3D track extraction from a fluorescent nuclear track detector via machine learning and an application to diagnostics of laser-driven ions.

We have developed an ion diagnostic method for laser-driven ion acceleration experiments that uses fluorescent nuclear track detectors (FNTDs). An FNTD records the particle tracks as color centers and does not require chemical etching, unlike CR-39 track detectors. The color centers are observed using a confocal laser microscope, and 3D particle tracks can be obtained by changing its focal position. The intensity of the color centers corresponds to the energy deposited by the ions. The nuclides of the ions can be determined from the intensity distribution of the color centers as a function of depth and the distance between the stopping point and the surface of the detector. To extract the intensity distribution, we must track the same ion tracks in the depth-layered microscopic images from the surface to the stopping point, even if they overlap with those of other ions. In addition, since an FNTD is sensitive not only to ions but also to electrons and photons, we must identify ion tracks among those from the latter particles. To analyze a statistical number of ion tracks, it is necessary to automate these processes. We have thus developed a method for automated ion detection and 3D tracking that relies on a support vector classifier and a kernelized correlation filter. This method was tested on a laser ion acceleration experiment performed using the J-KAREN-P laser. The method automatically detects ion tracks on FNTDs and tracks them in the depth direction. The training data are sampled from the Heavy-Ion Medical Accelerator in Chiba.

Automation of etch pit analyses on solid-state nuclear track detectors with machine learning for laser-driven ion acceleration.

Solid-state nuclear track detectors (SSNTDs) are often used as ion detectors in laser-driven ion acceleration experiments and are considered to be the most reliable ion diagnostics since they are sensitive only to ions and measure ions one by one. However, ion pit analyses require tremendous time and effort in chemical etching, microscope scanning, and ion pit identification by eyes. From a laser-driven ion acceleration experiment, there are typically millions of microscopic images, and it is practically impossible to analyze all of them by hand. This research aims to improve the efficiency and automation of SSNTD analyses for laser-driven ion acceleration. We use two sets of data obtained from calibration experiments with a conventional accelerator where ions with known nuclides and energies are generated and from actual laser experiments using SSNTDs. After chemical etching and scanning the SSNTDs with an optical microscope, we use machine learning to distinguish the ion etch pits from noises. From the results of the calibration experiment, we confirm highly accurate etch-pit detection with machine learning. We are also able to detect etch pits with machine learning from the laser-driven ion acceleration experiment, which is much noisier than calibration experiments. By using machine learning, we successfully identify ion etch pits ∼105 from more than 10 000 microscopic images with a precision of ≳95%. A million microscopic images can be examined with a recent entry-level computer within a day with high precision. Machine learning tremendously reduces the time consumption on ion etch pit analyses detected on SSNTDs.

Characterization of a retroreflector array for 320-GHz interferometer system in Heliotron J.

A retroreflector array, composed of a cluster of small retroreflectors, is experimentally studied for application to a Michelson-type interferometer system in the fusion plasma experiment. Such a new-type reflector has the potential to be a vital and effective tool at a spatially limited location, such as on the vacuum chamber wall of plasma experimental devices. To investigate the effect of retroreflector array on the reflected beam properties, a tabletop experiment is performed with the retroreflector array composed of 4 mm corner-cube retroreflectors and with a 320-GHz (λ ∼ 0.937 mm) submillimeter wave source. An imaging camera is utilized to measure the submillimeter wave beam profile and is scanned perpendicularly to the beam propagation direction if necessary. The experimental result exhibits a diffraction effect on the reflected beam, resulting in the emergence of discrete peaks on the reflected beam profile, as predicted in the past numerical study; however, the most reflected beam power converges on the one reflected into the incident direction, resulting from a property as a retroreflector. Furthermore, the dependence of the reflected beam on the incident beam angle is characterized while fixing the detector position, and the retroreflection beam intensity is found to vary due to the diffraction effect. Such an undesired variation of beam intensity induced by the diffraction can be suppressed with a focusing lens placed in front of the detector in the practical application to an interferometer.

Mass-resolved ion measurement by particle counting analysis for characterizing relativistic ion beams driven by lasers.

