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(OV2/1) Overview of Recent Results from the Alcator C-Mod Tokamak

E. Marmar, P. Acedo^a, O. Batishchev, R. Bengtson^b, R. L. Boivin, F. Bombarda^c, X. Bonnin^b, P. Bonoli, C. Boswell, R. Bravenec^b, N. Bretz^d, C. Changⁿ, C. Christensen, G. Cima^d, W. Dorland^d, J. Drake^e, E. Eisner^b, G. Esser^f, M. Finkenthal^g, C. Fiore, K. Fournier^h, T. Fredian, R. Gandyⁱ, S. Gangadhara, K. Gentle^b, J. Goetz, R. Granetz, M. Greenwald, H. Griem^e, G. Hallock, J. Harker^b, J. Heardⁱ, J. Hosea^d, A. Hubbard, I. Hutchinson, J. Irby, D. Johnson^d, J. Ke, J. Kesner, S. Krasheninnikov, B. LaBombard, H. Lamela^a, B. LeBlanc^d, Y. Lin, B. Lipschultz, S. Lisgo^j, R. Maqueda^k, M. May^g, A. Mazurenko, S. Migliuolo, E. Nelson-Melby, G. Miller, D. Mossessian, R. Nachtrieb, R. Nazikian^d, R. Neu^l, H. Ohkawa, P. O'Shea, T. S. Pedersen, D. Pappas, C. K. Phillips^d, A. Pigarov, C. S. Pitcher, M. Porkolab, J. Ramos, J. Reardon, J. Rice, B. N. Rogers^e, J. C. Rost, W. Rowan^b, J. Schachter, G. Schilling^d, H. Scott^h, C. Skinner^d, J. A. Snipes, V. Soukhanovskii^g, P. Stangeby^j, P. Stek, J. Stillerman, Y. Takase, G. Taylor^d, J. Terry, T. Tutt, M. Umansky, W. Wampler^m, A. Wan^h, C. Wattsⁱ, L. Weathers, J. Weaver^e, B. Welch^e, J. R. Wilson^d, S. Wolfe, K.-L. Wong^d, A. Wootton^b, S. Wukitch, G. Wurden^k, Y. In, H. Yuh, S. Zweben^d

MIT Plasma Science and Fusion Center, Cambridge, MA 02139, USA
^a Carlos III University, Madrid, Spain
^b U. Texas, Austin, TX, USA
^c ENEA Frascati, Italy
^d Princeton Plasma Physics Lab., Princeton, NJ, USA
^e U. Maryland, College Park, MD, USA
^f KFA Julich, Germany
^g Johns Hopkins U., Baltimore, MD, USA
^h Lawrence Livermore National Lab., Livermore, CA, USA
ⁱ Auburn U., Auburn, AL, USA
^j U. Toronto, Toronto, Ontario, Canada
^k Los Alamos National Lab., Los Alamos, NM, USA
^l MPIPP, Garching, Germany
^m Sandia National Lab., Albuquerque, NM, USA
ⁿ Courant Institute of Mathematical Sciences, New York University, New York, NY, USA

Abstract.  Recent results from the compact, high field, Alcator C-Mod tokamak program are summarized. H-mode threshold studies have demonstrated that the threshold appears to be closely related to local edge plasma parameters: for fixed field and plasma current, T_e($\psi_{95}^{}$) takes on a density independent value at the transition. The Enhanced D-Alpha H-Mode (EDA) regime has been investigated. EDA is distinct from ELM free H mode, in that there is no accumulation of impurities, and at the same time EDA does not exhibit large discrete ELMs. The energy confinement is degraded by only about 10%, compared to ELM free. Comparisons for EDA with ELMy H-Mode database scalings indicate $\tau$EDA $\sim$ 1.2$\tau_{ITER97H}^{}$. Strong toroidal rotation is observed in ICRF-only auxiliary heated plasmas; the rotation increases with plasma pressure, and decreases with increasing plasma current. The inferred radial electric field reaches the order of 30kV/m near the center of the plasma. Through feedback controlled nitrogen impurity puffing, steady state detached EDA H-Modes have been achieved with Z_eff < 1.5. $\tau_{E}^{}$ is reduced by about 10% in the detached case, compared to the confinement before the N₂ puff begins. The heat load to the divertor is reduced by a factor of 4. Volume recombination rates are measured in the divertor, using 2-d tomography of Balmer series TV movies. Volume recombination can be a significant contributor to the overall reduction in ion current to the divertor plates which occurs in detachment. Particle balance measurements indicate that the divertor and main chamber plasmas are largely isolated from one another, at least with regard to particle recycling, with most of the main chamber (core plus scrape-off) fueling coming from neutrals in the main chamber volume. With the addition of Lower Hybrid Current Drive, C-Mod would be an ideal vehicle for investigation of advanced tokamak operation with fully relaxed current profiles. Detailed modeling indicates that discharges approaching the $\beta$ limit ( $\beta_{N}^{}$ $\sim$ 3.7) with > 70% bootstrap fraction should be achievable.

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