Charge Exchange Neutral Particles Research Articles

Development of a quartz crystal microbalance diagnostic for measuring material erosion and deposition on the first wall in EAST.

A quartz crystal microbalance (QMB) diagnostic system has been established in Experimental Advanced Superconducting Tokamak (EAST) for real-time and in situ measurements of erosion and deposition rates of plasma-facing materials at the first wall. A ∼70 nm aluminum (Al) film has been coated on the QMB crystal surface to measure the erosion rate by charge exchange neutral particles. Dual sensors of the QMB system have been used with a closed sensor for reference. The stability and light sensitivity of the QMB system have been tested in the lab, demonstrating its feasibility on the application of EAST experiments. The QMB system with cooling water has been successfully applied in the 2018 EAST campaign. The net erosion thickness measured by the QMB has been well validated by thickness measurements using the Rutherford backscattering spectrometry. The developed QMB systems can help us to understand the physics processes of material erosion and deposition at main chamber walls for long pulse operations in EAST.

Enhancement of an E parallel B type neutral particle analyzer with high time resolution in the Large Helical Device

In order to study fast ion confinement and transport mechanisms both in quiescent plasmas and in the presence of magneto-hydrodynamic (MHD) activity, an electric field parallel to the magnetic field type neutral particle analyzer (E||B-NPA) was installed using Micro-Channel Plates (MCPs) in the Large Helical Device (LHD). The E||B-NPA is an established diagnostic used to measure the energy distribution of charge exchange neutral particles escaping from plasmas corresponding to hydrogen, deuterium, and tritium, respectively. The LHD deuterium experimental campaign started in 2017. To obtain more details of MHD behavior, we improved on time resolution performance of the E||B-NPA using a high-speed Pre-Amplifier and Discriminator (PAD) and Latching Scaler Module (LSM). For the last LHD experimental campaign, the E||B-NPA was installed on nearly the equatorial plane of the vacuum chamber to measure the passing energetic particles generated by tangentially injected neutral beams. As the results of the E||B-NPA commissioning, it is confirmed that the E||B-NPA successfully detected the energetic particles and obtained the energy distributions corresponding to hydrogen. Also, the effect of the neutron to E||B-NPA is estimated using the other neutron diagnostics during the experiments.

Ellipsometric and electron spectroscopic study of degradation of optical properties in Mo mirror irradiated with deuterium and/or helium ions

Degradation of optical properties in Mo mirrors irradiated with helium and/or deuterium ions has been studied by ellipsometric spectroscopy, electron energy loss spectroscopy, and electron microscopy. It was found that values of the optical constants (reflectivity, refractive index, and extinction coefficient) of the specimens decreased in the order of deuterium, deuterium and helium dual-beam, and helium irradiations. The penetration depth of the laser light in the specimen, estimated from the observed extinction coefficient, increased in the same order. Electron energy loss spectroscopic study on an individual helium bubble formed in a specimen without surface roughness clearly revealed that the reflectivity decreased owing to a change in the di-electric constants caused by the formation of the bubble, and it was associated with a decrease in the optical constants. These results and the microstructure observation indicate that the degradation of reflectivity originating from the helium bubble is the primary issue affecting a diagnostic mirror irradiated with charge-exchange neutral particles.

Design of a retarding potential grid system for a neutral particle analyzer.

The ion energy distribution in a magnetically confined plasma can be inferred from charge exchange neutral particles. On the Madison Symmetric Torus (MST), deuterium neutrals are measured by the Florida A&M University compact neutral particle analyzer (CNPA) and the advanced neutral particle analyzer (ANPA). The CNPA energy range covers the bulk deuterium ions to the beginning of the fast ion tail (0.34-5.2 keV) with high-energy resolution (25 channels) while the ANPA covers the vast majority of the fast ion tail distribution (∼10-45 keV) with low energy resolution (10 channels). Though the ANPA has provided insight into fast ion energization in MST plasma, more can be gained by increasing the energy resolution in that energy range. To utilize the energy resolution of the CNPA, fast ions can be retarded by an electric potential well, enabling their detection by the diagnostic. The ion energy distribution can be measured with arbitrary resolution by combining data from many similar MST discharges with different energy ranges on the CNPA, providing further insight into ion energization and fast ion dynamics on MST.

Deposition/erosion and H/D retention characteristics in gaps of PFCs in KSTAR studied by cavity technique

Stainless steel coupons with cavity structure were installed at different poloidal locations in KSTAR during 2009 campaign to study deposition and H/D retention inside the gap of PFCs. The deposition profiles at different poloidal locations indicate that the incoming species have high surface loss probability (β) and they are probably charge exchange neutral particles. The (H + D)/C ratios of layers inside the coupons were in a range from 0.04 to 0.37 at different poloidal locations which might be caused by different contribution of different hydrocarbon species. Campaign integrated, time averaged net C, H/D atom flux density towards outer walls are in the similar range with other carbon dominated machines.

