Bremsstrahlung Measurements Research Articles

Development of a multi-channel visible bremsstrahlung measurement system for the effective charge in the versatile experiment spherical torus (VEST).

A diagnostic system for measuring the effective charge in the versatile experiment spherical torus (VEST) has been developed. The system utilizes a toroidal array to observe the plasma radius on the low magnetic field side, providing a spatially resolved Zeff. The target wavelength of visible bremsstrahlung (VB) was carefully selected to avoid contamination by line emissions. The detector signal was calibrated using a halogen light source and an integrating sphere to obtain an absolute value of the radiative power from each chord. The local emissivity profile was reconstructed from the line-integrated VB emission using the Abel inversion method. Reconstruction tests were performed on various shapes of phantom profiles to effectively reconstruct the local emissivity from the measurements. We found that the initial measurements of the multi-channel VB system were consistent with the results of other independent measurements, supporting the validity of the new measurements. Finally, we obtained the initial result of Zeff in the VEST.

Radiation safety of ultra-high dose rate electron accelerators for FLASH radiotherapy.

An ultra-high dose rate (UHDR) electron accelerator for FLASH radiotherapy (RT) produces very intense bremsstrahlung by the interaction of the electron beam with objects both inside and outside of the accelerator. The bremsstrahlung dose per pulse is typically 1-2 orders of magnitude larger than that of conventional RT x-ray treatment of the same energy, and for electron energies above 10 MeV, the bremsstrahlung produces substantially more induced radioactivity outside the accelerator than for conventional RT. Therefore, a thorough radiation safety assessment is mandatory prior to the operation of a UHDR electron accelerator. To evaluate the radiation safety of a prototype FLASH-enabled Varian TrueBeam accelerator and to develop a general framework for assessment of all key radiation safety properties of a UHDR electron accelerator for FLASH RT. Production of bremsstrahlung and induced radioactivity by a UHDR electron accelerator is modeled by various analytical methods. The analytical modeling is compared with National Institute of Standards and Technology (NIST) bremsstrahlung yield data as well as measurements of primary bremsstrahlung outside the bunker and induced radioactivity of irradiated thick targets for a FLASH-enabled 16 MeV Varian TrueBeam electron accelerator. In addition, the analytical modeling is complemented by measurements of secondary bremsstrahlung inside/outside the bunker and neutrons at the maze entrance. Calculated bremsstrahlung yields deviate maximum 8.5% from NIST data, and all measurements of primary bremsstrahlung and induced radioactivity agree with calculations, validating the analytical tools. In addition, it is found that scattering foil bremsstrahlung dominates primary bremsstrahlung and the main source of secondary bremsstrahlung is the irradiated object outside the accelerator. It follows that primary and secondary bremsstrahlung outside the bunker can be calculated using the same simple formalism as that used for conventional RT. Measured primary bremsstrahlung tenth-value layers for concrete of the simple formalism are in good agreement with NCRP and IAEA data, while measured secondary bremsstrahlung tenth-value layers for concrete are considerably lower than NCRP and IAEA data. All calculations and measurements form a general framework for assessment of all key radiation safety properties of a UHDR electron accelerator. The FLASH-enabled Varian TrueBeam accelerator is safe for normal operation (max. 99 pulses per irradiation) in a bunker designed for at least 15 MV conventional x-ray treatment unless the UHDR workload is much larger than the x-ray workload. A similar finding applies to other UHDR electron accelerators. However, during beam tuning, radiation survey, or other tests with extended irradiation time, the UHDR workload may become very large, necessitating the implementation of additional safety measures.

Real-time bremsstrahlung detector as a monitoring tool for laser–plasma proton acceleration

Abstract Real-time evaluation of laser-driven byproducts is crucial for state-of-the-art facilities operating at high repetition rates. This work presents real-time measurements of hard X-rays (bremsstrahlung radiation) generated from the interaction of high-intensity laser pulses with solid targets in the target normal sheath acceleration regime using a scintillator stack detector. The detector offers insights into the effectiveness of laser–plasma interaction through measured fluctuations in bremsstrahlung radiation temperature and scintillation light yield on a shot-to-shot basis. Moreover, a strong correlation of the bremsstrahlung measurements (i.e., temperature and yield) with the cutoff energy of laser-driven protons was observed. The scintillator stack detector serves not only as a diagnostic for online monitoring of the laser–plasma interaction but also as a promising tool for estimating proton energy fluctuations in a non-disruptive manner, which is particularly important when direct proton source characterization is impractical, for example, during experiments aimed at irradiating user samples with the accelerated proton beam.

