X-ray Pulse Height Analysis Research Articles

Theoretical interpretation of W soft X-ray spectra collected by the pulse height analysis system on Wendelstein 7-X stellarator

In many fusion devices, such as tokamaks or stellarators like Wendelstein 7-X (W7-X), soft x-ray pulse height analysis (PHA) system diagnostics are routinely used during the experiments. The PHA system is dedicated to providing information about the impurity content, and average along line-of-sight electron temperature in the plasma conditions. Moreover, it is also able to estimate impurity density and an average effective charge from the comparison of experimental spectra with the modeled ones. However, the experimental x-ray spectra can be interpreted in terms of interesting plasma parameters only when the theoretical radiation models first identify and then take into account all the relevant factors that affect the spectrum. Therefore, for this purpose, a theoretical model has been applied. Flexible Atomic Code, which allows for calculation of various atomic properties such as energy levels, cross sections for excitation and ionization by electron impact, transition probabilities for radiative transitions and autoionization, and any others as needed in the collisional–radiative approximation. The chosen spectra collected during the W7-X campaign (OP1.2b) were examined, trying to obtain an agreement between the observed and simulated spectra. The analysis carried out allowed for a reliable interpretation of experimental x-ray spectra, estimation of the electron temperature, and obtaining information on the content of tungsten impurities.

Microcalorimeter measurement of x-ray spectra from a high-temperature magnetically confined plasma.

A NASA-built x-ray microcalorimeter spectrometer has been installed on the MST facility at the Wisconsin Plasma Physics Laboratory and has recorded x-ray photons emitted by impurity ions of aluminum in a majority deuterium plasma. Much of the x-ray microcalorimeter development has been driven by the needs of astrophysics missions, where imaging arrays with few-eV spectral resolution are required. The goal of our project is to adapt these single-photon-counting microcalorimeters for magnetic fusion energy research and demonstrate the value of such measurements for fusion science. Microcalorimeter spectrometers combine the best characteristics of the x-ray instrumentation currently available on fusion devices: high spectral resolution similar to an x-ray crystal spectrometer and the broadband coverage of an x-ray pulse height analysis system. Fusion experiments are increasingly employing high-Z plasma-facing components and require measurement of the concentration of all impurity ion species in the plasma. This diagnostic has the capability to satisfy this need for multi-species impurity ion data and will also contribute to measurements of impurity ion temperature and flow velocity, Zeff, and electron density. Here, we introduce x-ray microcalorimeter detectors and discuss the diagnostic capability for magnetic fusion energy experiments. We describe our experimental setup and spectrometer operation approach at MST, and we present the results from an initial measurement campaign.

Plasma impurities observed by a pulse height analysis diagnostic during the divertor campaign of the Wendelstein 7-X stellarator.

The paper reports on the optimization process of the soft X-ray pulse height analyzer installed on the Wendelstein 7-X (W7-X) stellarator. It is a 3-channel system that records X-ray spectra in the range from 0.6 to 19.6 keV. X-ray spectra, with a temporal and spatial resolution of 100 ms and 2.5 cm (depending on selected slit sizes), respectively, are line integrated along a line-of-sight that crosses near to the plasma center. In the second W7-X operation phase with a carbon test divertor unit, light impurities, e.g., carbon and oxygen, were observed as well as mid- to high-Z elements, e.g., sulfur, chlorine, chromium, manganese, iron, and nickel. In addition, X-ray lines from several tracer elements have been observed after the laser blow-off injection of different impurities, e.g., silicon, titanium, and iron, and during discharges with prefill or a gas puff of neon or argon. These measurements were achieved by optimizing light absorber-foil selection, which defines the detected energy range, and remotely controlled pinhole size, which defines photon flux. The identification of X-ray lines was confirmed by other spectroscopic diagnostics, e.g., by the High-Efficiency XUV Overview Spectrometer, HEXOS, and high-resolution X-ray imaging spectrometer, HR-XIS.

First Results from the Soft X-ray Pulse Height Analysis System on Wendelstein 7-X Stellarator

The paper presents experimental results obtained during the first operational phase (called OP1.1) of the Wendelstein 7-X stellarator (W7-X). One of W7-X diagnostics that delivers information about plasma impurities and was commissioned during the OP1.1, is the Pulse Height Analysis (PHA) system which provides soft X-ray spectra of highly ionized impurity species over a very broad energy range with energy resolution not worse than 200 eV and with temporal resolution of 100 ms. The line-of-sight integrated spectra, collected by a Silicon Drift Detector in the energy range between 0.9 and 10 keV, have been analyzed for various experimental conditions and are discussed with respect to results from other diagnostics. In this work, injected impurities (argon and neon) as well as intrinsic impurities (sulphur and chlorine) have been identified. Spectral lines corresponding to heavier impurities have not been observed.

