DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 115: Stable Impurity Enhanced Radiative Divertor Operation with Pellet Pacing | ||
| Name: | Richard A. Moyer ( |
Affiliation: | University of California, San Diego |
| Research Area: | Core-Edge Integration | Presentation time: | Requested | Co-Author(s): | Todd Evans, Tom Petrie, Jon Watkins, Larry Baylor, Tom Jernigan |
| Description: | This goal of this proposal is to develop a steady-state operating scenario compatible with Radiative Divertor operation using impurity seeding as envisioned for ITER. Stable, high performance H-modes are obtained, impurity is puffed (neon, argon) to detach the divertor legs and reduce strike point erosion presumably. Repetitive pellets (1-10 Hz) are used from the HFS and LFS to generate ELM-like transport events (ELM pacing) in order to control the accumulation of the impurity in the plasma core and provide a stable RD discharge. | ||
| Experimental Approach/Plan: | Use lower single null, high performance, co-NBI H-modes (4-5 sources) to produce a reference ELMing H-mode with cryopumping. This plasma should have a detached inner strike point and attached outer strike point, but characterize these conditions. Then, use impurity puffing to detach the outer leg (partial or full) and develop a Radiative Divertor with reduced average peak heat flux to the divertor. Add repetitive pellets (1- 10 Hz as needed) from the HFS or LFS to trigger "ELMs" of smaller amplitude and controlled frequency to flush impurities from the plasma edge and prevent accumulation in the core, which leads to an X-point MARFE, radiative collapse, and/or disruption. Time permitting, try varying the input neutral beam torque to more ITER-like rotation while maintaining the steady state RD operation in a high performance H-mode. | ||
| Background: | Although most poloidal divertor tokamaks (all?) have demonstrated that the time average peak heat flux to the divertor targets can be reduced by enhancing the level of radiation in the boundary with impurity puffing, this Radiative divertor operation has proven difficult to maintain in steady-state. The impurities tend to accumulate in the core, leading to an x-point MARFE, a radiative collapse or a disruption.
Pellets have been demonstrated to produce "ELMs" when injected into the plasma; this feature is used in e.g. ASDEX-Upgrade to mitigate large ELM heat flux impulses to the divertor by triggering ELMs at narrower spacing, and hence with lower amplitude ("pellet pacing"). However, fast IRTV measurements by Jakobowski et al and particle flux measurements with divertor probes and/or TV (Groth, Watkins, et al) suggest that these "ELMs" in DIII-D are much more "convective" than intrinsic, naturally occurring ELMs. Consequently, rapid, shallow pellets can be used to enhance pedestal particle transport while having only a minimal effect on the pedestal thermal transport, and may be a good tool for preventing the accumulation of the puffed impurity, leading to unstable RD operation. | ||
| Resource Requirements: | 5 co and 2 counter NB sources
neon and argon impurity injection HFS and LFS pellets at 1 �?? 10 Hz 1 full day of operations is envisioned Possible extension to include implimentation of a feedback control on the pellet injection frequency might be desirable but this would have to follow this first day to establish the range of conditions needed. | ||
| Diagnostic Requirements: | core, tangential and divertor Thomson scattering
CER system tuned for core impurity profiles Floor Langmuir probes Fast IRTV (Juelich; if available) or LLNL IRTV Full pedestal and divertor diagnostics Fast framing camera (UCSD) Fast and Mirnov magnetics Fluctuation diagnostics | ||
| Analysis Requirements: | Although I envision no specialized analysis requirements, these data would provide an excellent opportunity for edge modeling with e.g. UEDGE, SOLPS, etc. and may provide useful information on the physics mechanisms for pellet induced ELMs and how they are or aren�??t �??normal�?� ELMs (e.g. ELITE stability analysis; NIMROD?)
This experiment makes a good companion to the Petrie proposed experiment to provide steady-state RD operation with the RMP, the other tool DIII-D has which can enhance particle transport while leaving thermal transport relatively unaffected. | ||
| Other Requirements: | Development of a stable radiative divertor operating scenario, in addition to being of direct interest to steady-state integrated operation, would develop a reference discharge for use with DiMES and MiMES material sample exposures. | ||
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