s, generated using the Gemini laser system at the Rutherford Appleton Laboratory, UK. We studied DNA double strand break (DSB) repair kinetics using the p53 binding protein-1(53BP1) foci formation assay and observed a close similarity in the 53BP1 foci repair kinetics in the cells irradiated with 225 kVp X-rays and ultra- high dose rate protons for the initial time points. At the microdosimetric scale, foci per cell per track values showed a good correlation between the laser and cyclotron-accelerated protons indicating similarity in the DNA DSB induction and repair, independent of the time duration over which the dose was delivered.

Automated positioning of transparent targets using defocusing method in a laser proton accelerator

We report an automated target positioning method with an accuracy of 2 μm for a compact laser proton accelerator. This method is based on micrometer-scale dots on the images of transparent targets. In order to interpret the variation of these dots quantitatively, we developed a defocusing method making transparent targets visible as phase objects and applied it in target positioning. Automated positioning of transparent targets can be realized by means of minimizing the standard deviations σ of the pixel values of the images. Fast positioning based on the relationship between the radius of spherical aberration ring and defocusing distance is also discussed. Laser proton acceleration experiments have been carried out to demonstrate the performance of this positioning method.

Calibration of proton dispersion for the NIF electron positron proton spectrometer (NEPPS) for short-pulse laser experiments on the NIF ARC.

Experiments using the Advanced Radiographic Capability (ARC) laser at the National Ignition Facility (NIF) aim to characterize short-pulse-driven proton beams for use as both probes and drivers for high-energy-density physics experiments. Measurements of ARC-driven proton beam characteristics, such as energy spectrum and conversion efficiency, rely on the NIF Electron Positron Proton Spectrometer (NEPPS). The NEPPS diagnostic is a version of an existing particle spectrometer which is used for detecting MeV electron and positron spectra via permanent magnetic field dispersion. These spectrometers have not yet been calibrated for protons and instead use an analytical calculation to estimate the dispersion. Small variations in the field uniformity can affect the proton dispersion due to the relatively small resolving power (E/dE) for this diagnostic. A broadband energy, laser-accelerated proton source was produced at the Titan laser to experimentally calibrate the proton dispersion. These experimental data were used to test the theoretical dispersion. Numerical simulations using measurements of the magnetic field variation within the diagnostic were used to obtain a realistic proton dispersion curve for the new NEPPS units. This procedure for obtaining each independent dispersion is applicable to all EPPS and NEPPS diagnostics, given the axial magnetic field profile.

Effects of radiation reaction on laser proton acceleration in the bubble regime

We investigated laser proton acceleration in the bubble regime when radiation reaction (RR) effects become non-negligible. By using particle-in-cell simulations, it is shown that in addition to the bubble field, the RR trapped electrons form another charge-separation field, leading to fast acceleration of the background protons at the early stage. However, for insufficient laser intensities, the reduced bubble field and acceleration length originating from the laser depletion by gamma-photon emission do not allow for trapping and further acceleration of the protons. The final proton energy is significantly smaller than the one when RR is not considered. This effect can be mitigated by tuning the peak laser intensity (hence the pulse duration) for certain laser energy and plasma density. By increasing the laser intensity (shortening the pulse duration), we found that protons pre-accelerated by the RR-induced charge separation field gain enough momenta such that they are picked-up by the bubble field and continuously accelerated. In this case, the bubble field is also strong enough to trap protons located in the bubble front. Eventually, two groups of protons are effectively accelerated, resulting in a bump in the proton spectrum. These results can be used to design the future proton acceleration experiments in upcoming 100 PW laser facilities.

Transversal dose distribution optimization for laser-accelerated proton beam medical applications by means of Geant4

The main purpose of this paper is to quantitatively study the possibility of delivering dose distributions of clinical relevance with laser-driven proton beams. A Monte Carlo application has been developed with the Geant4 toolkit, simulating the ELIMED (MEDical and multidisciplinary application at ELI-Beamlines) transport and dosimetry beam line which is being currently installed at the ELI-Beamlines in Prague (CZ). The beam line will be used to perform irradiations for multidisciplinary studies, with the purpose of demonstrating the possible use of optically accelerated ion beams for therapeutic purposes. The ELIMED Geant4-based application, already validated against reference transport codes, accurately simulates each single element of the beam line, necessary to collect the accelerated beams and to select them in energy. Transversal dose distributions at the irradiation point have been studied and optimized to try to quantitatively answer the question if such kind of beam lines, and specifically the systems developed for ELIMED in Prague, will be actually able to transport ion beams not only for multidisciplinary applications, such as pitcher-catcher nuclear reactions (e.g. neutrons), PIXE analysis for cultural heritage and space radiation, but also for delivering dose patterns of clinical relevance in a future perspective of possible medical applications.

