Development and testing of secondary ion accelerator for ITER neutral particle diagnostic system.

This paper presents the results of the development and testing of a secondary ion accelerator for the diagnostic complex of neutral particle analyzers at ITER. The accelerator is part of an analyzer designed to measure fluxes of neutral deuterium and tritium particles escaping from reactor plasma in a total range of 10-200 keV. The aim of the accelerator is to improve the signal/background ratio of the analyzer. It is especially important for the region with a relatively low energy of 10-50 keV. The accelerator has the function of converting atoms into secondary ions and accelerating them in an electrostatic field with a voltage of up to +100 kV. The accelerator is based on two high-voltage accelerating tubes. A special unit is built into the central electrode of the accelerator, which provides the replacement of stripping foils used for the conversion of the flux of neutral particles into a flux of secondary ions. The high-voltage tube assembly is encompassed by a sealed steel housing, in which electrical insulation is provided by a gas gap (nitrogen at 6 bar). The results of tests of the accelerator under ITER relevant conditions, namely, under high-intensity gamma irradiation and seismic loads, have shown its high reliability for use in the diagnostic complex of the fusion reactor.

Neutral particle analyzer measurements on the SSPX spheromak.

A neutral particle analyzer is used to measure the time-resolved energy spectrum of neutral hydrogen leaving a spheromak plasma. A gas cell filled with 10-50 mTorr of helium is used to strip electrons from incoming neutral hydrogen, lowering the minimum detectable energy well below that obtained with thin foils. Effective neutral particle temperature is calculated by fitting a Maxwellian energy distribution to the measured energy spectrum above and below approximately 300 eV. A computational model with approximated profiles of plasma density and neutral density is used with the measured neutral hydrogen flux to estimate the ion temperature. Measurement of the power flux due to neutral hydrogen emitted at the measurement location is extended to the whole plasma surface to estimate the total charge exchange power loss from the plasma. The initial results indicate that the charge exchange power loss represents only 2% of the total input gun power during the sustainment phase of the discharge.