Charge-exchange Neutral Particle Analyzer Research Articles

Measurements of D/H Ratio Using Compact Neutral Particle Analyzer in LHD Deuterium Experiments

The deuterium/hydrogen ratio in the plasma strongly affects against the plasma heating and other behaviors. The ratio in high energy region is measured by the charge exchange neutral particle analyzer instead of the spectroscopic technique. The ratio has obtained by reducing the electrostatic plate voltage in the compact neutral particle analyzer, which is tuned for hydrogen measurement. When the voltage is adjusted for deuterium, hydrogen simultaneously enters as contamination because the hydrogen orbit is closed to the mid-plane. Here, the mass rejection factor has been evaluated and the time variation of the ratio in the low and high energy regions during hydrogen and deuterium neutral beam injection is described. The behavior of the ratio in high and low energy regions can be explained by the classical slowing down process of the energetic particle.

Recent Progress in Plasma Control Studies on the Improvement of Plasma Performance in Heliotron J

Recent progress in plasma control studies on the improvement of plasma performance in Heliotron J is reviewed. The supersonic molecular beam injection (SMBI) fueling is successfully applied to Heliotron J plasma. A supersonic H2-beam is effectively injected to increase fueling efficiency and generate a peaked density profile. Local fueling with a short-pulsed SMBI can increase the core plasma density and avoid the degradation arising from edge cooling. Second harmonic electron cyclotron current drive (ECCD) experiments were conducted by launching a focused Gaussian beam with a parallel refractive index of −0.05 ≤ N‖ ≤ 0.6. Results show that the electron cyclotron (EC) driven current is determined not only by N‖ but also by local magnetic field (B) structure where the EC power is deposited. Detailed analysis of the observed N‖ and B dependences is in progress with a ray-tracing simulation using the TRAVIS code. Fast ion velocity distribution was investigated using fast protons generated by ion cyclotron resonant frequency (ICRF) minority heating. For the standard configuration in Heliotron J, charge exchange neutral particle analysis (CX-NPA) measurements show higher effective temperature of fast minority protons in the on-axis resonance case compared to that in the HFS (high field side) off-axis resonance case. However, the increase in bulk ion temperature in the HFS resonance case is larger than that in the on-axis resonance.

Ion flow measurements and plasma current analysis in the Irvine Field Reversed Configuration

Measurements of the Doppler shift of impurity lines indicate that there is an ion flow of ∼7 km/s in the Irvine Field Reversed Configuration. A charge-exchange neutral particle analyzer shows the peak energy is below the 20 eV minimum detectable energy threshold, which is in agreement with the spectroscopic data. By evaluating the collision times between the impurities and hydrogen, the dominant plasma ion species, it is concluded that the ions rotate with an angular frequency of ∼4×104 rad/s. Estimates of the ion current in the laboratory frame indicate it is one to two orders of magnitude larger than the measured plasma current of 15 kA. Electron drifts are expected to cancel most of the ion current based on the measured magnetic fields and calculated electric fields.

Comparison of the IOFFE neutral particle analyser with the Princeton analyser on the Mega Amp Spherical Tokamak

Charge exchange-neutral particle analysis (CX-NPA) will be applied for determining the ion temperature in the stellarator Wendelstein 7-X (W7-X), which is under construction, now, at the Max-Planck-Institute of Plasma Physics, Greifswald, Germany. Three ACORD-type analyser and one compact analyser (CNPA) are foreseen for the active NPA diagnostics at W7-X, i.e. NPA in combination with a 60keV neutral beam injector, thus enabling locally resolved CX-NPA measurements. For an intermediate period the CNPA is now installed on the Mega Amp Spherical Tokamak (MAST) in Culham Science Centre, UK.The arrangement of the CNPA on MAST and results of energy spectra of plasma ions in MAST are presented. They are in a good agreement with measurements by the Princeton particle analyser.

Chapter 8: Particle Diagnostics

Three techniques for particle diagnostics of magnetically confined fusion plasmas are reviewed: charge exchange neutral particle analysis, Rutherford scattering, and heavy ion beam probes. The physical basis and instrumentation for each technique are described. Typical examples obtained by these diagnostics are presented. Charge exchange analysis is used for ion temperature measurements in small- and medium-sized plasma devices and for the study of the ion energy distribution function, especially in the suprathermal energy range. It also provides an ion isotope composition determination that is a high-priority task for ITER. Rutherford scattering enables local measurements of the ion temperature as well as measurements of the ion collective velocity. The heavy ion beam probe diagnostic gives a unique opportunity to measure locally the electric plasma potential and, by that, the electric field profile in a plasma and its influence on confinement. Other significant options of this diagnostic are the measurements of the local electron density and fluctuations in these quantities. Prospects for the application of the various techniques to ITER are discussed.