Graphitization of diamond by laser-accelerated proton beams

In this work, we investigate the interaction between fast energetic proton beams generated by laser-plasma based accelerators with carbon lattices of graphite and diamond. Theoretical and experimental results indicate that these proton beams cause both, a low sputtering of carbon atoms from lattices (estimated to be in the order of 3 × 10−8 displacements per atom for each single shot), and a heating (inside the target and within a depth of about 10 μm below the surface) at a temperature close to the sp3/sp2 transition. The defects generated inside the lattices by the atom displacements cause the random formation of amorphous carbon islands, while the higher temperature causes the start of diamond partial graphitization. All the results suggest the impossibility to use diamond as detector in laser-driven accelerators when placed close to the source, while they confirm that proton beams can be used as alternative to classical laser or ion beam methods to graphitize diamond in specific optoelectronic applications.

Enhanced laser proton acceleration by target ablation on a femtosecond laser system

Proton acceleration during the interaction of an ultraintense (6 × 1019 W/cm2) femtosecond (fs) laser pulse with a thin (2.5 μm) foil target pre-ablated by a picosecond (ps) pulse is experimentally and numerically investigated. Enhancements in both proton cut-off energy and charge are observed with the target ablation due to a large number of energetic electrons generated from the preformed preplasma in front of the target. The enhanced proton beams are successfully collected at 4–9 MeV with ±4% energy spread and then transported to the irradiating platform. The results show that for the interaction between fs laser pulse and μm-thickness target, proton energy and charge can be enhanced by target ablation using a ps laser pulse, which is valuable for application like cancer radiotherapy.

Optical probing of high intensity laser interaction with micron-sized cryogenic hydrogen jets

Probing the rapid dynamics of plasma evolution in laser-driven plasma interactions provides deeper understanding of experiments in the context of laser-driven ion acceleration and facilitates the interplay with complementing numerical investigations. Besides the microscopic scales involved, strong plasma (self-)emission, predominantly around the harmonics of the driver laser, often complicates the data analysis. We present the concept and the implementation of a stand-alone probe laser system that is temporally synchronized to the driver laser, providing probing wavelengths beyond the harmonics of the driver laser. The capability of this system is shown during a full-scale laser proton acceleration experiment using renewable cryogenic hydrogen jet targets. For further improvements, we studied the influence of probe color, observation angle of the probe and temporal contrast of the driver laser on the probe image quality.

Design and optimization of a compact laser-driven proton beamline

Laser-accelerated protons, generated by irradiating a solid target with a short, energetic laser pulse at high intensity (I > 1018 W·cm−2), represent a complementary if not outperforming source compared to conventional accelerators, due to their intrinsic features, such as high beam charge and short bunch duration. However, the broadband energy spectrum of these proton sources is a bottleneck that precludes their use in applications requiring a more reduced energy spread. Consequently, in recent times strong effort has been put to overcome these limits and to develop laser-driven proton beamlines with low energy spread. In this paper, we report on beam dynamics simulations aiming at optimizing a laser-driven beamline - i.e. a laser-based proton source coupled to conventional magnetic beam manipulation devices - producing protons with a reduced energy spread, usable for applications. The energy range of investigation goes from 2 to 20 MeV, i.e. the typical proton energies that can be routinely obtained using commercial TW-power class laser systems. Our beamline design is capable of reducing the energy spread below 20%, still keeping the overall transmission efficiency around 1% and producing a proton spot-size in the range of 10 mm2. We briefly discuss the results in the context of applications in the domain of Cultural Heritage.