First results of a new quadrupole minimum-B permanent magnet electron cyclotron resonance ion source

The first experimental results of a novel permanent magnet electron cyclotron resonance ion source (ECRIS) with a minimum-B quadrupole magnetic field topology are presented. Bremsstrahlung measurements indicate that the electrons can reach energies of 150–200 keV. Ion charge states up to Ar11+ and Kr15+ were detected in the charge state distributions of the extracted beams. Measurements in pulsed operation mode reveal an afterglow peak, which implies that high charge state ions are confined electrostatically similar to a conventional ECRIS. The potential applications and future development of the ion source are discussed.

Characterization of sub-picosecond laser-produced fast electrons by modeling angularly resolved bremsstrahlung measurements with 3D hybrid particle-in-cell code

Hard x-rays produced by intense laser-produced fast electrons interacting with solids are a vital source for producing radiographs of high-density objects and implosion cores for inertial confinement fusion. Accurate calculation of hard x-ray sources requires a three-dimensional (3D) simulation geometry that fully models the electron transport dynamics, including electron recirculation and the generation of absolute photon yields. To date, 3D simulations of laser-produced bremsstrahlung photons over tens of picoseconds and code benchmarking have not been performed definitively. In this study, we characterize sub-picosecond laser-produced fast electrons by modeling angularly resolved bremsstrahlung measurements for refluxing and non-refluxing targets using the 3D hybrid particle-in-cell (PIC), Large Scale Plasma code. Bremsstrahlung radiation and escaped electron data were obtained by focusing a 50-TW Leopard laser (15 J, 0.35 ps, 2 × 1019 W/cm2) on a 100-μm-thick Cu foil and a Cu with a large plastic backing (Cu–CH target). Data for both the Cu and Cu–CH targets were reproduced for simulations with a given set of electron parameters. Comparison of the simulations revealed that the hard x-ray emission from the Cu target was significantly longer in duration than that from the Cu–CH target. The benchmarked hybrid PIC code could prove to be a powerful tool in the design and optimization of time- and angular-dependent bremsstrahlung sources for flash x-ray and gamma-ray radiography.

Soft photon bremsstrahlung at next-to-leading power

The emission of soft radiation provides a fundamental probe of the consistency of the underlying quantum field theory. Correspondingly, the measurement of the soft photon bremsstrahlung, such as the one proposed with the planned future upgrade of the ALICE experiment at the LHC, is of great interest. In this letter we explore the possibility to implement analytic techniques that have been recently developed for soft gluon resummation at Next-to-Leading-Power (NLP) in the context of the soft photon spectrum. We provide a formula for the differential cross-section with shifted kinematics that is particularly suitable for numerical implementations. We also discuss the impact of loop corrections to Low's theorem due to radiative jet functions.

Diagnostic techniques of minimum-B ECR ion source plasma instabilities.

The performance of a minimum-B Electron Cyclotron Resonance Ion Source (ECRIS) is traditionally quantified by measuring the beam current and quality of the extracted ion beams of different charge state ions. The stability of the extracted ion beam currents has drawn more attention recently as the technology is pushing its limits toward higher ion charge states and beam intensities. The stability of the extracted beam is often compromised by plasma instabilities manifesting themselves as rapid oscillations of the beam currents in millisecond scale. This paper focuses on practical aspects of diagnostics techniques of the instabilities, showcases examples of instability-related diagnostics signals, and links them to the plasma physics of ECR ion sources. The reviewed techniques include time-resolved microwave emission diagnostics, bremsstrahlung measurements, direct measurement of electron and ion fluxes, measurement of the ion beam energy spread, and optical emission diagnostics. We list the advantages and disadvantages of each technique and outline the development needs of further diagnostics. Finally, we discuss the implications of the instabilities in both historical and forward-looking context of ECRIS development.

