Time-of-flight neutral particle analyzer for Alcator C-Mod

Abstract

A neutral particle analyzer, based on the time-of-flight technique is scheduled to be installed on the Alcator C-Mod tokamak. The instrument was originally designed and used in the ZT-40 experiment, and later, briefly used on the CTX experiment at Los Alamos National Laboratory. The design consists of a chopper wheel mounted on a turbomolecular pump, a ∼2 m long flight tube, a Cu–Be secondary electron emitting surface, and an electron multiplier, which together are used as the neutral particle detector. Among the changes introduced in the system from the original design, of particular interest is the location of the instrument. The instrument is mounted behind a magnetic-type neutral particle analyzer and shares the same line of sight. Both of them can be scanned, poloidally down to the X point and tangentially to R/R0=0.7. This will enable us to compare neutral flux through two different techniques with an overlap in energies from ∼0.5–4 keV, with the instrument capable of detecting neutral particles with energies as low as ∼20 eV. This should aid in characterizing neutral and ion behavior at the edge of the tokamak plasma, especially near the X point. We will also describe changes made to the controls for improved versatility and ease of operation.