Efficient proton acceleration from a 3 TW table-top laser interacting with submicrometric mass-produced solid targets

Thin layer membranes with controllable features and material arrangements are often used as target materials for laser driven particle accelerators. Reduced cost, large scale fabrication of such membranes with high reproducibility, and good stability are central for the efficient production of proton beams. These characteristics are of growing importance in the context of advanced laser light sources where increased repetition rates boost the need for consumable targets with design and properties adjusted to study the different phenomena arising in ultra-intense laser-plasma interaction. We present the fabrication of sub-micrometric thin-layer gold or aluminum membranes in a silicon wafer frame by using nano/micro-electro-mechanical-system (N/MEMS) processing which are suitable for rapid patterning and machining of many samples at the same time and allowing for high-throughput production of targets for laser-driven acceleration. Obtained targets were tested for laser-proton acceleration through the Target Normal Sheath Acceleration mechanism (TNSA) in a series of experiments carried out on a purpose-made table-top Ti:Sa running at 3 TW peak power and 10 Hz diode pump rate with a contrast over ASE of 108.

On-shot characterization of single plasma mirror temporal contrast improvement

We report on the setup and commissioning of a compact recollimating single plasma mirror (PM) for temporal contrast enhancement at the Draco 150 TW laser during laser-proton acceleration experiments. The temporal contrast with and without PM is characterized single-shot by means of self-referenced spectral interferometry with extended time excursion at unprecedented dynamic and temporal range. This allows for the first single-shot measurement of the PM trigger point, which is interesting for the quantitative investigation of the complex pre-plasma formation process at the surface of the target used for proton acceleration. As a demonstration of high contrast laser plasma interaction we present proton acceleration results with ultra-thin liquid crystal targets of ∼ 1 μm down to 10 nm thickness. Focus scans of different target thicknesses show that highest proton energies are reached for the thinnest targets at best focus. This indicates that the contrast enhancement is effective such that the acceleration process is not limited by target pre-expansion induced by laser light preceding the main laser pulse.

Efficient monoenergetic proton beam from ultra-fast laser interaction with nanostructured targets

The broad energy spectrum of laser-accelerated proton beams is the most important difficulty associated with such particle sources on the way to future applications such as medical therapy, proton imaging, inertial fusion, and high-energy physics. The generation of proton beams with enhanced monoenergetic features through an ultra-intense laser interaction with optimized nanostructured targets is reported. Targets were irradiated by 40 fs laser pulses of intensity 5.5×1020 W cm−2 and wavelength 1 μm. The results of multi-parametric Particle-in-Cell calculations showed that proton beams with considerably reduced energy spread can be obtained by using the proposed nanostructured target. At optimized target dimensions, the proton spectrum was found to exhibit a narrow peak at about 63 MeV with a relative energy spread of ΔE/Epeak∼ 5% which is efficiently lower than what is expected for unstructured double layer targets (∼70%).

Spectral characterization of laser-accelerated protons with CR-39 nuclear track detector.

CR-39 nuclear track material is frequently used for the detection of protons accelerated in laser-plasma interactions. The measurement of track densities allows for determination of particle angular distributions, and information on the kinetic energy can be obtained by the use of passive absorbers. We present a precise method of measuring spectral distributions of laser-accelerated protons in a single etching and analysis process. We make use of a one-to-one relation between proton energy and track size and present a precise calibration based on monoenergetic particle beams. While this relation is limited to proton energies below 1 MeV, we show that the range of spectral measurements can be significantly extended by simultaneous use of absorbers of suitable thicknesses. Examples from laser-plasma interactions are presented, and quantitative results on proton energies and particle numbers are compared to those obtained from a time-of-flight detector. The spectrum end points of continuous energy distributions have been determined with both detector types and coincide within 50-100 keV.

Transverse expansion of the electron sheath during laser acceleration of protons

The transverse expansion of the electrostatic sheath during target normal sheath acceleration of protons is investigated experimentally using a setup with two synchronized laser pulses. With the pulses spatially separated by less than three laser spot diameters, the resulting proton beam profiles become elliptical. By introducing a small intensity difference between the two pulses, the ellipses are rotated by a certain angle, except if the spatial separation of the two laser pulses is in the plane of incidence. The rotation angle is shown to depend on the relative intensity of the two pulses. The observed effects are found to require high temporal contrasts of the laser pulses. A simple model describing how the transverse shape of the electron sheath on the rear of the target depends on the relative intensity between the foci is presented. The model assumptions are verified, and the unknown dependence of the transverse extents of the sheaths are estimated self-consistently through a series of high resolution, two-dimensional particle-in-cell simulations. The results predicted by the model are also shown to be consistent with those obtained from the experiment.