Megavolt bremsstrahlung measurements from linear induction accelerators demonstrate possible use as a FLASH radiotherapy source to reduce acute toxicity

Recent studies indicate better efficacy and healthy tissue sparing with high dose-rate FLASH radiotherapy (FLASH-RT) cancer treatment. This technique delivers a prompt high radiation dose rather than fractional doses over time. While some suggest thresholds of > 40 Gy s−1 with a maximal effect at > 100 Gy s−1, accumulated evidence shows that instantaneous dose-rate and irradiation time are critical. Mechanisms are still debated, but toxicity is minimized while inducing apoptosis in malignant tissue. Delivery technologies to date show that a capability gap exists with clinic scale, broad area, deep penetrating, high dose rate systems. Based on these trends, if FLASH-RT is adopted, it may become a dominant approach except in the least technologically advanced countries. The linear induction accelerator (LIA) developed for high instantaneous and high average dose-rate, species independent charged particle acceleration, has yet to be considered for this application. We review the status of LIA technology, explore the physics of bremsstrahlung-converter-target interactions and our work on stabilizing the electron beam. While the gradient of the LIA is low, we present our preliminary work to improve the gradient by an order of magnitude, presenting a point design for a multibeam FLASH-RT system using a single accelerator for application to conformal FLASH-RT.

Bayesian inference of spatially resolved Zeff profiles from line integrated bremsstrahlung spectra.

In nuclear fusion research, the effective ion charge Zeff, which characterizes the overall content of impurities, can be experimentally derived from the plasma electron-ion bremsstrahlung, given the electron density ne and temperature Te. At Wendelstein 7-X, a multichannel near-infrared spectrometer is installed to collect the plasma bremsstrahlung along 27 lines of sight covering more than half the plasma cross section, which provides information on Zeff over the entire plasma radius. To infer spatially resolved Zeff profiles, a Bayesian model is developed in the Minerva framework. Zeff, ne, and Te profiles are modeled as Gaussian processes, whose smoothness is determined by hyperparameters. These profiles are transformed to fields in Cartesian coordinates, given the poloidal magnetic flux surfaces calculated by the variational moments equilibrium code. Given all these physical quantities, the model predicts line-of-sight integrals of near-infrared bremsstrahlung spectra. The model includes the predictive (forward) models of the interferometer, Thomson scattering system, and visible and near-infrared spectrometers. Given the observations of all these diagnostics, the posterior probability distribution of Zeff profiles is calculated and shown as an inference solution. The smoothness (gradient) of the profiles is optimally chosen by Bayesian Occam's razor. Furthermore, wall reflections can significantly pollute the measurements of the plasma bremsstrahlung, which leads to over-estimation of Zeff values in the edge region. In the first results presented in this work, this problem does not appear, and the posterior samples of Zeff profiles are overall plausible and consistent with Zeff values inferred, given the data from the single-channel visible spectrometer.

Impact of boronizations on impurity sources and performance in Wendelstein 7-X

The low-Z oxygen and carbon were the main plasma impurities in the Wendelstein 7-X (W7-X) stellarator in the last experimental campaign with the passively cooled graphite divertor. To tackle this issue boronization (Winter et al 1989 J. Nucl. Mater. 162–4 712–23) was applied, which has led to one of the main achievements of the campaign: plasma operation at high core densities of more than 1020 m−3 in hydrogen fueled plasmas due to the reduced radiation-induced density limit.In total three boronizations were applied. After the first boronization the oxygen to hydrogen flux ratio (normalized influx of oxygen) at the divertor substantially decreased by a factor of 10 and the carbon to hydrogen flux ratio (normalized influx of carbon) decreased by a factor of 4 as obtained from spectroscopy. In the same time, boron emission appeared in the spectra. Between the boronizations oxygen and carbon normalized influxes increased but never reached the pre-boronization values. With each subsequent boronization O level decreased even more, reaching the lowest values after the third boronization which were more than a factor of 100 lower than before the first boronization.Such a decrease in low-Z impurity concentration significantly extended the operation window of W7-X in terms of line-integrated electron density (from 4ċ1019 m−2 to more than 1ċ1020 m−2) and diamagnetic energy (from 330 kJ up to 510 kJ). Zeff decreased from 4.5 down to values close to 1.2 as obtained from bremsstrahlung measurements. The above mentioned values are given for the two reference discharges before and after boronization.

