Bremsstrahlung Measurements Research Articles

Study of runaway electrons in TUMAN-3M tokamak plasmas

Studies of runaway electrons in present day tokamaks are essential to improve theoretical models and to support possible avoidance or suppression mechanisms in future large-scale plasma devices. Some of the phenomena associated with the runaway electrons take place at faster time scales, and thus it is essential to probe the runaway electrons to investigate underlying physics. The present article reports a few experimental observations of runaway electron associated events, at fast time scales, using a state-of-the-art multi-detector system developed at the Ioffe Institute and recently deployed on the TUMAN-3M tokamak. The system is based on the high-performance scintillation gamma-ray spectrometers for measurements of bremsstrahlung generated during the interaction of accelerated electrons with plasma and materials of the tokamak chamber. It includes a total three detectors configured in the spectroscopic mode having different lines of sight. Along with this hardware, dedicated algorithms were developed and validated that enables the separation of piled-up pulses, maximize the dynamic range of the detector and provides a counting rate as high as 107 counts per second. The inversion code, DeGaSum, has been used for the reconstruction of a runaway electron energy distribution function from the measured gamma-ray spectra. Using this tool, experimental analysis of the runaway electron beam generation and evolution of their energy distribution in the TUMAN-3M representative plasma discharges is performed. The effect on gamma-ray count rate during the magnetohydrodynamic activities and possible changes in the runaway electron energy distribution function during sawtooth oscillations is discussed in detail. Possible maximum limit of the runaway electron energy in TUMAN-3M is investigated and compared with the numerical analysis. In addition, the probability of the runaway electron generation throughout the plasma discharge is estimated analytically and compared with the experimental observation that suggests a balance between production and loss of the runaway electrons.

Dependence of the Bremsstrahlung Spectral Temperature in Minimum-B Electron Cyclotron Resonance Ion Sources

The spectral temperature $T_{s}$ obtained from bremsstrahlung spectra emitted from electron cyclotron resonance (ECR) ion sources (ECRISs), in which the plasma is confined in a minimum-B magnetic field, is used as a relative indication of the temperature of the plasma hot electrons. Past bremsstrahlung measurements taken on ECRISs indicate that $T_{s}$ is strongly dependent on the magnetic field gradient at the resonance zone or $({B_{\mathrm{ min}}}/{B_{\mathrm{ ECR}}})$ . However, this dependence was never fully proven or explained. To further our understanding a more detailed study of the bremsstrahlung radiation for X-rays above 10 keV is underway using VENUS, a third-generation ECRIS at Lawrence Berkeley National Laboratory. Initial analysis of previous and new data has revealed that $T_{s}$ appears to be dependent solely on the minimum magnetic field $B_{\mathrm{ min}}$ rather than $({B_{\mathrm{ min}}}/{B_{\mathrm{ ECR}}})$ and the microwave frequency $\omega$ . Decoupling $T_{s}$ from $({B_{\mathrm{ min}}}/{B_{\mathrm{ ECR}}})$ , mainly $B_{\mathrm{ ECR}}$ , implies that $T_{s}$ does not depend on the magnitude of the heating frequency $\omega$ . While it certainly appears to be true that plasma density $n_{e}(\propto \omega ^{2}\propto B^{2})$ increases with heating frequency, as was postulated by Geller in 1987, a more careful consideration into the heating mechanism of the plasma electrons is warranted. The disassociation of $T_{s}$ from the heating frequency $\omega$ , while an interesting discovery, implies that we must change the way we understand how ECRISs operate. This paper presents new bremsstrahlung measurements, analyses, and discussions of the results.

Zeff profile measurement system using a Thomson polychromator

To measure the effective ion charge Zeff profile, most plasma machines are equipped with a bremsstrahlung measurement system as a filter-scope diagnostic device. In Korean Superconducting Tokamak Advanced Research (KSTAR) machine, a new type of bremsstrahlung measurement system, called hybrid-type polychromator system, was developed and tested at a single position during the 10th KSTAR plasma campaign [1]. The main idea of this hybrid-type polychromator system is to combine a polychromator system, which can measure the Thomson scattering signal, with a bremsstrahlung signal measurement system. The advantages of this hybrid-type polychromator system include the reduction of diagnostic vacuum windows in the Tokamak or Stellarator, and the measurement of the Zeff parameter with electron temperature (Te) and electron density (ne) at a same position. In this study, we improve last year's hybrid-type polychromator system. A major improvement is the change in the structure of the optical system. This allows measuring the bremsstrahlung signals more clearly, and four hybrid-type polychromators were equipped to measure the Zeff profile preliminarily. In this paper, we explain the structure of the hybrid-type polychromator and the system's measurement results.