Study of Inverted Sawtooth Activities in EAST Plasma

On the EAST tokamak, during whole off-axis LHCD (low hybrid current drive) limiter discharge, inverted sawteeth oscillations on the SXR (soft X-ray camera) signals appear continuously, and no positive sawtooth is observed. It is thought that this phenomenon is caused by the curvature pinch, though it could partly be explained by the electron temperature profile observed on the PHA (soft X-ray pulse height analyzer) system. The off-axis LHCD and Ohmic heating generate a non-monotonic q profile. According to the curvature pinch effect, this q profile leads to a special electron density profile that has a valley ring. The non-monotonic q profile and the special electron density profile lead to this interesting phenomenon.

Development of a soft X-ray pulse height analyzer on the J-TEXT tokamak

A soft X-ray pulse height analyzer has been designed and installed on the J-TEXT tokamak for measuring the electron temperature and possibly for monitoring the impurity content and concentration. A multi-channel analyzer, a set of amplifiers, and three silicon drift detectors constitute the detection system. An off-line analysis code was developed to analyze the raw data recorded by the system. The system can measure a minimum electron temperature of 400eV, and its routine operation provides electron temperature measurements at three different positions with a temporal resolution of 60ms and channel separation of 18cm. Line radiation from some moderate and high Z impurities are observable by the system. The results show that the intensity of line radiation from heavy impurity ions is inversely proportional to the plasma density under similar discharge conditions.

Soft X-Ray Diagnostics on LHD

Several types of diagnostic system for soft X-ray radiation (SXR) have been developed and installed on the Large Helical Device (LHD). Magnetohydrodynamic (MHD) activities are studied by the fluctuation component of SXR, and the electron temperature is measured from the spectrum of the SXR. PIN photodiode arrays are installed at three poloidal cross sections to study the equilibrium and fluctuations. The spatial resolution of an array is a few centimeters, and its frequency response is up to ~300 kHz. Absolute extreme ultraviolet diode arrays are also used for lower-energy radiation, and edge MHD instabilities are thereby studied. There are tangentially viewing two-dimensional soft X-ray camera systems by which more detailed spatial structure can be studied. Radial profile of X-ray spectrum is measured by X-ray pulse height analyzer systems.

Preliminary observation of medium-Z impurities by means of the soft x-ray pulse height analysis diagnostic on EAST

A soft x-ray pulse height analysis (PHA) system, based on a silicon drift detector (SDD), has been successfully implemented on EAST to measure spectra of soft x-ray emission. Utilizing the soft x-ray PHA diagnostic, the behavior of intrinsic and injected medium-Z impurities is first investigated on EAST and the preliminary observations are presented. There are hardly any Kα emission peaks of metallic impurities including Ti (Z = 22), Cr (Z = 24), Fe (Z = 26), Ni (Z = 28) and Cu (Z = 29) in the soft x-ray spectra for an ohmic heating plasma due to lower electron temperatures, while remarkable peaks can be seen in a lower hybrid wave (LHW) heated plasma. The profiles of Kα intensities of these metallic elements have been obtained with about 7 cm spatial resolution. It is observed that the Kα emissions come mainly from the core plasma column of ρ ⩽ 0.3 and found that their intensities are roughly proportional to the power of the LHW launched into the plasma. The Ar Kα peaks can also be monitored when argon gas is puffed into the plasma. It is qualitatively confirmed that the Kα lines of argon emitted from the lower ionic states are predominantly located in the outer plasma region, whereas the Kα lines with higher energies are mainly from high ionization stages in the central region.

First results obtained from the soft x-ray pulse height analyzer on experimental advanced superconducting tokamak

An assembly of soft x-ray pulse height analyzer system, based on silicon drift detector (SDD), has been successfully established on the experimental advanced superconducting tokamak (EAST) to measure the spectrum of soft x-ray emission (E=1-20 keV). The system, including one 15-channel SDD linear array, is installed on EAST horizontal port C. The time-resolved radial profiles of electron temperature and K(alpha) intensities of metallic impurities have been obtained with a spatial resolution of around 7 cm during a single discharge. It was found that the electron temperatures derived from the system are in good agreement with the values from Thomson scattering measurements. The system can also be applied to the measurement of the long pulse discharge for EAST. The diagnostic system is introduced and some typical experimental results obtained from the system are also presented.

