Abstract
Solid state neutral particle analyzer (ssNPA) arrays are operated in current mode on the DIII-D tokamak and the National Spherical Torus Experiment (NSTX). Compared with conventional pulse-counting NPAs, current-mode operation sacrifices energy resolution to obtain economical, high-bandwidth, pitch-angle resolved measurements. With the success from a new three-channel near-vertical-view current mode ssNPA on DIII-D, the apertures on an existing array on NSTX were expanded to increase the particle influx. The sightlines of both arrays intersect heating beams, enabling both active and passive charge exchange measurements. The spatial resolution at beam intersection is typically 5 cm on both devices. Directly deposited ultra-thin foils on the detector surface block stray photons below the energy of 1 keV and also set low energy threshold about 25 keV for deuterium particle detection. Oscillations in neutral flux produced by high frequency magnetohydrodynamics (MHD) instabilities are readily detected.
Highlights
The neutral particle analyzer (NPA), introduced in the early 1960s initially as an ion temperature diagnostic, has evolved through the years to meet new demands in fusion plasma research. It is based on charge exchange (CX) process whereby an ion gains an electron from a background atom to get neutralized and ceases to be confined by the magnetic field; NPA measures the lost neutrals and their distribution to deduce the ion energies, plasma temperature and their profile
As shown in a recent review paper[1] and some contemporary publications,[2,3,4,5] a variety of NPAs have been developed based on different neutral re-ionization techniques and spectrometer designs, including electrostatic, E//B and time-of-flight spectrometers
Compact NPA system based on absolute extreme ultraviolet (AXUV) silicon photodiode[8] has been demonstrated on National Spherical Torus Experiment (NSTX) (Refs. 9 and 10) and successfully applied on Alcator C-Mod.[11]
Summary
The neutral particle analyzer (NPA), introduced in the early 1960s initially as an ion temperature diagnostic, has evolved through the years to meet new demands in fusion plasma research. As shown in a recent review paper[1] and some contemporary publications,[2,3,4,5] a variety of NPAs have been developed based on different neutral re-ionization techniques and spectrometer designs, including electrostatic, E//B and time-of-flight spectrometers. These conventional NPA systems are well understood but are relatively complex, bulky and expensive.
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