Peculiarities of glow discharge constriction in helium

The work experimentally demonstrates the fundamental difference between the constriction in helium and other inert gases (argon, neon). Despite the visual similarity, during the constriction of the positive column of a glow discharge in helium, only a compression of the emission of spectral lines is observed. In the constricted helium discharge there is no formation of a thin channel of current which is evident from the bremsstrahlung measurements. This phenomenon can be called the optical constriction. Unlike neon and argon, where the current cord is formed because of losses of charged particles in the volume due to the dissociative recombination, the discharge in helium is controlled by the ambipolar diffusion. Moreover, while in neon and argon constriction arises in times of hundreds of microseconds—units of milliseconds after the ionization balance establishment, constricted discharge in helium forms in times about 2 min after the establishment of the thermal balance between the discharge tube and the environment. It was shown that in helium the main mechanism of constriction is the inhomogeneous heating of the neutral gas which leads to the redistribution of the neutrals over the volume, in contrast to argon and neon, where thermal effects are almost insignificant and constriction is due to the peculiarities of electron kinetics. The absence of constriction of the emission of spectral lines during the cooling of the tube walls is demonstrated. The interpretation of the observed phenomena is given on the basis of a simple qualitative model. Experiments in helium were performed at reduced pressure pR = 200 torr cm and reduced currents up to 150 mA cm−1.

Absolute cross sections of ultra-soft x-ray radiation in electron scattering upon atoms

SynopsisThe soft-photon approximation (SPA) is shown to give quantitative interpretation of absolute measurement of bremsstrahlung (BrS) cross section, σ, on atomic targets in the region of large wavelengths λ, for which σ∼ 1/λ2.

Measurements of visible bremsstrahlung and automatic Bayesian inference of the effective plasma charge Zeff at W7-X

The effective charge Zeff indicates the overall impurity contamination of a plasma. Zeff can be derived experimentally from the intensity of the plasma bremsstrahlung emission. We describe here the diagnostic set-ups and the Bayesian modeling allowing the inference of Zeff at W7-X. First results from the operational campaigns in 2017 and 2018 are shown. Measurements of the visible plasma radiation along a single line-of-sight traversing the core plasma has been carried out using a compact USB-spectrometer with a time resolution of 100 ms. A spectral region (627–641 nm) that is free from line emission is selected for the analysis of the bremsstrahlung emission, which also depends on electron temperature and density profiles. Electron temperature profiles are derived from either the electron cyclotron emission or the Thomson scattering diagnostic. Electron density profiles, however, have their shape information derived from Thomson scattering measurements and absolute values from single line-of-sight interferometry measurements. The Minerva framework is used to infer the profiles with Gaussian processes and develop a Bayesian model of the bremsstrahlung emission to infer line averaged Zeff. The sensitivity of the diagnostic enables Zeff measurements down to the lowest core electron densities observed in the last campaign of 0.75×1019 m−3 with a statistical relative error of ≈50% (Zeff = 3.2, 100 ms integration time). The analysis is automated to routinely compute Zeff after every plasma discharges.