Calculation of low-Z impurity pellet induced fluxes of charge exchange neutral particles escaping from magnetically confined toroidal plasmas

Measurements of energy- and time-resolved neutral hydrogen and helium fluxes from an impurity pellet ablation cloud, referred to as pellet charge exchange or PCX experiments, can be used to study local fast ion energy distributions in fusion plasmas. The estimation of the local distribution function f(i)(E) of fast ions entering the cloud requires knowledge of both the fraction F(0)(E) of incident ions exiting the cloud as neutral atoms and the attenuation factor A(E,rho) describing the loss of fast atoms in the plasma. Determination of A(E,rho), in turn, requires the total stopping cross section sigma(loss) of neutral atoms in the plasma and the Jacobian reflecting the measurement geometry and the magnetic surface shape. The obtained functions F(0)(E) and A(E,rho) enter multiplicatively into the probability density for escaping neutral particle kinetic energy. A general calculation scheme has been developed and realized as a FORTRAN code, which is to be applied for the calculation of f(i)(E) from PCX experimental results obtained with low-Z impurity pellets.

Neutron Emission Profile Measurement and Fast Charge Exchange Neutral Particle Flux Measurement for Transport Analysis of Energetic Ions in JT-60U

This paper describes diagnostics for transport analysis of energetic ions in JT-60U. Line integrated neutron emission profiles are measured using Stilbene neutron detectors, which are installed as a neutron emission profile monitor. The flux and energy distribution of charge exchange neutral particles are measured using a natural diamond detector (NDD). These measurements in the presence of bursting instabilities in the frequency range of Alfven Eigenmode induced by negative-ion-based neutral beam injection, which are called Abrupt Large-amplitude Events, indicate that energetic ions are transported from the core region in the plasma due to a resonance interaction between energetic ions and the instabilities. Development of a digital signal processing (DSP) system for a neutron detector and a NDD enabling a sampling rate in the MHz class is proceeding. The DSP system for the neutron detector successfully discriminates between neutrons and gamma-rays in tests using neutron/gamma-ray sources.

New 20-Channel Diagnostic for Angle-Resolved Fast Particles Measurements in LHD

A new multi-channel diagnostic for fast particles has been developed and successfully tested on the Large Helical Device (LHD) during 2005-2006 experimental campaign. The number of simultaneously used channels was significantly improved from 2 to 20 channels and additional improvements for noise reduction have been made. Same time location of the diagnostic has been changed that allowed one to make measurements in a much wider range of pitch angles from perpendicular to tangential (90-160 degrees). All these improvements allow one to make time, energy, and angle-resolved measurements of charge exchange neutral particles in a single plasma discharge and to check the presence of fast particles loss-cones from LHD plasma in different heating regimes. This new diagnostic can be a very helpful and powerful tool in studying of fast particle distribution in such a complex helical plasma geometry like the one of LHD. Example data from plasma discharges are presented.

Initial angle resolved measurements of fast neutrals using a multichannel linear AXUV detector system on LHD

A new multichannel diagnostic for fast ion distribution studies has been developed and successfully tested on the Large Helical Device (LHD) in different plasma heating conditions. The diagnostic is based on a linear array AXUV detector consisting of 20 segments, charge sensitive preamplifiers, and a set of pulse height analysis channels. The main advantage of this system is the possibility to make time, energy, and angle-resolved measurements of charge exchange neutral particles in a single plasma discharge. This feature makes the new diagnostic a very helpful and powerful tool intended to contribute to the understanding of fast ion behavior in a complex helical plasma geometry like the one of LHD.

Experimental estimation of the local energy balance of the potential-confining electrons in tandem-mirror plasmas

Our proposed “matrix-type” semiconductor detectors are applied for studying the local energy balance of bulk electrons in the tandem-mirror GAMMA 10. The matrix-type detector array consists of compactly produced six “rows” having different thicknesses of thin dead layers (SiO2) on its surface. Each row has seven channel units (“columns”) for measuring radiation profiles in the radial direction of plasmas. These various SiO2 layers are, thus, employed as “unbreakable ultrathin radiation-absorption filters” having various thicknesses to distinguish x rays from charge-exchange neutral particles and analyze the radial profiles of both plasma ion and electron temperatures simultaneously. The radial profiles of the energy confinement time and the thermal diffusivity obtained from the local energy balance analysis imply that the improvement of the plasma confinement is associated with the strong shear of radial electric fields due to a high plasma confining potentials.