Particle counting analysis is a possible way to characterize GeV-scale, multi-species ions produced in laser-driven experiments. We present a multi-layered scintillation detector to differentiate multi-species ions of different masses and energies. The proposed detector concept offers potential advantages over conventional diagnostics in terms of (1) high sensitivity to GeV ions, (2) realtime analysis, and (3) the ability to differentiate ions with the same charge-to-mass ratio. A novel choice of multiple scintillators with different ion stopping powers results in a significant difference in energy deposition between the scintillators, allowing accurate particle identification in the GeV range. Here, we report a successful demonstration of particle identification for heavy ions, performed at the Heavy Ion Medical Accelerator in Chiba. In the experiment, the proposed detector setup showed the ability to differentiate particles with similar atomic numbers, such as C6+ and O8+ ions, and provided an excellent energy resolution of 0.41%-1.2% (including relativistic effect, 0.51%--1.6%).

Development and initial results of 320 GHz interferometer system in Heliotron J.

A new 320 GHz solid-state source interferometer is installed in the Heliotron J helical device to explore the physics of high-density plasmas (ne > 2-3 × 1019 m-3, typically) realized with advanced fueling techniques. This interferometry system is of the Michelson type and is based on the heterodyne principle, with two independent solid-state sources that can deliver an output power of up to 50 mW. A high time resolution measurement of <1 µs can be derived by tuning the frequency of one source in the frequency range of 312-324 GHz on the new system, which can realize the fluctuation measurement. We successfully measured the line-averaged electron density in high-density plasma experiments. The measured density agreed well with a microwave interferometer measurement using a different viewing chord, demonstrating that the new system can be used for routine diagnostics of electron density in Heliotron J.

Measurement of Paα line from pellet ablation cloud in Heliotron J.

The Paα line (1875.13 nm) in the near-infrared (NIR) region was evaluated to apply Stark broadening of the line spectrum to the electron density measurement of the small-pellet ablation cloud in Heliotron J, a medium-sized helical-axis heliotron device. Paα is three-to-four times broader than the visible Hß line (486.13 nm) for the same electron density. Using a portable NIR spectrometer, preliminary proof-of-concept experiments determined the marginal density, below which the broadening was undetectable. The lower detection density limit can be decreased using a narrower entrance slit or a denser grating.

Predictive capability of material screening by fast neutron activation analysis using laser-driven neutron sources.

Bright, short-pulsed neutron beams from laser-driven neutron sources (LANSs) provide a new perspective on material screening via fast neutron activation analysis (FNAA). FNAA is a nondestructive technique for determining material elemental composition based on nuclear excitation by fast neutron bombardment and subsequent spectral analysis of prompt γ-rays emitted by the active nuclei. Our recent experiments and simulations have shown that activation analysis can be used in practice with modest neutron fluences on the order of 105 n/cm2, which is available with current laser technology. In addition, time-resolved γ-ray measurements combined with picosecond neutron probes from LANSs are effective in mitigating the issue of spectral interference between elements, enabling highly accurate screening of complex samples containing many elements. This paper describes the predictive capability of LANS-based activation analysis based on experimental demonstrations and spectral calculations with Monte Carlo simulations.

High-power laser experiment on developing supercritical shock propagating in homogeneously magnetized plasma of ambient gas origin.

A developing supercritical collisionless shock propagating in a homogeneously magnetized plasma of ambient gas origin having higher uniformity than the previous experiments is formed by using high-power laser experiment. The ambient plasma is not contaminated by the plasma produced in the early time after the laser shot. While the observed developing shock does not have stationary downstream structure, it possesses some characteristics of a magnetized supercritical shock, which are supported by a one-dimensional full particle-in-cell simulation taking the effect of finite time of laser-target interaction into account.

Three-dimensional dynamics of fluctuations appearing during pellet ablation process around a pellet in a fusion plasma experiment.

Understanding pellet ablation physics is crucial to realizing efficient fueling into a high temperature plasma for the steady state operation of ITER and future fusion reactors. Here we report the first observation of the formation of fluctuation structures in the pellet plasmoid during the pellet ablation process by a fast camera in a medium-sized fusion device, Heliotron J. The fluctuation has a normalized fluctuation level of ~ 15% and propagates around the moving pellet across the magnetic field. By comparing the fluctuation structures with the shape of magnetic field lines calculated with the field line tracing code, we successfully reconstruct the spatio-temporal structure of the fluctuations during the pellet ablation process. The fluctuations are located at the locations displaced toroidally from the pellet and propagate in the cross-field direction around the pellet axis along the field line, indicating a three-dimensional behavior and structure of fluctuations. The fluctuation would be driven by a strong inhomogeneity formed around the pellet and invoke the relaxation of the gradient through a cross-field transport induced by the fluctuations, which could affect the pellet ablation and pellet fueling processes. Such fluctuations can be ubiquitously present at the inhomogeneity formed around a pellet in the pellet ablation process in fusion devices.