Detection of Bounce Ions by Use of Charge Exchange Bounce Ion Analyzer

Existence of the plug potential (PP) bounce ion is quite essential for effective improvement of axial confinement in the tandem mirror, which is bounced by the confining potential hill. We paid attention to the neutral particles changed from the bounce ions through the charge exchange process and measured simultaneously both energy and emergence angle of the neutral particles by use of a charge exchange neutral particle analyzer for measuring bounce ions located near the inner mirror throat (IMT) of the plug barrier cell. We detected successfully the bounce ions during confining potential formation and assigned the bounce ions to the PP bounce ion and the OMT bounce ion which is bounced near the outer mirror throat (OMT) of the plug/barrier cell. The trajectories of the PP bounce ion were calculated, and it was found that the confinement of the PP bounce ion was sensitive to the radial profile of the confining potential in relation to the radial transport.

Direct measurements of the electrostatically and magnetically bounced ions in the tandem mirror

Existence of the bounced ion by the plug potential (PP) bounce ion is essential in order to improve the ion confinement in the tandem mirror. The trajectories of the PP bounce ion were calculated on the assumption that the equipotential surface was slightly deviated from the equilibrium state, and it was found that the radial electric field affected the radial transport of the bounce ion. A charge exchange neutral particle analyzer was located at the inner mirror throat region as a bounce ion measurement device. The energy distribution function of the bounce ion was measured using the device, and compared with the energy distribution function of the end-loss ion.

Charge exchange neutral particle measurements with natural diamond detector under the deuterium–deuterium neutron field on JT-60U tokamak

A natural diamond detector (NDD) has been installed on the JT-60U tokamak to measure the flux and the energy distribution of charge exchange (CX) fast neutral particles. A NDD has many important advantages to be used as a CX neutral particle analyzer, for example very compact size, high energy resolution, and high radiation hardness etc., while the neutrons and γ rays are a large noise source in the deuterium plasma. The shield was set up around the NDD to reduce those noises. Time-resolved energy distribution of CX neutral particles corresponding to injected beam energy have been successfully obtained under high intensity neutron yield Yn>1015 n/s. Further enhanced neutral particle fluxes during sawtooth oscillation and Alfvén eigenmodes were observed with the NDD. The performance of the NDD as CX neutral particle spectrometer under high intensity neutron yield was demonstrated for the first time on JT-60U in this work.

Loss cone boundary measurement using diagnostic neutral beam and neutral particle analyzer in a compact helical system

A horizontally scannable diagnostic neutral beam (DNB) has been installed on the compact helical system (CHS) in order to study the confinement of energetic ions with different pitch angles by varying the injection angle. The DNB has been designed to provide energetic ions as a test particle source with (1) no heating to background plasma and (2) small divergence angle. A charge-exchange neutral particle analyzer (NPA) to measure energetic ions injected by the DNB is also scannable and varies the observation angle on the equatorial plane in CHS. A combination of horizontally scannable DNB and NPA provides information on whether the energetic ions with different pitch angle are confined in the plasma or immediately lost. The experimental results are consistent with the prediction of single particle full orbit calculation in CHS.

Edge ion heating and parametric decay during injection of ion cyclotron resonance frequency power on the Alcator C-Mod tokamak

A population of suprathermal ions has been observed in the scrape off layer during injection of power in the ion cyclotron frequency range on the Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)]. These ions generated energetic neutrals which were measured with a charge exchange neutral particle analyzer. In the hydrogen minority heating regime, fast particles were generated only when the injected power was above 500 kW. Fast particles were observed at a low level of injected power when the rf was a multiple of the ion cyclotron frequency in the scrape off layer. The power deposited in the edge was always less than five percent of the injected power and did not increase impurity levels. The theoretical threshold of parametric decay instabilities for decay of the pump wave in the antenna near field into an ion Bernstein wave and an ion cyclotron quasimode matches the observed threshold for fast ion production under a variety of conditions.

Charge exchange neutral particle analysis with natural diamond detectors on LHD heliotron

Semiconductor detectors based on natural diamonds have been installed on the Large Helical Device (LHD) heliotron to measure the energy distribution of charge exchange fast neutral particles from different viewing angles. Advantages of a natural diamond detector (NDD) are (1) very compact size, (2) relatively easy handling, and (3) high energy resolution. Although NDDs are sensitive to visible light, vacuum ultraviolet, and soft x rays, unfavorable pulses produced by such radiation were greatly reduced by choosing an appropriate stainless steel shield in this experiment. In LHD, the time-resolved energy distribution of counter-going beam ions and ion cyclotron range of frequency-produced energetic ions have been successfully obtained by means of an NDD. The performance of NDDs as a neutral particle analyzer and its good suitability to LHD plasmas were demonstrated throughout this work.

