MST Diagnostic Neutral Beams

A Collaboration between UW and the Budker Institute of Nuclear Physics, Novosibrsk, Russia

UW contracted the Budker Institute in November 1998 for the delivery of two neutral beam sources, to be used on MST in two diagnostics (CHERS and Rutherford Scattering) described below. Two neutral particle analyzers, for use with the Rutherford Scattering diagnostic, were also contracted. All four pieces were delivered to Madison in June 1999, reassembled, and brought to full operation by engineers from the Budker Institute, who continue to provide technical advice on optimization of the beams. The delivery of all equipment occurred on schedule and under budget.


CHERS stands for Charge Exchange Recombination Spectroscopy. This diagnostic technique uses a beam of neutral atoms projected through the plasma, and an array of spectrometers. Some of the neutral atoms charge exchange with (donate an electron to) fully- stripped plasma impurity ions, which will then emit line radiation as the electron recombines (decays to the ground state). The MST Chers source produces a beam of 30 KeV H atoms with an equivalent current of 4 A for a duration of 3 ms. Click here to see a picture of the MST Chers beam mounted on MST. An array of spectrometers is being designed which will observe this line radiation. From the Doppler widths and broadenings of the visible lines, the temperature and flow velocity of the impurity ions is deduced. Preliminary CHERS measurements on MST were made in April 2000.

Rutherford Scattering

Rutherford Scattering (RS) refers to an interaction of two nuclei mediated by the Coulomb force. In this case one nucleus belongs to a neutral atom from the beam and the other is an ion in the plasma. The neutral atom (which moves much faster than the plasma ion) is deflected slightly but not ionized when it collides with the plasma ion, and loses some energy. The energy loss depends on the velocity of the plasma ion. If the plasma ions have a Maxwellian velocity distribution, their temperature (and mean drift velocity) can be deduced from the energy spectrum of the scattered atoms. Accordingly, the MST RS diagnostic consists of a source, which produces a 4-A-equivalent-current beam of 20 KeV He atoms for 3 ms, and a pair of 12-channel electrostatic neutral particle energy analyzers. The axis of the source and the analyzers are approximately 10 degrees off from colinear. Click here to see a picture of the RS source and one of the analyzers, mounted on a test stand prior to installation on MST. RS has been operating routinely on MST since December 1999.

For more information on the MST Rutherford Scattering diagnostic, look here.

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