Soft x-ray PHA measurement of the long pulse discharge in HT-7 tokamak

By analyzing the soft x-ray energy spectrum measured by the soft x-ray pulse height analysis (PHA) system, the electron temperature ( T e ) and the effective charge number ( Z eff ) of the ultra-long pulse discharge driven by lower hybrid wave (LHW) were obtained in the HT-7 tokamak. Moreover, the information of medium- Z impurities such as Ti, Cr, Fe, and Ni intrinsic to HT-7 tokamak can also be inspected. The accuracy of the electron temperature derived from the soft x-ray energy spectrum measurements is verified by comparing with the temperature measured by the Thomson scattering system for various plasmas and electron cyclotron emission diagnostic system for ohmic plasmas. The bulk electron temperature of about 1 keV and Z eff ≈ 2 were achieved for long pulse plasma. The appreciable K α lines of Ti, Cr, Fe and Ni metallic impurities released from the antennas of radio frequency wave and/or the first wall and Ar injected into plasma can be observed, and they kept stable during the long duration discharges. As a result, the longest pulse discharge with relatively high temperature of T e ( 0 ) ∼ 1 keV , and n e ( 0 ) ∼ 0.5 × 10 19 m − 3 has been achieved with a duration of 400 s in the HT-7 experimental campaign in 2008.

Observation of Space and Energy Distributions of High Energy Electrons Produced in ECH Plasmas of LHD

Hard x-ray spectrum has been studied with x-ray pulse-height analyzer (PHA) in Large Helical Device (LHD). The hard x-ray emissions have been observed in high temperature plasmas heated by electron cyclotron heating (ECH). The radial distribution of the hard x-ray emissions is quantitatively derived from the line-integrated spectrum through an Abel inversion. When the bulk electron density ranges at 2.0 × 1018 m−3 , the hard x-ray emissions are measured to be the most intense. From an analysis on the spectrum it is quantitatively proved that the energy-distribution function of the suprathermal electrons produced by ECH remarkably deviates from that of thermal electrons. The relaxation time of the high energy electrons is estimated to be 70 ms from the exponential decay rate after turning off the ECH pulse.

A new soft x-ray pulse height analysis array in the HL-2A tokamak

A new soft x-ray pulse height analysis (PHA) array including nine independent subsystems, on basis of a nonconventional software multichannel analysis system and a silicon drift detector (SDD) linear array consisting of nine high performance SDD detectors, has been developed in the HL-2A tokamak. The use of SDD has greatly improved the measurement accuracy and the spatiotemporal resolutions of the soft x-ray PHA system. Since the ratio of peak to background counts obtained from the SDD PHA system is very high, p/b > or = 3000, the soft x-ray spectra measured by the SDD PHA system can approximatively be regarded as electron velocity distribution. The electron velocity distribution can be well derived in the pure ohmic and auxiliary heating discharges. The performance of the new soft x-ray PHA array and the first experimental results with some discussions are presented.

Soft X-ray PHA Diagnostic for the Electron Temperature Measurements on EAST

Two soft X-ray pulse height analysis (PHA) systems, based on silicon drift detector (SDD), were established on the experimental advanced superconducting tokamak (EAST) to measure spectrum of soft X-ray emission. In this paper, the soft X-ray PHA systems on EAST are introduced and some experimental results of electron temperature in EAST obtained from the soft X-ray PHA diagnostic are presented.

Calibration of an electron cyclotron emission diagnostic system and electron temperature measurements on HT-7 tokamak

Recently, a broadband heterodyne radiometer has been installed on HT-7 tokamak to measure second harmonic X-mode electron cyclotron emission (ECE) at a frequency range 98–126 GHz, which is the only ECE detectable mode with optical depth greater than 1 in the present study. The high temporal resolution (4 µs) diagnostic system is now operational with 16 detection channels separated by approximately 2 cm on the radial axis for measuring electron temperature (Te) on the low-field side for typical machine operation at a toroidal magnetic field (BT) of 1.0–2.5 T. The results of electron temperature measurements are presented along with the calibration of the diagnostic system. In order to cross calibrate the ECE system, the electron temperature data obtained have been compared to other Te measuring diagnostics, i.e. Thomson scattering and soft x-ray pulse height analysis (PHA). A radial electron temperature profile during the ohmic heating phase has been presented and analyzed. Future plans for the diagnostics of temperature fluctuations and measurements are also discussed.

