Low-energy neutral particle analysis by a time-of-flight method on JFT-2M

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Abstract
A time‐of‐flight method is used to measure the energy spectrum of charge exchange atoms emitted from the JFT‐2M plasma in the energy range below 1000 eV/amu. The neutral flux is chopped in bunches of about 0.8 μs by a slotted cylinder. The rotating cylinder is driven by a turbomolecular pump. After the flight of 1.5 m the particles are detected by a secondary particle detector. This is an electron multiplier modified from a photomultiplier which can be gated for an intense photon. The signal from the electron multiplier is digitized at a sampling time of 0.2 μs. To get good statistics of the arrival time distribution (ATD), eight or nine ATDs are averaged. Then the energy spectrum of charge exchange neutrals is measured by a 2 ms interval. The fast change (≤2–6 ms) of energy spectra at the L/H and H/L transition is also reported. The average energy defined by ∫E(dΓ/dE)dE/∫(dΓ/dE)dE increases by about 100 eV at the H transition like the increase of the ECE electron temperature at the edge. The neutral flux defined by ∫(dΓ/dE)dE decreases like the divertor Hα/Dα signal. In the energy distribution of the flux, the number of particles whose energy less than about 400 eV decrease by the H transition but those greater than 500 eV do not change or increase. The flux at 200 eV in the H phase is two times lower than that in the L phase.