Bremsstrahlung In Plasma Research Articles

Correlation of bremsstrahlung and energy distribution of escaping electrons to study the dynamics of magnetically confined plasma

A combination of electron and bremsstrahlung diagnostics was used to study the properties of the highly charged plasma of an electron cyclotron resonance ion source (ECRIS). The bremsstrahlung radiation in both axial and radial directions was correlated with the energy distribution of the electrons escaping axially from the confined plasma, i.e. the lost electron energy distribution (LEED), to achieve a more comprehensive view of the plasma. The evolution of the bremsstrahlung spectra and the LEED were determined as a function of the main operating parameters of the ion source. Also, the effects of the transition from a stable to an unstable plasma regime were determined for the bremsstrahlung and LEED changes in the shape of the measured LEED were found to correlate with changing bremsstrahlung spectral temperature. It was also found that the magnetic field has a strong impact on the axial and radial bremsstrahlung spectra and the LEED affecting the bremsstrahlung intensity, spectral temperatures and the LEED high energy local maximum. A comprehensive discussion is provided to explain the observed different behaviour of axial and radial bremsstrahlung emissions with varying magnetic field based on the directionality of plasma bremsstrahlung in ECRIS specific magnetic confinement scheme. Furthermore, it was observed that the onset of kinetic plasma instabilities has a clear impact on the shape of the LEED

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.

Generation of a Beam of Fast Electrons, Plasma Bremsstrahlung, and Characteristic Radiation in a High-Current Z-Pinch

The generation of accelerated electron beams in a high-current Z-pinch formed by the implosion of wire cylindrical tungsten arrays on an Angara-5-1 facility is studied. The most intense characteristic and bremsstrahlung X-ray radiation of fast electrons is recorded from the central region of the pinch at the pin-ching stage. The transverse size of the radiation source in the characteristic tungsten radiation Lα is ~1.5‒1.8 mm, which is close to the transverse size of the radiation source in the soft X-ray emission (1‒1.5 mm). The current of accelerated electrons generated during the implosion of the wire array and the formation of a pinch is on the order of the Alfven current (IA). This result is consistent with the estimate of the current of accelerated electrons made from the measured characteristic radiation intensity Lα of tungsten under the assumption that the mean free path of fast electrons in a pinch is on the order of its length.

Time-resolved study of Bremsstrahlung emission and spectra at the early stage in a nanosecond laser ablated tungsten plasma

Laser-Induced Breakdown Spectroscopy (LIBS) has been considered as a crucial tool for element analysis in plasma wall interaction (PWI) of fusion devices. In this work, a one-dimensional gas-dynamic numerical model has been optimized to simulate the Bremsstrahlung emission and the spectral lines of W and W+ ranging from 180 nm to 450 nm at the early stage of plasma in a nanosecond laser ablation. In the literature, most of the researchers pay more attentions on the spectral lines but the Bremsstrahlung emission (continue background) is neglected. In our study, the relationship between plasma temperature and the continue Bremsstrahlung background is found. The numerical results show that the plasma Bremsstrahlung emission dominated in the early time of laser ablation but exponentially decreases from 10 ns to 160 ns. The intensity of spectral lines of W increase while that of W+ increase firstly, then decrease gradually from 10 ns to 160 ns. In addition, a Q-switch Nd:YAG pulsed laser with 8 ns FWHM and 1064 nm wavelength is used to experimentally study the plasma emission processes. The results indicate that the plasma Bremsstrahlung domains during the early stage of laser ablation, which is agreement with numerical prediction. The spatial integrated intensity of Bremsstrahlung, W-atomic line emission and W-ionic line emission were investigated. We hope that the results would provide a good guidance for optimizing LIBS application for PWI diagnosis of fusion devices.

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.

On the influence of the condensed particles on the absorption properties of plasma created by ablation controlled arc in a capillary

The results of experimental studies of the absorption properties of plasma created by ablation controlled arc in a capillary are presented. It is shown that the dominant influence on the plasma absorption properties is exerted by condensed particles formed in relatively low-temperature zones in the vicinity of the capillary wall and on the periphery of the plasma jet, whereas the plasma bremsstrahlung is optically thin. The nonmonotonic behavior of the plasma optical thickness in the spectral range Δλ=400−700 nm, as well as amplification of the probing radiation in a relatively narrow wavelength interval Δλ=628±5 nm, caused, probably, by resonant excitation of condensed particles by electromagnetic radiation, are detected. The estimations of the condensed particles parameters (the average size dD≈2−4 nm, the concentration ND=(1−5)·1013 cm−3, the volume fraction fV≈(0.1−3)·10−6), which quantitatively consistent with the results of studies of the microstructure of the condensed phase on scanning electron microscope, have been obtained.