A multi-stage scintillation counter for GeV-scale multi-species ion spectroscopy in laser-driven particle acceleration experiments.

Particle counting analysis (PCA) with a multi-stage scintillation detector shows a new perspective on angularly resolved spectral characterization of GeV-scale, multi-species ion beams produced by high-power lasers. The diagnosis provides a mass-dependent ion energy spectrum based on time-of-flight and pulse-height analysis of single particle events detected through repetitive experiments. With a novel arrangement of multiple scintillators with different ions stopping powers, PCA offers potential advantages over commonly used diagnostic instruments (CR-39, radiochromic films, Thomson parabola, etc.) in terms of coverage solid angle, detection efficiency for GeV-ions, and real-time analysis during the experiment. The basic detector unit was tested using 230-MeV proton beam from a synchrotron facility, where we demonstrated its potential ability to discriminate major ion species accelerated in laser-plasma experiments (i.e., protons, deuterons, carbon, and oxygen ions) with excellent energy and mass resolution. The proposed diagnostic concept would be essential for a better understanding of laser-driven particle acceleration, which paves the way toward all-optical compact accelerators for a range of applications.

Direct observations of pure electron outflow in magnetic reconnection.

Magnetic reconnection is a universal process in space, astrophysical, and laboratory plasmas. It alters magnetic field topology and results in energy release to the plasma. Here we report the experimental results of a pure electron outflow in magnetic reconnection, which is not accompanied with ion flows. By controlling an applied magnetic field in a laser produced plasma, we have constructed an experiment that magnetizes the electrons but not the ions. This allows us to isolate the electron dynamics from the ions. Collective Thomson scattering measurements reveal the electron Alfvénic outflow without ion outflow. The resultant plasmoid and whistler waves are observed with the magnetic induction probe measurements. We observe the unique features of electron-scale magnetic reconnection simultaneously in laser produced plasmas, including global structures, local plasma parameters, magnetic field, and waves.

High-power laser experiment forming a supercritical collisionless shock in a magnetized uniform plasma at rest.

We present an experimental method to generate quasiperpendicular supercritical magnetized collisionless shocks. In our experiment, ambient nitrogen (N) plasma is at rest and well magnetized, and it has uniform mass density. The plasma is pushed by laser-driven ablation aluminum (Al) plasma. Streaked optical pyrometry and spatially resolved laser collective Thomson scattering clarify structures of plasma density and temperatures, which are compared with one-dimensional particle-in-cell simulations. It is indicated that just after the laser irradiation, the Al plasma is magnetized by a self-generated Biermann battery field, and the plasma slaps the incident N plasma. The compressed external field in the N plasma reflects N ions, leading to counterstreaming magnetized N flows. Namely, we identify the edge of the reflected N ions. Such interacting plasmas form a magnetized collisionless shock.

Robustness of large-area suspended graphene under interaction with intense laser.

Graphene is known as an atomically thin, transparent, highly electrically and thermally conductive, light-weight, and the strongest 2D material. We investigate disruptive application of graphene as a target of laser-driven ion acceleration. We develop large-area suspended graphene (LSG) and by transferring graphene layer by layer we control the thickness with precision down to a single atomic layer. Direct irradiations of the LSG targets generate MeV protons and carbons from sub-relativistic to relativistic laser intensities from low contrast to high contrast conditions without plasma mirror, evidently showing the durability of graphene.

Development of a multi-channel 320 GHz interferometer for high density plasma measurement in Heliotron J.

We report the development of a new interferometer with two stable, high-power, 320 GHz solid-state sources in Heliotron J. A heterodyne Michelson interferometer optical scheme is employed. Two solid-state oscillators are utilized as sources with a fixed frequency at 320 GHz and frequency tunable of 312-324 GHz. Quasi-optical techniques are used for beam transmission. The beam is elongated in the vertical direction with two off-axis parabolic mirrors and injected into the plasma as a sheet beam for the multi-channel measurement (>5 ch.). Passing through the plasma, the beam is reflected at a retroreflector-array installed at the vacuum chamber wall. The retroreflector-array is a bunch of retroreflector structures, which can suppress the beam refraction caused by plasma without much space inside a vacuum chamber unlike a single retroreflector and can facilitate the system design. The source, detectors, and the retroreflector-array are tested to evaluate their basic performance on a tabletop experiment.

