Study of Density Fluctuations and Particle Transport at the Edge of I-Mode Plasmas (original) (raw)

2012, Proquest Dissertations and Theses Thesis Massachusetts Institute of Technology 2012 Publication Number Aat 0829158 Source Dissertation Abstracts International Volume 74 08 Section B

The wide range of plasma parameters available on Alcator C-Mod has led to the accessibility of many regimes of operation. Since its commissioning, C-Mod has accessed the Linear ohmic confinement, Saturated ohmic confinement, L-Mode and ELM-free, ELMy and Enhanced D, H-Mode regimes. Recently, another novel regime, the I-Mode, has been identified[1][2][3][4]. I-modes feature the presence of steep H-Modelike electron and ion temperature gradients at the edge of the plasma with L-Mode-like density profiles. The I-Mode, in contrast to the Hl-mode, shows very weak degradation of energy confinement with increased input power, and routinely reaches H 98 > 1 while operating at low edge collisionalities (*edf~).1), making it a good candidate for reactor relevant tokamaks. Also relevant for reactors, this regime can be sustained in steady state for more than-15 energy confinement times without the need for ELMs to regulate particle and impurity confinement. Changes in edge density, temperature and magnetic field fluctuations accompany the L-mode to I-mode transition, with reduction of fluctuations in the 50-150kHz range as well as the appearance of a Weakly Coherent Mode (WCM) in the 200-300kHz range, analogous to the Quasi-Coherent Mode (QCM) characteristic of the Enhanced D, H-mode. Previous work[4] has established a connection between the midrange fluctuation suppression and reduction in the effective thermal diffusivity, Xye, in the pedestal region. The mechanism in I-mode for maintaining sufficient particle transport to avoid impurity accumulation and instabilities has been unclear. The O-mode reflectometry system has been extensively used for the characterization and detection of the I-mode and the WCM, in part, enhanced by upgrades to the system which enabled the baseband detection of density fluctuations at an array of cutoff locations at the edge of the plasma[5] [6] [7]. Using a novel model, the autopower signals of reflectometry channels detecting the density fluctuations have been decomposed into a broadband component and a WCM component. The latter is then used to estimate the intensity of the WCM. In parallel, the particle transport across the LCFS in I-mode plasmas has been estimated using a volume integrated particle transport model, where ionization source measurements are acquired using D, profiles measured near the outboard midplane. This model Last month, President Obama made a gaffe when lie was talking about people starting their own businesses. He said that they don't do it alone, that they need the help of many people. He said "If you start a business... you didn't build that." Now, I agree with his opponent, Mitt Romney: I wrote my thesis alone, without anybody's helpV Well, other than... My advisor, Earl Marmar. As the director of the C-Mod program, Earl has a million things on his plate, yet that never stopped me from being at his door almost every day, especially in this last year; and he's been constantly there for me, giving me the support I've needed to get to this point I'm at now. His door has always been open to my incessant questions, which always seem to arrive in bulk and close to a deadline; and he always opens up some time in his packed schedule to show me where I'm right and where I've gone wrong, but to always show me the way forward. During these last few months, he's been marching side by side next to me, giving me his valuable feedback on both, the thesis research and the dissertation, even, once, through iPhone pictures from Vermont. Thanks Earl, for everything. Gerrit Kramer, who's taught me about 5 times more than what I currently know about reflectometry, thanks for picking up the phome all of those countless times that I've called you to talk about the research. You've been there, always available, for the last 8 years, and don't think it's over, you've still got lots to teach me, and I'll make sure you do ;) Nils Basse, the man who got me going in the research and who first suggested the upgrade that ultimately led to my dissertation. 'We shared too few moments here, but in the few months we did share, you really put me on the right track. Thanks. To the thesis readers, Miklos Porkolab and Rick Temkin, thank you for your help in this process. Knowing that you will be two of the few people that will actually read this document in its entirety, I hope it wasn't too boring of a read.