Groomed jets in heavy-ion collisions: sensitivity to medium-induced bremsstrahlung

We argue that contemporary jet substructure techniques might facilitate a more direct measurement of hard medium-induced gluon bremsstrahlung in heavy-ion collisions, and focus specifically on the “soft drop declustering” procedure that singles out the two leading jet substructures. Assuming coherent jet energy loss, we find an enhancement of the distribution of the energy fractions shared by the two substructures at small subjet energy caused by hard medium-induced gluon radiation. Departures from this approximation are discussed, in particular, the effects of colour decoherence and the contamination of the grooming procedure by soft background. Finally, we propose a complementary observable, that is the ratio of the two-pronged probability in Pb-Pb to proton-proton collisions and discuss its sensitivity to various energy loss mechanisms.

Feasibility studies of the diffractive bremsstrahlung measurement at the LHC

Feasibility studies of an observation of the exclusive diffractive bremsstrahlung in proton-proton scattering at the LHC are reported. A simplified approach to the photon and the scattered proton energy reconstruction is used. The background influence is discussed.

Improved spectrum simulation for validating SEM-EDS analysis

X-ray microanalysis by SEM-EDS requires corrections for the many physical processes that affect emitted intensity for elements present in the material. These corrections will only be accurate provided a number of conditions are satisfied and it is essential that the correct elements are identified. As analysis is pushed to achieve results on smaller features and more challenging samples it becomes increasingly difficult to determine if all conditions are upheld and whether the analysis results are valid. If a theoretical simulated spectrum based on the measured analysis result is compared with the measured spectrum, any marked differences will indicate problems with the analysis and can prevent serious mistakes in interpretation. To achieve the necessary accuracy a previous theoretical model has been enhanced to incorporate new line intensity measurements, differential absorption and excitation of emission lines, including the effect of Coster-Kronig transitions and an improved treatment of bremsstrahlung for compounds. The efficiency characteristic has been measured for a large area SDD detector and data acquired from an extensive set of standard materials at both 5 kV and 20 kV. The parameterized model has been adjusted to fit measured characteristic intensities and both background shape and intensity at the same beam current. Examples are given to demonstrate how an overlay of an accurate theoretical simulation can expose some non-obvious mistakes and provide some expert guidance towards a valid analysis result. A new formula for calculating the effective mean atomic number for compounds has also been derived that is appropriate and should help improve accuracy in techniques that calculate the bremsstrahlung or use a bremsstrahlung measurement for calibration.

Application of visible bremsstrahlung to Z(eff) measurement on the Experimental Advanced Superconducting Tokamak.

The multi-channel visible bremsstrahlung measurement system has been developed on Experimental Advanced Superconducting Tokamak (EAST). In addition to providing effective ion charge Zeff as a routine diagnostic, this diagnostic can also be used to estimate other parameters. With the assumption that Zeff can be seen as constant across the radius and does not change significantly during steady state discharges, central electron temperature, averaged electron density, electron density profile, and plasma current density profile have been obtained based on the scaling of Zeff with electron density and the relations between Zeff and these parameters. The estimated results are in good coincidence with measured values, providing an effective and convenient method to estimate other plasma parameters.

Reduction of X-ray generation in high-intensity laser ion acceleration

In this paper, we report on measurements of bremsstrahlung in laser ion acceleration experiments from ultra-thin, polymer-based target foils. The influence of laser polarization on the generated \(\gamma\) radiation, the maximum achievable proton energy and the total proton number is investigated. A clear benefit in terms of \(\gamma\) radiation reduction by the use of circular polarized light can be observed. At the same time, the total number of accelerated protons was increased.

Electromagnetic transition between molecular resonances in 8Be

The nucleus 8Be has been conjectured to resemble a molecule of two interacting α-particles. A crucial test of this conjecture is the electromagnetic transition between the molecular resonances. This paper discusses the earlier indirect bremsstrahlung measurements and describes a recent experiment on the direct measurement of γ-transition between the 4+ and 2+ resonances. Experimental results are compared with various theories. The outlook on the measurement of 2+ → 0+ transition will be presented.

Measurement of Inner Bremsstrahlung in Polarized Muon Decay with MEG

A muon decay accompanied by a photon through the inner Bremmstrahlung process (μ→eνν¯γ, radiative muon decay) produces a time-correlated pair of positron and photon which becomes one of the main backgrounds in the search for μ→eγ decay. This channel is also an important probe of timing calibration and cross-check of whole the experiment. We identified a large sample (∼ 13000) of radiative muon decays in MEG data sample. The measured branching ratio in a region of interest in the μ→eγ search is consistent with the standard model prediction. It is also the first measurement of the decay from polarized muons. The precision measurement of this mode enables us to use it as one of the normalization channels of μ→eγ decay successfully reducing its uncertainty to less than 5%.

