Thomson scattering upgrades on Alcator C-Mod

Abstract

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 (T e ) and density (n e ) 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 < ∼ τ (s) < ∼ 10 −2 ) 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 n e and T e 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 ∼ 1 cm. 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 n e 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 n e . 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.

Characterization of the pedestal in Alcator C-Mod ELMing H-modes and comparison with the EPED model

“…Two Nd:YAG lasers fired vertically through the plasma provide the scattering photons, which are collected in core and edge fibers running to a set of polychromators [18,19]. The beam path is shown in figure 1, overlaid on C-Mod equilibria with normal and the altered shaping used for ELMing H-modes.…”

Validation of nonlinear gyrokinetic simulations of L- and I-mode plasmas on Alcator C-Mod

“…20 Thomson scattering also measured the electron temperature profile, as well as the electron density profile. 21 Ion temperature profiles and the toroidal rotation were measured with a high resolution X-ray crystal spectrometer (XRCS). 22 Electron temperature fluctuations were measured with a correlation electron cyclotron emission (CECE) radiometer, described in Refs.…”

Nonlinear gyrokinetic simulations of the I-mode high confinement regime and comparisons with experimenta)

“…30 The electron temperature profile was also measured with a Grating Polychromator (GPC) electron cyclotron emission (ECE) diagnostic. 31 Ion temperature and toroidal rotation were measured with a high resolution x-ray spectrometer 32 outside of r=a ¼ 0:35 in these plasmas, and the radial electric field profile was calculated using TRANSP.…”