Space-Resolved VUV and EUV Spectrometers in HL-2A

A normal incidence vacuum ultraviolet (VUV) and a grazing incidence extreme ultraviolet (EUV) spectrometers have been developed for the edge and core impurity measurements in HL-2A tokamak. The VUV and the EUV spectrometers cover wavelength ranges of 300∼3200 Å and 50∼500 Å, respectively. A spatial resolution of 2 mm has been achieved for the VUV spectrometer when a space-resolved slit 50 μm in width is used. The space-resolved slit is placed between the entrance slit and the grating of the spectrometer. The spectral resolutions of 0.15 Å for the VUV spectrometer in the wavelength coverage of the concave 1200 grooves/mm grating and of 0.22 Å for the EUV spectrometer at λ=200 Å with a flat-field laminar-type holographic grating are obtained. The sensitivity of the VUV spectrometer was calibrated in situ with the plasma bremsstrahlung radiation. The experimental results from both spectrometers are presented, especially the line intensity radial profiles measured by the VUV spectrometer.

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.

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.

The effect of magnetic field strength on the time evolution of high energy bremsstrahlung radiation created by an electron cyclotron resonance ion source

An electron cyclotron resonance (ECR) ion source is one of the most used ion source types for high charge state heavy ion production. In ECR plasma the electrons are heated by radio frequency microwaves in order to provide ionization of neutral gases. As a consequence, ECR heating also generates very high electron energies (up to MeV region) which can produce a vast amount of bremsstrahlung radiation causing problems with radiation shielding and heating superconducting cryostat of an ECR ion source. To gain information about the time evolution of the electron energies in ECR plasma radial bremsstrahlung measurements were performed. JYFL 14 GHz ECR ion source was operated in pulsed mode and time evolution measurements were done with different axial magnetic field strengths with oxygen and argon plasmas. Bremsstrahlung data were analyzed with a time interval of 2 ms yielding information at unprecedented detail about the time evolution of high energy bremsstrahlung radiation from an ECR ion source. It was observed, for example, that reaching the steady state phase of the plasma bremsstrahlung requires several hundred milliseconds and the steady state time can be different with different gases.

A study of the Bremsstrahlung of plasma at about 1 A.U. in times of quiet sun and flare activity

In space plasma, especially in the interior of plasma with weak magnetic field, Bremsstrahlung is the main mechanism of loss of the plasma energy. As shown by our calculations of the Bremsstrahlung of plasma at 1 A.U. in times of quiet sun and flare activity, the variation of intensity of plasma radiation If is directly related to the frequency of the electromagnetic wave. When the frequency f is close to that of plasma fpe, the intensity of radiation is greatly enhanced. With increasing frequency, the enhancement of the radiation intensity with frequency exhibits a slow logarithmic decrease. The brightness temperature Tb is directly proportional to the electron temperature of plasma Te and to the optical depth of medium. The overall tendencies of variations of Tb and If with frequency are identical. For the same frequency, the values of If and Tb are lower in time of quiet sun than in time of flare activity.

Reply to ‘Comment on “Collective effects in bremsstrahlung in plasmas” ’

We reply here to a criticism of our paper (Tsytovich et al. 1996) by Iglesias (1997).In our paper we present a very general formulation of collective effects in bremsstrahlung that is valid for any non-equilibrium non-Maxwellian particle distribution. This result is given in (2.20) early in the paper. The standard treatments of bremsstrahlung found in books like Bekefi (1966) are only for thermal plasmas, where the fluctuation–dissipation theorem is valid. Note that the fluctuation–dissipation theorem cannot be used for non-thermal or non-dipole fields, and in this respect the method we use is more general. Our method is the more complex of the approaches used, but, as stated, it can handle situations that cannot be treated by the standard approach. Our main result is the formula (2.20), which is valid for any non-equilibrium non-Maxwellian particle distribution, and which cannot be found anywhere else in the literature. Furthermore, we find new qualitative effects indicating that the ion–ion bremsstrahlung (which is always neglected in the literature) is not small in the case where the collective effects are taken into account, and is in fact, for certain frequencies, of the order of the electron–electron bremsstrahlung. The other qualitatively new result is that, where collective effects are important, the electron–electron bremsstrahlung is not of the order v2Te/c2, as it is for the case in the absence of collective effects, but of the order ω2pe/ω2 times less – which, for example in the solar interior, where ω2pe/ω2 is of the order of v2Te/c2, is then of the order of v4Te/c4.

Comment on ‘Collective effects in bremsstrahlung in plasmas’

In a recent study Tsytovich et al. [J. Plasma Phys. 56, 127 (1996)] claimed to obtain new expressions for electron–ion bremsstrahlung in plasmas. It is shown, however, that they interpreted earlier work incorrectly and simply reproduced well-known results.

