Initial Study Comparing the Radiating Divertor Behavior in Single-Null and Double-Null Plasmas in DIII-D
'Puff and pump' radiating divertor scenarios [1,2] were applied to upper SN and DN H-mode plasmas. Under similar operating conditions, argon (Ar) accumulated in the main plasma of single-null (SN) plasmas more rapidly and reached a higher steady-state concentration when the B x {del}B ion drift direction was toward the divertor than when the B x {del}B ion drift direction was out of the divertor. The initial rate that Ar accumulated inside double-null (DN) plasmas was more than twice that of comparably-prepared SNs with the same B x {del}B direction. One way to reduce power loading at the divertor targets is to 'seed' the divertor plasma with impurities that radiatively reduce the conducted power. Studies have shown that the concentration of impurities in the divertor are increased by raising the flow of deuterium ions (D{sup +}) into the divertor by a combination of upstream deuterium gas puffing and active particle exhaust at the divertor targets, i.e., puff-and-pump. An enhanced D{sup +} particle flow toward the divertor targets exerts a frictional drag on impurities, and inhibits their escape from the divertor. A puff-and-pump approach using Ar as the impurity was successfully applied in recent DIII-D experiments to SN plasmas [3] while maintaining good H-mode performance. Studies on DIII-D and other tokamaks have shown that both the direction of the toroidal magnetic field B{sub T} and the degree of magnetic balance between divertors [i.e., the degree to which the plasma shape is considered SN or DN] are important factors in determining recycling and particle pumping [4,5]. It is unclear whether the favorable results of Ref. [3] can be extended to cases with different magnetic balance and/or B{sub T} direction. We show in this paper that reversing the direction of B{sub T} or altering the divertor magnetic balance does have an impact on how plasmas behave under puff-and-pump conditions. Our study takes advantage of DIII-D's capabilities to actively pump SN and DN shapes with high-triangularity. In-vessel pumping of deuterium and Ar, shown schematically in Fig. 1, was done by cryopumps located inside the upper outer ('plus'), upper inner ('minus'), and lower outer (dotted) divertor pumping plenums. To increase the ion D{sup +} flow toward these pumps, deuterium gas was introduced at an outboard location, as shown. Argon was injected directly into the private flux region (PFR) of the upper outer divertor.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 940874
- Report Number(s):
- UCRL-JRNL-232262; TRN: US0807235
- Journal Information:
- Proceedings of 34th EPS Conference on Plasma Physics, vol. 31F, na, July 2, 2007, P2.032, Vol. 31F
- Country of Publication:
- United States
- Language:
- English
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Comparison of radiating divertor behaviour in single-null and double-null plasmas in DIII-D
A Comparison of Radiating Divertor Behavior in Single- and Double-Null Plasmas in DIII-D