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The Thomson scattering (TS) diagnostics set [Rev. Sci. Instrum.
70, 759 (1999); Rev. Sci. Instrum. 72, 1107 (2001)]
on the Alcator C-Mod tokamak [Phys. Plasmas 1, 1511 (1994)]
has been upgraded to deliver measurements of electron temperature (Te)
and density (ne) with improved temporal and spatial resolution.
Two 30 Hz, 1.3 J Nd-doped yttrium aluminum garnet (Nd:YAG) lasers share
a vertical beam path through the tokamak plasma and are fired in alternating
fashion, allowing collection of scattered light with a 60 Hz rep rate.
Also, plasma variation occurring on faster time scales (10-5-10-2
s) can be studied by closely spacing the pulses from either laser for operation
in 30 Hz burst mode. Eight compact polychromators [Rev. Sci. Instrum.
61, 2858 (1990)] have been added to the system, better than doubling
the spatial resolution of core plasma TS measurements and allowing better
diagnosis of core ne and Te gradients. Up to 14 fiber
bundles view the core plasma, and a linear array of up to 22 single fibers
of 1 mm diameter observes the plasma edge. Each fiber holder has been upgraded
to incorporate a cam key and follower, so that the holders can be mounted
to a cammed plate that automatically orients each holder to the correct
viewing angle as its height is adjusted. This allows the fiber bundles
to be repositioned easily between tokamak discharges, making it possible
to concentrate core measurements in regions of particular interest, such
as the vicinity of an internal transport barrier, with spatial resolution
of ~ 1cm.
The newly designed mount can be positioned with sub-millimeter precision,
and its alignment with respect to the Nd-YAG beam can be adjusted between
discharges using remotely controlled stepper motors. This capacity for
correcting misalignment increases the reliability of density measurements.
Cross-calibration of TS ne measurements with electron cyclotron
emission (ECE) diagnostics is performed using dynamic ramps of both plasma
density and magnetic field to force ECE signal cutoffs at known values
of ne. This technique, which can be performed on any given day
of an experimental campaign, supplements an absolute calibration based
on Raman scattering in hydrogen and deuterium gas, which normally can not
be done during a campaign. |