Collective effects in bremsstrahlung in plasmas

The results of recent developments in the theory of fluctuations in plasmas show that the previously used theory of bremsstrahlung is incomplete and the exact expressions for bremsstrahlung should include transition bremsstrahlung. The collective effects in bremsstrahlung known previously as Debye screening are changed to a qualitatively different structure, which removes the effect of ion polarization in bremsstrahlung and introduces a new effective polarization which depends on an effective ion charge and electron velocity. The results may be relevant for applications in plasmas when the wavelength is greater than the Debye length. It is shown that for the problem of photon transport in the solar interior the correct collective corrections to the bremsstrahlung change the opacity by only about −0·35%, which is less than was calculated previously when collective effects in bremsstrahlung where estimated without taking recent results of plasma fluctuation theory into account.

Camera for imaging hard x rays from suprathermal electrons during lower hybrid current drive on PBX-M

During lower hybrid current drive on tokamaks, suprathermal electrons are generated that emit hard x-ray bremsstrahlung. A pinhole camera has been installed on the PBX-M tokamak that records 128×128 pixel images of the hard x-ray plasma bremsstrahlung emitted in the 30–200 keV photon energy range. The camera has a temporal resolution of 3 ms and a spatial resolution of 3–4 cm in the plasma. The detector is a 230-mm-diam hard x-ray image intensifier. This instrument has identified hollow radiation profiles on PBX-M, indicating off-axis current drive. A detailed account of the construction of the hard x-ray camera, its operation, and its performance is given.

Continuum wave functions and bremsstrahlung in plasmas based on the ion-sphere and jellium models

We determine the density of states and subject the continuum wave functions to normalization and asymptotic conditions consistent with the ion-sphere and jellium models of the plasma. To illustrate the difference between the two models, we present bremsstrahlung calculations for aluminum and iron plasmas. We also compare the exact quantum-mechanical results with the predictions of the Born-Elwert approximation and with some predictions based on phase-shift calculations. Finally, we discuss briefly the effect of a non-Maxwellian electron distribution.

BREMSSTRAHLUNG IN PLASMAS

This paper investigates the Bremsstrahlung in plasmas from the point of view of quantum statistics, thus eliminating the divergence which exists in the classical theory. We have treated the ion shielding effect rather strictly and obtained an expression clearly showing its influence. Finally, the corresponding absorption coefficient is given.

Correct values for high-frequency power absorption by inverse bremsstrahlung in plasmas

Numerical corrections are made to previous values for the power absorption coefficient and some common errors noted.

X-Ray Production in Laser-Heated Plasmas

The efficiency of converting focused laser radiation into kilovolt x rays by means of a high-z planar plasma has been calculated. For electron temperatures of several keV, the rapid expansion of the plasma limits the effective absorption and reradiation times to a few tenths of a nanosecond. This short time limits the fractional degree of ionization to 30%–60%, depending on the temperature, for heavy target ions such as gold, and in this case the predominant ions have ionization energies less than the electron temperature. For plasma temperatures of 3 up to 10 keV the total radiation output will range from 6 down to 2 times that of plasma bremsstrahlung alone. The conversion efficiency is relatively insensitive to the laser frequency and to the plasma temperature, and amounts to 2%–5%. Overall efficiency for a Nd-glass laser system turns out to be about 10−3.

Space, Time, and Energy Distributions of Neutrons and X Rays from a Focused Plasma Discharge

The energy spectrum and spatial distribution of neutrons emitted by a plasma focus device were measured with nuclear emulsions and scintillation detectors, and the results are reported. The average energy of the D-D neutrons emitted along the axis was shifted ∼500 keV corresponding to an axial center-of-mass velocity of 2 × 108 cm/secfor the reacting deuterons. An 11% anisotropy in the neutron fluxes measured in the forward and radial directions correlates with an axially streaming plasma which is not isotropic; the reacting deuterons collide predominantly in the radial direction. Time-resolved collimation measurements of the emitted neutrons showed an axial translation of the neutron source corresponding to velocities up to 2 × 108 cm/sec; this correlates with the nonsimultaneous pinching of the noncylindrical plasma along the axis. Plasma densities of ∼ 2 × 1020cm−3 and ion temperatures of 2 keV were consistent with observed neutron yields of 1010 and pulse widths of 60 nsec. X-ray intensity measurements were made for x-ray energies of 7 < Ep < 30 keV and showed an Ep−2 dependence which does not agree with plasma bremsstrahlung, but appears to result from anode bombardment of axially accelerated electrons with energies > 200 keV. The line radiation corresponded to that of fairly cold ions.