Analysis of charge-exchanged neutral particles during IBW heating in the HT-7 tokamak

Abstract The charge-exchange neutral particles fluxes and energy distribution in IBW heated plasma were investigated in the HT-7 tokamak. The RF frequency was 30 MHz and with an injecting power up to 200 kW. It is observed that the plasma performance is obviously enhanced by IBW heating. The electron temperature was increased by 0.5 keV and the central line averaged electron density was doubled. The neutral particle fluxes of high-energy increased and the bulk ions were heated during IBW heating. The ion temperature was increased by 0.3 keV and the ion heating efficiency of (2–3) eV kW−1 × 1013 cm−3 was achieved. The velocity distribution of charge-exchanged neutral particles appears to be Maxwellian without high-energy tail ions up to the maximum RF power.

A proposal for both plasma ion- and electron-temperature diagnostics under simultaneous incidence of particles and x-rays into a semiconductor on the basis of a proposed model for a semiconductor detector response

A method is proposed for obtaining radial profiles of plasma temperatures of both plasma ion (Ti) and electron (Te) simultaneously by the use of a semiconductor detector array. The method is based on our developed particle-response model for a semiconductor detector; in particular, the response theory is constructed for giving the applicability in particle energies ranging down to a kiloelectronvolt. Calculated results from our model are in fairly good agreement with experimental data on the detector response of incident particle beams with energies in the range 100 eV to a few kiloelectronvolts. On the basis of the verification of the proposed model, an idea of the use of a developed semiconductor detector array covered with `reliably unbreakable' ultrathin SiO2 `dead-layer filters' having various nanometre-order thicknesses is applied for simultaneous Ti and Te analyses by using charge-exchange neutral particles and x-rays from plasmas. Radial profiles of Ti and Te are obtained in a single plasma discharge alone, and the data reliability is independently cross-checked by a radial scan of a conventional charge-exchange neutral-particle analyser system as well as a 50-channel microchannel plate x-ray diagnostics system in the GAMMA 10 tandem mirror.

Simultaneous measurements of temporally and spatially resolved ion and electron temperatures using a semiconductor detector array in a single plasma discharge

A method is proposed for obtaining radial profiles of both plasma ion (Ti) and electron temperatures (Te) simultaneously using a semiconductor detector. This method for semiconductor Ti diagnostics is proposed on the basis of an alternative “positive” use of a semiconductor “dead layer” as an energy-analysis filter. Filtering dependence of charge-exchange (cx) neutral particles from plasmas on the thickness on the order of a nanometer-thick SiO2 layer is used for analyzing Ti in the range from hundreds to thousands of electron volts. Even under the circumstances of simultaneous incidence of such particles and x rays along the same lines of sight of a semiconductor detector array, it is found that the different dependence on their penetration lengths and deposition depths in semiconductor materials makes it possible to distinguish cx neutral particles for Ti diagnostics from x rays for Te diagnostics. Experimental verification of this concept of the simultaneous Ti and Te diagnostics is carried out in the GAMMA 10 tandem mirror by the use of a matrix-type semiconductor detector. The detector is characterized in terms of compact formation of six rows with different thicknesses of thin dead layers (SiO2) on its surface. Each row has seven channels (columns) for measuring radiation profiles in the radial direction of plasmas so as to make tomographic reconstructions for attaining the actual emissivity profiles in the plasmas. These various SiO2 layers are proposed to be applied as “unbreakable ultrathin radiation-absorption filters” having various thicknesses to distinguish x rays from neutral particles simultaneously.

Software and hardware developments for remote participation in TJ-II operation. Proof of concept using the NPA diagnostic

The operation protocols for the TJ-II require that all diagnostics be controlled remotely during machine operation. For this reason, most diagnostics have control systems that permit remote actions to be performed on their various subsystems. An Internet browser, which connects to a server where the human machine interfaces (HMIs) for all diagnostics are situated, is used for this. Real time control has been resolved using programmable automates for each diagnostic. Dedicated application software is in operation to provide a user interface for programming digitisers, for signal visualisation and for data processing during TJ-II discharges. This software is an event-based application that can be remotely launched from any X terminal. Other resources provided to users are graphical and computational tools for data analysis, data compression, off-line access to databases, reception of data and its integration in the databases at any moment. Two neutral particle analysers (NPAs) are operating on TJ-II to measure the energy spectra of charge exchange (CX) neutral particles. Protocols require remote control over several actions and subsystems of this diagnostic. With all these tools a proof of the concept of remote participation has being performed during the present experimental campaign. The NPA diagnostic has been operated from UPC-ETSEIB in Barcelona, 600 km of distance from the machine.