Efficiency calibration for a NaI scintillation detector based on Monte-Carlo process and preliminary measurements of bremsstrahlung

In order to better apply the NaI scintillation spectrometer to bremsstrahlung measurements, the energy response function of a NaI detector spectrometer system is studied by using 137Cs and 60Co sources based on Monte Carlo N particle transport code (MCNP) process. Simulated and measured almighty peak efficiency are in good agreement. An energy response matrix (ERM) is obtained by simulating photons with a certain energy incident on the NaI crystal in MCNP process, through deconvoluting the detected spectrum of NaI using the ERM, and the results of the deconvolution accord well with those from the original spectrum. Furthermore, the NaI detector is used to preliminarily detect its response to bremsstrahlung generated by high intensity electrons bombarding a target of 1.5 mm thickness.

Fast electron beam measurements from relativistically intense, frequency-doubled laser–solid interactions

Experimental measurements of the fast electron beam created by the interaction of relativistically intense, frequency-doubled laser light with planar solid targets and its subsequent transport within the target are presented and compared with those of a similar experiment using the laser fundamental frequency. Using frequency-doubled laser light, the fast electron source size is significantly reduced, while evidence suggests the divergence angle may be reduced. Pyrometric measurements of the target rear surface temperature and the Cu Kα imager data indicate the laser to fast electron absorption fraction is reduced using frequency doubled laser light. Bremsstrahlung measurements indicate the fast electron temperature is 125 keV, while the laser energy absorbed into forward-going fast electrons was found to be 16 ± 4% for frequency doubled light at a mean laser intensity of 5 ± 3 × 1018 W cm−2.

Using bremsstrahlung for electron density estimation and correction in EAST tokamak

In EAST electron density (ne) is measured by the multi-channel far-infrared (FIR) hydrogen cyanide (HCN) interferometer and Thomson scattering (TS) diagnostics. However, it is difficult to obtain accurate ne profile for that there are many problems existing in current electron density diagnostics. Since the visible bremsstrahlung (VB) emission coefficient has a strong dependence on electron density, the visible bremsstrahlung measurement system developed to determine the ion effective charge (Zeff) may also be used for ne estimation via inverse operations. With assumption that Zeff has a flat profile and does not change significantly in steady state discharges, line averaged electron density (n¯e) has been deduced from VB signals in L-mode and H-mode discharges in EAST. The results are in good coincidence with n¯e from FIR, which proves that VB measurement is an effective tool for ne estimation. VB diagnostic is also applied to n¯e correction when FIR n¯e is wrong for the laser phase shift reversal together with noise causes errors when electron density changed rapidly in the H-mode discharges. Typical ne profiles in L-mode and H-mode phase are also deduced with reconstructed bremsstrahlung profiles.

Zeff first measurements in EAST with a multi-channel visible bremsstrahlung new system

A multi-channel visible bremsstrahlung measurement system was developed to measure the ion effective charge (Zeff) in EAST tokamak. The system has a temporal resolution of 0.05ms and spatial resolution of 3cm. The measurement principle and the design of the 8-channel fiber-photomultiplier tubes (PMTs) coupled system are described, including the calibration process of the measurement system with an integrating sphere. Preliminary experimental results of line integrated bremsstrahlung profile and Zeff derived from the system are reported.

Comparisons of angularly and spectrally resolved Bremsstrahlung measurements to two-dimensional multi-stage simulations of short-pulse laser-plasma interactions

A 2-D multi-stage simulation model incorporating realistic laser conditions and a fully resolved electron distribution handoff has been developed and compared to angularly and spectrally resolved Bremsstrahlung measurements from high-Z planar targets. For near-normal incidence and 0.5-1 × 1020 W/cm2 intensity, particle-in-cell (PIC) simulations predict the existence of a high energy electron component consistently directed away from the laser axis, in contrast with previous expectations for oblique irradiation. Measurements of the angular distribution are consistent with a high energy component when directed along the PIC predicted direction, as opposed to between the target normal and laser axis as previously measured.

Calibration techniques for fast-ion Dα diagnostics

Fast-ion D(α) measurements are an application of visible charge-exchange recombination (CER) spectroscopy that provide information about the energetic ion population. Like other CER diagnostics, the standard intensity calibration is obtained with an integrating sphere during a vacuum vessel opening. An alternative approach is to create plasmas where the fast-ion population is known, then calculate the expected signals with a synthetic diagnostic code. The two methods sometimes agree well but are discrepant in other cases. Different background subtraction techniques and simultaneous measurements of visible bremsstrahlung and of beam emission provide useful checks on the calibrations and calculations.

