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Title 255: Role of turbulent momentum transport for intrinsic rotation
Name:Stefan Muller () Affiliation:University of California, San Diego
Research Area:Rotation Physics Presentation time: Requested
Co-Author(s): J. Boedo, C. Holland, R. Moyer, D. Rudakov, G. R. Tynan, J. H. Yu
Description: The goal of this experiment is to investigate the role of turbulent momentum transport for intrinsic rotation, with the emphasis on the role of the SOL (Is the SOL spinning up the core? Are there indications of a recoil action from SOL ejection events?). Measurements of the vr-vpar Reynolds stress (= radial transport of parallel momentum) and SOL flows are performed with the reciprocating Langmuir probes and CER. A scan of nu* over a wide range allows to influence the relative importance of neoclassical and anomalous terms in the parallel-momentum transport equation. Both terms are inferred independently from the measurements, allowing to search for possible "source-like" contributions ("residual stresses"). The question of boundary conditions for rotation at the separatrix is addressed by measuring the sign and gradient of stresses across the separatrix. A second goal consists of the investigation of the relation between minority-ion and bulk-ion rotation by comparing CER and Mach-probe measurements on the same flux surface.
Experimental Approach/Plan: Use LSN divertor discharges with ion gradB drift toward the X-point to produce a low L-H power threshold (about 250 kW). Use the Reynolds-stress head of the midplane probe to measure turbulent momentum flux and parallel flows in low-power L-mode discharges. Select outer gap to optimize high spatial density chords of CER (separatrix = 2.30 m), which, for LSN discharges that "fit" into the lower divertor, allows probe access to the same flux surfaces for Mach probe measurements. Use lower divertor cryopump to perform pedestal collisionality scan by varying the pumping. Time permitting, increase NB power to cross L-H threshold into H-mode and repeat measurements in H-mode.
Background: As ITER will be a low-torque input machine, it is of crucial importance whether the intrinsic plasma rotation will be sufficiently fast to stabilize Resistive Wall Modes. It arises the need to understand and predict the magnitude of intrinsic rotation in tokamaks. Experiments have shown that the intrinsic toroidal rotation typically exceeds neoclassical levels significantly, suggesting an important role of turbulent fluctuations, analogously to their well established importance for particle and heat transport. Emerging theories include variations of neoclassical theory, the existence of "source-like" contributions to momentum transport ("residual stresses"), and core rotation as a recoil action of SOL ejection events. A parameter scan of nu* is an excellent way to achieve contrast between these mechanisms. In order to quantitatively predict the magnitude of core rotation, theories need boundary conditions at the separatrix, which are presently under debate. CER and Mach probe measurements across the separatrix could contribute significantly to clarify this question. Finally, most available data on toroidal velocities is obtained by using CER on carbon impurities, and it is not clear whether the velocities of the bulk ion species can be reliably inferred. Simultaneous measurements with CER and Mach probes at the same flux surface could clarify this point.
Resource Requirements: Optimized error field correction with i-coil; balanced NBI; lower divertor cryopump at liquid helium temperature
Diagnostic Requirements: CER: ion temperature, rotation and Er; Thomson scattering: core, tangential and divertor; BES; PCI; Doppler reflectometry; FIR scattering (if possible); Correlation ECE (Te fluctuations)
Analysis Requirements: --
Other Requirements: Due to the limited flexibility and high vulnerability of the reciprocating Langmuir probes, the success of the proposed experiments depends crucially on the possibility to fine tune the power input and the plasma shape. For these reasons, the session leadership for at least one day of operation is requested.