Rutherford Scattering on MST

A poloidal cross-section of MST, in which the locations of the Rutherford Scattering beam source and analyzers are shown, is given here.

The beam source (hanging from the bottom of MST) provides a 4 Ampere-equivalent beam of 20 KeV neutral Helium atoms. On the order of 50% of these neutrals will go straight through the plasma to the attenuation detector. Of the remainder, perhaps 50% will undergo Rutherford Scattering (RS) from Hydrogen ions in the plasma (the rest will be lost to charge exchange). The neutral analyzers observe those neutrals which undergo RS.

Since the beam source produces a beam which is effectively monoenergetic, the width of the energy distribution of the neutrals undergoing RS will be related to the ion temperature of the plasma. The peak of the energy distribution will be Doppler shifted by an amount proportional to the ion flow velocity (equations given here).

The energy width of the beam source is found by puffing Argon gas into MST (no plasma) and measuring the scattering. The angular spread of the beam source (as well as the effect of finite scattering volume) are accounted for by measuring the scattering from Hydrogen gas. The non-zero ion temperature and flow velocity in a Hydrogen plasma are responsible for the difference between scattering from Hydrogen plasma and Hydrogen gas. Look at a plot of scattering data from Argon gas, Hydrogen gas, and Hydrogen plasma.

We calculate that the intensity of the beam (and the sensitivity of the analyzers) will allow an energy spectrum to be recorded every 100 microseconds for typical MST plasmas, which will allow the fluctuations in ion temperature and flow velocity to be measured during the 3 ms duration of the beam. Ion density fluctuations () will also be diagnosed, from variations in the beam intensity.

This is the first attempt to use RS on a Reverse-Field Pinch. RS has seen limited use on tokamaks (TEXTOR [1] and JT-60 [2]). It was first proposed as a plasma diagnostic in 1972 [3].

1. A.A.E. van Blokland, et al. "Ion Temperature Measurements in Tokamak Plasmas by Rutherford Scattering", Review of Scientific Instruments, Vol. 63, No. 6, 1992 (pp. 3359-3368).

2. K. Tobita, et al. "Ion Temperature Measurements in JT-60 Plasmas by Active Beam Scattering", Nuclear Fusion, Vol. 28, No. 10, 1988 (pp. 1719-1726).

3. V.G. Abramov, et al. "Temperature Measurement in a Low-Density Medium with an Atomic Beam", Soviet Physics-Technical Physics, Vol. 16, No. 9, 1972 (pp. 1520-1525).

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