Temperature diagnostics of ECR plasma by measurement of electron bremsstrahlung

The x-ray bremsstrahlung spectrum emitted by the electron population in a 14.5 GHz ECR plasma source has been measured using a NaI(Tl) detector, and hence the electron temperature of the higher energy electron population in the plasma has been determined. The x-ray spectra for Ne and Ar gases have been systematically studied as a function of inlet gas pressure from 7 × 10(-7) mbar to 7 × 10(-5) mbar and for input microwave power ∼1 W to ∼300 W. At the highest input power and optimum pressure conditions, the end point bremsstrahlung energies are seen to reach ∼700 keV. The estimated electron temperatures (T(e)) were found to be in the range 20 keV-80 keV. The T(e) is found to be peaking at a pressure of 1 × 10(-5) mbar for both gases. The T(e) is seen to increase with increasing input power in the intermediate power region, i.e., between 100 and 200 W, but shows different behaviour for different gases in the low and high power regions. Both gases show very weak dependence of electron temperature on inlet gas pressure, but the trends in each gas are different.

Studies of plasma breakdown and electron heating on a 14 GHz ECR ion source through measurement of plasma bremsstrahlung

Temporal evolution of plasma bremsstrahlung emitted by a 14 GHz electron cyclotron resonance ion source (ECRIS) operated in pulsed mode is presented in the energy range 1.5–400 keV with 100 µs resolution. Such a high temporal resolution together with this energy range has never been measured before with an ECRIS. Data are presented as a function of microwave power, neutral gas pressure, magnetic field configuration and seed electron density. The saturation time of the bremsstrahlung count rate is almost independent of the photon energy up to 100 keV and exhibits similar characteristics with the neutral gas balance. The average photon energy during the plasma breakdown is significantly higher than that during the steady state and depends strongly on the density of seed electrons. The results are consistent with a theoretical model describing the evolution of the electron energy distribution function during the preglow transient.

Further investigating the determination of phosphorus in plants by INAA using bremsstrahlung measurement

Phosphorus is an essential element for plants and animals, playing a fundamental role in the production of biochemical energy. Despite its relevance, phosphorus is not commonly determined by instrumental neutron activation analysis (INAA), because 32P does not emit gamma-rays in its decay. There are alternative methods for the determination of phosphorus by INAA, such as the use of beta counting or the measurement of bremsstrahlung originated from the high energy beta particle from 32P. Here the determination of phosphorus in plant materials by measuring the bremsstrahlung production was further investigated, to optimize an analytical protocol for minimizing interferences and overcoming the poor specificity. Eight certified reference materials of plant matrices with phosphorus ranging between 171 and 5,180 mg kg−1 were irradiated at a thermal neutron flux of 9.5 × 1012 cm−2 s−1 and measured with a HPGe detector at decay times varying from 7 to 60 days. Phosphorus solutions added to a certified reference material at three levels were used for calibration. Counts accumulated in the baseline at four different regions of the gamma-ray spectra were tested for the determination of phosphorus, with better results for the 100 keV region. The Compton scattering contribution in the selected range was discounted using an experimental peak-to-Compton factor and the net areas of all peaks in the spectra with energies higher than 218 keV, i.e. Compton edge above 100 keV. Amongst the interferences investigated, the production of 32P from sulfur, and the contribution of Compton scattering should be considered for producing good results.

Studies of electron heating on a 6.4 GHz ECR ion source through measurement of diamagnetic current and plasma bremsstrahlung

Diamagnetic current and low energy (2–70 keV) x-ray bremsstrahlung measurements taken on a 6.4 GHz electron cyclotron resonance ion source (ECRIS) are presented as a function of microwave power, neutral gas pressure and magnetic field configuration. X-ray flux from confined electrons and plasma energy density depend logarithmically on microwave power. This result differs from previous studies performed on ECRISs that operate at higher microwave frequencies, in which the x-ray power increases in an essentially linear fashion with the microwave power. X-ray power and plasma energy density both saturate as the neutral pressure is increased beyond a certain value. The gradient of the magnetic field is shown to have a large effect on both x-ray power and plasma energy density. Lastly, it is observed that the peak in x-ray power efficiency (x-ray power per unit of absorbed microwave power) and the peak in extracted ion current efficiency (recorded Faraday cup current per unit of absorbed microwave power) occur at different absorbed microwave powers.