Characteristics of 63.6° cylindrical energy analyzer used as charge exchange neutral particle analyzer

A charge exchange neutral particle analyzer including a newly designed multichannel 63.6° cylindrical electrostatic energy analyzer was constructed. Since the energy analyzer can simultaneously adopt a small radius of cylindrical electrodes and a large width of an entrance slit, high incident current can be analyzed while the size of the analyzer remain compact. Characteristics of the analyzer were investigated using hydrogen beam with energies from about 0.5 to 5 keV. The experimental results are compared well with the numerical analysis.

Charge exchange neutral particle analysis on ICRF-heated plasmas in the GAMMA10 tandem mirror

The ion energy balance in the central cell of the GAMMA10 tandem mirror is investigated for ICRF-heated plasmas on the basis of ion temperatures determined from charge exchange neutral particle measurements. The radial profiles of perpendicularly heated ions are measured using a neutral particle energy analyser (NPA). An experiment on ICRF power modulation is performed in order to evaluate the energy confinement. The resultant temporal change in the ion temperature is analysed from the charge exchange neutrals and central cell diamagnetism. A time evolution code of ion energy balance based on the radial profiles of measured plasma parameters is developed to predict the temporal behaviour of the ion stored energy and the calculated results are compared with the measurements.

Charge-exchange neutral particle measurement in MeV energy range on JT-60U

A charge-exchange neutral particle analyzer for the measurement of the MeV energy range ions produced by nuclear fusion or radio frequency heating has been developed and installed in JT-60U. Neutral particles entering the analyzer are ionized with a carbon foil of thickness 400 Å. The energy and mass of the stripped ions are resolved by magnetic and electrostatic fields (E∥B type). The analyzer has eight CsI(Tl) scintillator detectors. The energy range is 0.5–4 MeV for 4He0, the dynamic range is 4.08 and the energy resolution is 6%–11%. The detection efficiency for 4He0 with energy above 1 MeV is 30%–40%. A pulse height analysis (PHA) with 16 channels was adopted to distinguish particle signals from noise arising from neutrons, γ rays and optical lights emitted by JT-60U plasmas. The validity of the PHA was confirmed in a calibration experiment using a neutron source and in a high power heating experiment in JT-60U.

Compact and wide-range charge-exchange neutral particle analyzer with an acceleration tube

An E//B type neutral particle analyzer with an acceleration tube between a stripping cell and a deflection magnet has been developed to measure ion energy distributions in a wide energy range from thermal ions to fast ions up to several hundred kiloelectron volts simultaneously without making the analyzer size large. In the acceleration tube, ions are accelerated up to the detectable energy before coming into the deflection magnetic field. The acceleration method enables us to detect low-energy ions even for a strong deflection magnetic field to measure high-energy ions. Calibration experiments confirmed that ions were well accelerated according to expectation without degrading the abilities of the analyzer. The energy dynamic range Emax/Emin was improved up to 100 by the acceleration method. Using the analyzer, ion energy distributions have been measured in various heating experiments of JT-60.

Hot Ion Plasma Heating Experiments in SUMMA

Initial empirical results are presented for the hot-ion plasma heating experiments conducted in the new SUMMA (Superconducting Magnetic Mirror Apparatus) at NASA Lewis Research Center. A discharge was formed by applying a radially inward DC electric field near the mirror throats. Data were obtained at midplane magnetic flux densities from 1.0 to 3.5 tesla. Charge-exchange neutral particle energy analyzer data were reduced to ion temperatures using a plasma model that included a Maxwellian energy distribution super-imposed on an azimuthal drift, finite ion orbits, and radial variations in density and electric field. Using this plasma model, the highest ion temperatures computed were 5 keV, 1.2 keV, and 1 keV for He+, H2+, and H+, respectively. These were obtained at a mid-plane magnetic flux density of 1.6 T. Ion temperature was found to scale roughly as (P/B)n, where P/B is the ratio of power input to magnetic flux density and n is about 1 for hydrogen and 2 for helium. Optical spectroscopy line-broadening measurements yielded ion temperatures about 15 percent higher than the charge-exchange neutral particle analyzer results for hydrogen and about 50 percent higher for helium. Spectroscopically obtained electron temperatures ranged from 3 to 30 eV.