Improvement in multipass Thomson scattering system comprising laser amplification system developed in GAMMA 10/PDX.

The multipass Thomson scattering (MPTS) technique is one of the most useful methods for measuring low-electron-density plasmas. The MPTS system increases Thomson scattering (TS) signal intensities by integrating all multipass (MP) signals and improving the TS time resolution by analyzing each pass signal. The fully coaxial MPTS system developed in GAMMA 10/potential-control and diverter-simulator experiments has a polarization-based configuration with image-relaying optics. The MPTS system can enhance Thomson scattered signals for improving the measurement accuracy and megahertz-order time resolution. In this study, we develop a new MPTS system comprising a laser amplification system to obtain continuous MP signals. The laser amplification system can improve degraded laser power and return an amplified laser to the MP system. We obtain continuous MP signals from the laser amplification system by improving the laser beam profile adjuster in gas scattering experiments. Moreover, we demonstrate that more MP signals and stronger amplified MP signals can be achieved via multiple laser injections to the laser amplification system in the developed MP system comprising a laser amplification system.

Reformation of the Electron Internal Transport Barrier with the Appearance of a Magnetic Island.

When realising future fusion reactors, their stationary burning must be maintained and the heat flux to the divertor must be reduced. This essentially requires a stationary internal transport barrier (ITB) plasma with a fast control system. However, the time scale for determining the position of the foot point of an ITB is not clearly understood even though its understanding is indispensable for fast profile control. In this study, the foot point of the electron ITB (eITB) was observed to be reformed at the vicinity of a magnetic island when the island started to form. In addition, the enhanced confinement region was observed to expand during the eITB formation according to the radial movement of the magnetic island toward the outer region. Compared to the time scales of the local heat transport, the faster time scales of the movement of the eITB foot point immediately after island formation (~0.5 ms) suggest the importance of the magnetic island for plasma profile control to maintain stationary burning.

Direct observation of imploded core heating via fast electrons with super-penetration scheme.

Fast ignition (FI) is a promising approach for high-energy-gain inertial confinement fusion in the laboratory. To achieve ignition, the energy of a short-pulse laser is required to be delivered efficiently to the pre-compressed fuel core via a high-energy electron beam. Therefore, understanding the transport and energy deposition of this electron beam inside the pre-compressed core is the key for FI. Here we report on the direct observation of the electron beam transport and deposition in a compressed core through the stimulated Cu Kα emission in the super-penetration scheme. Simulations reproducing the experimental measurements indicate that, at the time of peak compression, about 1% of the short-pulse energy is coupled to a relatively low-density core with a radius of 70 µm. Analysis with the support of 2D particle-in-cell simulations uncovers the key factors improving this coupling efficiency. Our findings are of critical importance for optimizing FI experiments in a super-penetration scheme.

Density profile measurement with a heavy ion beam probe in a toroidal plasma of the compact helical system.

A possibility of electron density measurements with heavy ion beam probes (HIBPs) has been demonstrated, along with their capability to measure the potential and magnetic field. A method has been proposed to reconstruct the electron density profile [A. Fujisawa et al., Rev. Sci. Instrum. 74, 3335 (2003)]. In the method, the profile of secondary beam currents is converted into a local density profile by taking into account local brightness and so-called path integral effects which mean the effect of beam attenuation along the beam orbit. Here the article presents the HIBP measurement of the electron density profile after the proposed method was first applied on the real experimental data of compact helical system plasmas. In the real application, the hollow density and the peaked profiles are successfully obtained with sufficiently high temporal resolution (a few ms), in accordance with the electron density profile measured with Thomson scattering for electron cyclotron resonance heating and neutral beam injection plasmas.

Development of a laser amplification system for the multi-pass Thomson scattering system for GAMMA 10/PDX.

The multi-pass Thomson scattering (MPTS) system is a useful technique for increasing the Thomson scattering (TS) signal intensities and improving the TS diagnostic time resolution. The MPTS system developed in GAMMA 10/PDX has a polarization-based configuration with an image relaying system. The MPTS system has been constructed for enhancing the Thomson scattered signals for the improvement of measurement accuracy and the megahertz sampling time resolution. However, in the normal MPTS system, the MPTS signal intensities decrease with the pass number because of the damping due to the optical components. Subsequently, we have developed a new MPTS system with the laser amplification system. The laser amplification system can improve the degraded laser power after six passes in the multi-pass system to the initial laser power. For the first time worldwide, we successfully obtained the continued multi-pass signals after the laser amplification system in the gas scattering experiments.