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ECE, Li pellet injector, BZN & ECE fluctuations

60 R-0

BZN

BZN is a system to deposit a thin film of elemental boron on the walls of the DIII-D tokamak, in order to reduce the influx of impurities during plasma discharges. Subsequently new regimes of substantially improved tokamak energy confinement were obtained. The deposition of the boron layer is achieved during a glow-discharge session using a helium-diborane gas mixture and a film of [approximately-equal-to]100 nm is deposited. The boronization system includes special storage and handling precautions for the diborane, a delivery and metering system for the glow discharge, modifications to the tokamak's residual gas analyzer system, and a dedicated system for handling and neutralizing the exhaust gas from the tokamak. Tokamak discharges with similar parameters before and after boronization are used to characterize the effects of the boron film. Nickel has been reduced by a factor of 30, while impurities such as oxygen and carbon are reduced fivefold. A system of pulsing the glow discharge has been developed in order to improve the uniformity of the film applied.

ECE

The DIII-D ECE radiometer instrument is a multichannel heterodyne receiver consisting of three local oscillator/mixer bands with frequency selection by intermediate frequency (IF) bandpass filters. The local oscillator and filter frequencies are fixed and hence the system has had pretty much the same set of frequencies for its lifetime at DIII-D.

The ECE radiation is collected from the tokamak by a optimum conical horn antenna in the 60 degree port on DIII-D. The antenna views a major radial chord at a z height of 1.5 cm above the midplane. The radiation is transmitted to the instrument via a low loss (~4dB) transmission line of about 16 m length.

The spatial resolution of the radiometer is determined by several factors but primarily by the bandwidth of the IF filters and the spot size of the antenna. For the current system with IF bandwidth of 1 GHz the spatial resolution is about 2 cm at the half radius and 4 cm at the center and low-field side edge.

The DIII-D ECE radiometer is a multichannel heterodyne system that provides Te(r,t) from measurements of optically thick, second harmonic (X-mode) electron cyclotron emission. The instrument currently has 40 channels; viewing is along a horizontal chord at the tokamak midplane (antenna at 60 deg port). The system was installed in 1995, data acquired starting with shot 84000. This is a routine diagnostic, data is available for most shots.

Li pellet injector

The Lithium Pellet injector (LPI) was constructed in 1995 and first operated in October 1995. It follows closely the C-Mod Pellet Injector design. Location on DIII-D is 60 R0. It consists of 4 barrels. Each barrel is aligned with one of 90 pellets (360 total) on a pellet wheel and pellets are fired using helium gas as a propellent. There are two barrel sizes: 0.040" (small, 1 barrel) and 0.077" (large, 3 barrels). The thickness of the pellets can be varied, giving the parameters listed in the table below. Maximum velocity is 600 m/sec and slower velocities are possible by changing orifice sizes in the fast helium pneumatic valves (requires overnight turnaround).