Impurity Transport Study of Medium-Z Argon by Means of Soft X-Ray Pulse Height Analyzer in LHD

Radial profiles of impurity transport coefficients of argon have been successfully obtained in Large Helical Device, using an assembly equipped with conventional semiconductor detectors and soft x-ray pulse-height analyzers. Several fixed argon discharges have enabled the radial scanning of the assembly to measure the radial profiles of argon-Kα lines. The present experimental results indicate that the impurity transport study becomes possible with the soft x-ray pulse-height analyzers.

Hard X-Ray PHA System on the HT-7 Tokamak

A new hard X-ray pulse-height analysis (PHA) system has been established on HT-7 tokamak for long pulse steady-state operation. This PHA system consists of hard X-ray diagnostics and multi-channel analysers (MCA). The hard X-ray diagnostics consists of a vertical X-ray detector array (CdTe) and a horizontal X-ray detector array (NaI). The hard X-ray diagnostics can provide the profile of power deposition and the distribution function of fast electron during radio frequency (RF) current drive. The MCA system is the electronic part of the PHA system, which has been modularized and linked to PC through LAN. Each module of MCA can connect with 8 X-ray detectors. The embedded Ethernet adapter in the MCA module makes the data communication between PC and MCA very convenient. A computer can control several modules of MCA through certain software and a hub. The RAM in MCA can store 1024 or more spectra for each detector and therefore the PHA system can be applied in the long pulse discharge of several minutes.

The Electron Temperature Estimation Using Soft X-Ray Imaging in HT-7 Tokamak

In order to estimate the electron temperature soft x-ray imaging diagnostics using a double filter technique has been developed in the HT-7 tokamak. The chosen thicknesses of the Be foil are 12.5 μm and 70 μm, respectively. In this article both the main design of the diagnostic configuration and the method to estimate the electron temperature are presented. The results agree with those estimated from the soft x-ray pulse height analyzer (PHA). The main causes of systematic error have also been investigated.

New Soft x-ray PHA Diagnostic in the HT-7 Tokamak

A soft x-ray Pulse Height Analysis (PHA) diagnostic with the recently introduced Silicon Drift Detector (SDD) and a new MCA system has been installed in HT-7 for the measurements of electron temperature. With the SDD detector, the x-ray emission can be detected in the energy range from 1 keV up to 20 keV. The extremely low anode capacity of SDD allows very high counting rates of x-ray above 200 kHz and short shaping times below 0.25 μs to be measurede. In routine operation a counting rate of 120 kHz has been normally obtained with an energy resolution of better than 180 eV at 5.9 keV. The assembly is equipped with six SDD detectors measuring the soft x-ray emission integrated along six chords of the upper half plasma cross section. The preliminary results from the new soft x-ray PHA diagnostic are also presented in this paper.

Soft x-ray pulse height analyzer in the HT-7 tokamak

A new soft x-ray pulse height analyzer (PHA), based on a Silicon Drift Detector (SDD) linear array consisting of 15 SDD detectors, has been installed in the HT-7 tokamak. The energy resolution of 150–180 eV at photon energy of 6 keV is achieved for the SDD with Peltier cooling. The effective time response of the SDD PHA is 50 ms. The profiles of electron temperature and the intensity of metallic impurities can be obtained with a spatial resolution of 3 cm. The performance and first experimental results from the new PHA system are presented.

Concepts of x-ray diagnostics for WENDELSTEIN 7-X

The use of superconducting coils in W7-X will allow to run very long pulses (up to 30 min). Therefore enhanced demands have to be met for plasma diagnostics with respect to hardware components exposed to steady state heat fluxes, but also to the data acquisition and analysis. A multicamera x-ray tomography system inside the vacuum vessel—because of lack of suitable ports—with about 400 viewing chords distributed along the poloidal direction is proposed for magneto hydrodynamics investigations. The system will consist of about 20 compact cameras containing linear photodiode arrays, beryllium-transmission foils, and preamplifiers. A particular technical issue is the control of detector/amplifier offset and gains in steady state plasmas. Another multipurpose x-ray camera system with interchangeable filters will be installed using a particular port combination. In addition x-ray pulse height analysis will be used to deduce spectroscopic information and to provide the central electron temperature.