Integrated Zeff analysis on the DIII-D Tokamak

Determination of effective ion charge, or Zeff, from measurements of visible brems\\-strahlung radiation made by multiple diagnostics is now integrated into a single analysis workflow within the plasma profile analysis module in OMFIT [Logan et al. Fus. Sci. Technol. 74 (2018) 125]. Measurements are made across the midplane of DIII-D with independent filterscope and spectrometer based diagnostics. Emission from 5215–5245 Å is measured by 16 filtered visible bremsstrahlung measurements (filterscopes) with a sampling rate of 20 kHz. The charge exchange recombination spectroscopy (CER) system utilizes scanning spectrometers, and typically measures emission near 5291 Å with approximately 20 active spatial views. Both Abel inversion and least squares minimization analyses are performed on these chordal measurements to infer the plasma emissivity profile with respect to magnetic flux coordinate ρ. Zeff is derived from this emissivity profile from a calculation involving electron density and temperature from the Thomson scattering diagnostic. This analysis has been applied for a range of plasma conditions including L-mode, H- mode and H-mode with impurity seeding. The Zeff inferred from VB data is shown to be in good agreement with the Zeff computed from CER impurity density profile measurements when C6+ is assumed to be the dominant impurity species of the plasma. This new analysis is useful for cross checking diagnostics and measuring Zeff when all important impurities cannot be monitored directly with CER measurements.

Measurements of the fast electron bremsstrahlung during lower hybrid current drive in the HL-2A tokamak

Physics related to fast electrons in lower hybrid (LH) current drive (LHCD) plasma is a very important issue, since these particles will play an important role in runaway electron (RE) generation and lower hybrid wave (LHW)-related physics. Utilizing a new hard X-ray (HXR) pinhole camera, recent HL-2A tokamak experiments have devoted to enhancing the understanding of the physics on fast electrons and LHW. The fast electron bremsstrahlung (FEB) emission in the HXR energy range between 20 and 200 keV was measured by the HXR camera. To study the conversion of LHW-produced fast electrons into REs, a very short pulse of LHW, so-called “blip”, with duration of 5 ms was injected into the plasma during the current flattop phase. A strong enhancement of REs was induced by the blip injection. Measurements from the HXR camera show that the fast electrons generated by LHWs is mainly concentrated in 40-60 keV, which is well consistent with the calculated value based on Landau damping theory. The energy of these seed electrons is higher than the critical runaway energy. This phenomenon may be come from the synergetic effects of Dreicer and avalanche RE generation. Moreover, the measurements indicate that the spatial distribution of the fast electrons during LHCD has a peaked profile, implying that the fast electrons are mainly produced in the plasma core. It also suggests that the energy of the LHW mainly deposited in the plasma core region.

A Simulation Study on Estimation of Bragg-Peak Shifts via Machine Learning Using Proton-Beam Images Obtained by Measurement of Secondary Electron Bremsstrahlung

We investigated an estimation method of Bragg-peak shifts via machine learning using proton-beam images obtained by measurement of secondary electron bremsstrahlung (SEB) by Monte Carlo simulation. Proton beams having energy of 139 MeV were incident on a water phantom with randomly placed air spheres inside, and 6400 pairs of “proton-beam images” and “a Bragg-peak shift” were prepared and then multiple linear regression analysis was carried out. A good agreement was found between the actual Bragg-peak shifts and predicted values in both the training and test sets. The coefficients of determination of the obtained prediction model were 0.899 for the training set and 0.894 for the test set. Consequently, we found that a prediction model with small variance and high prediction performance could be obtained using the SEB data.

Monte Carlo MCNP-based simulation and radiation measurement of Yttrium-90 bremsstrahlung produced from radiation shielding apparatus

Yttrium-90 (90Y) has been widely used in therapeutic nuclear medicine including in the selective internal radiation therapy with 90Y-microsphere. In the procedure 90Y source is protected within the radiation shielding apparatus made of perspex. However, bremsstrahlung radiation may be generated when 90Y interacts with matter. The aim of this study was to evaluate bremsstrahlung radiation produced from radiation shielding apparatus using a scintillation detector and Monte Carlo simulation. The 90Y source in glass vial was placed in the radiation shielding apparatus. Bremsstrahlung measurement and spectrum were performed and recorded using thallium-doped sodium iodide (NaI(Tl)) scintillation detector. Counting time and distance between the detector and apparatus were varied to obtain the optimal condition for bremsstrahlung measurement. Monte Carlo N-Particle (MCNP) 5 was used to simulate bremsstrahlung radiation with the same geometry used in the measurement. For experimental measurement results, the optimal distance and counting time were 10 cm and 120 s, respectively. Although the result of bremsstrahlung spectrum from the detector and simulation exhibited similar trend, markedly greater count rate was observed in simulations than in detector measurements. In conclusion, 90Y bremsstrahlung can be produced from radiation protection apparatus and this can be evaluated by either measurement or simulation.