Some characteristics of the predisruption phase in the Damavand tokamak

The time evolution of magnetohydrodynamic activity, charge exchange neutral particles spectra, the line emission of the impurities in the visible spectral range and soft x-ray radiation were investigated in the predisruption phase of the small Damavand tokamak [Amrollahi et al., Plasma Phys. Rep. 23, 561 (1997)]. The experimental data show that the main disruptive processes, particularly the kinetic, take place near the plasma periphery. The experimental results also show that the interaction between high energy ions of bulk plasma and impurity ions may play an important role in the plasma radiation processes. A new mechanism for the plasma radiation processes in the predisruption phase may be obtained that can be considered for the investigation of the disruption instability.

Simultaneous Measurements of Spatial Profiles of Ion and Electron Temperatures Using a Semiconductor Detector Array

A new method is proposed for obtaining radial profiles of both plasma ion (Ti) and electron temperatures (Te) simultaneously using one semiconductor detector array alone. Furthermore, availability of the new idea of the simultaneous Ti and Te diagnostics is experimentally demonstrated by the use of a small-sized semiconductor detector array. This novel method for semiconductor Ti diagnostics is proposed on the basis of an alternative “positive” use of a semiconductor “dead layer” as an energy-analysis filter. Filtering dependence of charge-exchange neutral particles from plasmas on the thickness on the order of nm thick SiO2 layer is used for analyzing Ti ranging from hundreds to thousands eV. In this report, proof-of-principle plasma experiments for the proposed idea are, at first, demonstrated in the GAMMA 10 tandem mirror to verify the availability of this novel idea of distinguishing and identifying each value of Ti and Te by the use of various thin filtering materials. Furthermore, novel experimental data on radial profiles of Ti and Te are simultaneously observed and analyzed using a semiconductor detector array along with the development of a Monte-Carlo computer simulation code for analyzing interactions between semiconductor materials and incident particles. The radial profiles of Ti and Te obtained from semiconductor detectors by the use of the proposed method are found to be in good agreement with those from a charge-exchange neutral-particle Ti analyzer and a microchannel-plate Te detector. Detailed data and analysis method are represented in the paper.

A diagnostic method for both plasma ion and electron temperatures under simultaneous incidence of charge-exchange particles and x rays into a semiconductor detector array

An idea for using semiconductor detectors to simultaneously observe both plasma ion Ti and electron Te temperatures is proposed. The idea is also experimentally verified in tandem-mirror plasma shots. This method is developed on the basis of an alternative “positive” use of a semiconductor “dead layer” as an energy-analysis filter. Filtering dependence of charge-exchange neutral particles from plasmas on the thickness of a thin (on the order of nm thick) SiO2 layer is employed for analyzing Ti in the range from hundreds to thousands of eV. Even under the conditions of simultaneous incidence of such particles and x rays into semiconductor detectors, the different dependence on their penetration lengths and deposition depths in semiconductor materials makes it possible to distinguish particles (for Ti) from x rays (for Te). In this letter, proof-of-principle plasma experiments for the proposed idea are carried out to verify the availability of this concept of distinguishing and identifying each value of Ti and Te by the use of various thin filtering materials prior to the use of thinner dead layers.

New type of compact electrostatic energy analyzer adequate to measurement of charge exchange neutral particles with a low counting yield

A new type of compact electrostatic cylindrical energy analyzer originally providing a deflection angle of π/22 has been designed with the aim of measuring energy spectra of charge exchange neutral particles in the tandem mirror GAMMA10. The essential advantage of the use of this analyzer is that a large entrance aperture can be adopted with high energy resolution so that high incident current is analyzed, while the radius of the cylindrical electrodes remain compact. Characteristics of the analyzer are discussed using computer simulations of ion trajectories including an effect of fringing field obtained by finite-element method as well as small-angle scattering in stripping collisions of hydrogen atoms.

Impurity behavior in Heliotron E

The reduction of metallic impurity was found in a carbon limiter configuration in Heliotron E. In order to investigate the impurity penetration into the plasma, the distance between the outermost magnetic surface and the vacuum vessel wall was varied by inserting a carbon rail limiter. The analysis of vacuum ultraviolet impurity spectra and charge exchange neutral fluxes shows that the decrease of the impurity production rates by charge exchange neutral particles is the most probable reason to explain the experimental results. Moreover the electron temperature and density profiles in the limiter shadow provide an ability to screen metallic impurity influxes from the wall.

Wall conditioning and its effect on RFP plasma performance in REPUTE-1

Results are presented from experiments on the effects of glow discharge and carbonization on RFP plasma performance in REPUTE-1. With the proper choice of wall conditioning, control of plasma density behavior was demonstrated. Radiated power and carbon V intensity decreased by a factor of two after He glow. Using the carbonization technique, decreases in the plasma resistance and ion temperature as derived by the CV line and charge exchange neutral particles were observed, whereas the electron temperature did not change as much.