A Collaboration between UW and the Budker Institute of Nuclear Physics,
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 (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
Questions or comments? Send mail to Jim Reardon (firstname.lastname@example.org).
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