Recent DIII-D advances in runaway electron measurement and model validation

Novel measurements and modeling of runaway electron (RE) dynamics in DIII-D have resolved experimental discrepancies and validated predictions for ITER, improving confidence that RE avoidance and mitigation can be predictably achieved. Considering RE formation, first experimental assessments of the RE seed current demonstrates that present hot-tail theories are not yet accurate and require improved treatment of the pellet dynamics. Novel measurements of kinetic instabilities in the MHz-range have been made in the RE formation phase, with the intensity of these modes correlated with previously unexplained empirical thresholds for RE generation. Controlled RE dissipation experiments in quiescent regimes have validated RE distribution function dependencies on collisional and synchrotron damping, both in terms of distribution function shape and dissipation rates. Measurements of RE bremsstrahlung and synchrotron emission are now used in tandem to resolve energy and pitch-angle effects. A resolution to long-standing dissipation anomalies in the quiescent regime is offered by taking into account kinetic instability effects on RE phase-space dynamics. Kinetic instabilities in the 100–200 MHz range are directly observed, though modeling finds the largest dissipation arises from GHz range instabilities that are beyond the reach of existing diagnostics. Kinetic instabilities are also observed in the mature post-disruption RE plateau phase, so long as the collisional damping rate is reduced with low-Z injection. Experiments with high-Z injection find that the dissipation rate saturates with injection quantity, likely due to neutral diffusion rates being slower than vertical instability rates in DIII-D. Considering the final loss, a 0D model for first-wall Joule heating is found to be in agreement with experiment, and controlled access to RE equilibria with edge safety factor of two identifies novel dynamics brought about by large-scale kink instabilities. These dynamics are typified by fast (tens of microseconds) RE loss rates without RE beam regeneration. The above measurements and comparison with theory represent significant advances in the understanding of RE dynamics and indicate possible new opportunities for RE avoidance or mitigation via kinetic instabilities.

Characterization of fast electron divergence and energy spectrum from modeling of angularly resolved bremsstrahlung measurements

Characteristics of fast electrons generated in an intense laser-solid target interaction are studied by modeling angularly resolved bremsstrahlung measurements with a hybrid Particle-In-Cell code, Large Scale Plasmas. The experiment was performed using the 50 TW Leopard laser at the Nevada Terawatt Facility. A 100 μm thick Cu foil was irradiated by the 15 J, 0.35 ps laser at a peak laser intensity of 2 × 1019 W/cm2. Bremsstrahlung produced by transport and recirculation of the fast electrons in the foil was measured with two differential filter-stack x-ray spectrometers at 22° and 40° from the laser axis. The two-spectrometer signals simultaneously fit by varying single slope temperatures (Thot) and divergence angles (θ) enable for determining Thot and θ to be 1.1 ± 0.3 MeV and 15° ± 8°, respectively. The Thot inferred from the bremsstrahlung signals agrees with that from an escaped electron measurement, suggesting that that the bremsstrahlung is predominantly produced by the transport of the high energy fast electrons in the first pass.

Development of a new compact gamma-ray spectrometer optimised for runaway electron measurements.

A new compact gamma-ray spectrometer was developed in order to optimise the measurement of bremsstrahlung radiation emitted from runaway electrons in the MeV range. The detector is based on a cerium doped lutetium-yttrium oxyorthosilicate (LYSO:Ce) scintillator coupled to a silicon photomultiplier and is insensitive to magnetic fields. A dedicated electronic board was developed to optimise the signal readout as well as for online control of the device. The detector combines a dynamic range up to 10 MeV with moderate energy non-linearity, counting rate capabilities in excess of 1 MHz, and an energy resolution that extrapolates to a few % in the MeV range, thus meeting the requirements for its application to runaway electron studies by bremsstrahlung measurements in the gamma-ray energy range.