The motional Stark effect (MSE) diagnostic on DIII-D has been expanded to take advantage of a change in the neutral beam geometry, adding 24 new MSE channels viewing a beam injected counter to the plasma current. When data from these channels are used with those from two older MSE arrays viewing a different beam, the overall radial resolution improves near the magnetic axis at least a factor of 2, and the uncertainty in calculations of vertical magnetic field and radial electric field decreases in the edge at least a factor of 4. The new design uses two optical systems mounted on the same vacuum port with a common shutter and shielding.
DII-D currently operates with a single-or double-null open divertor and graphite walls. Active particle control with a divertor cryopump has demonstrated density control, efficient helium exhaust, and reduction of the inventory of particles in the wall. Gas puffing of D2 and impurities has demonstrated reduction of the peak divertor heat flux by factors of 3-5 by radiation. A combination of active cryopumping and feedback-controlled D2 gas puffing has produced similar divertor heat flux reduction with density control. Experiments with neon puffing have shown that the radiation is equally-divided between a localized zone near the X-point and a mantle around the plasma core. The density in these experiments has also been controlled with cryopumping. These experimental results combined with modeling were used to develop the new Radiative Divertor for DIII-D. This is a double-null slot divertor with four cryopumps to provide particle control and neutral shielding for high-triangularity advanced tokamak discharges. UEDGE and DEGAS simulations, benchmarked to experimental data, have been used to optimize the design.
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Abstract. The advanced tokamak research program at DIII-D relies critically on the measurement of the current density profile. This was made possible by the development of a Motional Stark Effect (MSE) polarimeter that was first installed in 1992. Three major upgrades have since occurred, and improvements in our understanding of critical performance issues and calibration techniques are ongoing. In parallel with these improvements, we have drawn on our DIII-D experience to begin studies and design work for MSE on burning plasmas and ITER. This paper first reviews how Motional Stark Effect polarimetry (MSE) is used to determine the tokamak current profile. It uses the DIII-D MSE system as an example, and shows results from the latest upgrade that incorporates an array of channels from a new counter-Ip injected neutral beam. The various calibration techniques presently used are reviewed. High-leverage or unresolved issues affecting MSE performance and reliability in ITER are discussed. Next, we show a four-mirror collection optics design for the two ITER MSE views. Finally, we discuss measurements of the polarization properties of a few candidate mirrors for the ITER MSE. Work supported by the U.S. Department of Energy under W-7405-ENG-48.
Accurate measurement of internal magnetic field direction using motional Stark effect (MSE) polarimetry in the edge pedestal is desired for nearly all tokamak scenario work. A newly installed 500 kHz 32-channel digitizer on the MSE diagnostic of DIII-D allows full spectral information of the polarimeter signal to be recovered for the first time. Fourier analysis of this data has revealed magnetohydrodynamic (MHD) fluctuations in the plasma edge pedestal at ρ ≥ 0.92. By correlating edge localized mode fluctuations seen on lock-in amplifier outputs with MSE spectrograms, it has been shown that edge pedestal tearing mode fluctuations cause interference with MSE second harmonic instrument frequencies. This interference results in unrecoverable errors in the real-time polarization angle measurement that are more than an order of magnitude larger than typical polarimeter uncertainties. These errors can cause as much as a 38% difference in local q. By using a redundant measure of the linear polarization found at the fourth harmonic photo-elastic modulator (PEM) frequency, MHD interference can be avoided. However, because of poorer signal-to-noise the fourth harmonic signal computed polarization angle shows no improvement over the MHD polluted second harmonics. MHD interference could be avoided in future edge pedestal tokamak polarimeters by utilizing PEMs with higher fundamental frequencies and a greater separation between their frequencies.
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