Internal transport barriers (ITBs) in tokamak plasmas are characterized by the reduction of transport in one or more of the particle, momentum, or energy channels in the core plasma region. On Alcator C-Mod, significant contributions to ITB studies were made possible with the core Thomson scattering (TS) diagnostic, which measures profiles of electron temperature (0.03 5 Te[keV] 5 10) and density (0.05 5 ne[10 20m"3] 5 5) with -1 cm resolution in the ITB region. For the transport analysis purposes, TS profiles are complemented by measurements from other diagnostics and fitted with smooth curves. This thesis research examines the plasma conditions necessary to trigger ITBs on C-Mod. ITBs can be routinely produced in C-Mod steady enhanced Da (EDA) highconfinement (H-mode) plasmas by applying off-axis ion cyclotron range of frequencies (ICRF) heating (Ir/al > 0.5). They are observed primarily in the electron particle channel and are marked by the steepening of the density and pressure profiles. ITB formation in C-Mod appears to be the result of competition between an inward particle pinch and the outward diffusion caused by various fine scale plasma instabilities. Several experiments were performed on C-Mod to verify whether ITB formation can be explained within a paradigm of marginal stability. Analyses of the temperature profiles reveal that the Ti profile widens and the T. profile exhibits flattening in the ITB region when the ICRF resonance is moved off-axis. Transport and gyrokinetic stability analyses demonstrate that reduction of the temperature gradient suppresses the temperature gradient driven instabilities. Nonlinear gyrokinetic simulations of plasma microturbulence show that this results in a significant reduction of the outward diffusion, and allows the inward particle pinch to dominate, peaking the density profile. Thesis Supervisor: Catherine Fiore Title: Research Scientist, Plasma Science and Fusion Center Thesis Reader: Ian Hutchinson Title: Professor of Nuclear Science and Engineering, Department Head Thesis Reader: Darin Ernst Title: Research Scientist, Plasma Science and Fusion Center Acknowledgements Now, as I prepare to leave MIT after spending so many years here, I feel nostalgia. This thesis research would not be possible without all the great people I've met at and beyond the PSFC. I am glad that I can take my time now to thank them. At this moment, I can completely appreciate the importance of the acknowledgement section in the thesis. First and foremost, I am in great debt to Dr. Catherine Fiore, my supervisor. She has been very helpful and encouraging, patient but persistent. She has always been available to answer my numerous questions and open for discussions. It was her wise guidance that helped plan my thesis work and steered it in the right direction. She helped me see the big picture and the light at the end of the tunnel. Many thanks go to Dr. Darin Ernst, the reader of this thesis. He spent many hours teaching me to how to use the GS2 code, discussing the results, and answering my innumerable and sometimes silly questions. If it were not for Darin, profiles fitting software would never have seen the light of day. I would like to thank Prof. Ian Hutchinson for the financial support he secured for me when he was the head of the Alcator project and for being the reader of this thesis. He gave me criticism when I deserved it and I saw it as a challenge and my professional duty not to let him down. Only now do I understand how wise some of his pieces of advice or decisions were. I constantly felt his presence and support behind the scene, which stimulated my work. I have to thank Dr. Dmitri Mossessian, who was my thesis supervisor for almost three years. It was his energy and dedication that made Thomson scattering upgrade a success. I learned a lot about laboratory techniques and work ethics from him. I would like to thank Prof. Ron Parker and Prof. Jeff Freidberg for sitting on my thesis committee. I am also in debt to them for their personal advice and words of wisdom when I needed it. Dr. Earl Marmar, as the head of Alcator, provided financial support not only for my thesis research but also for the undergraduate students who worked to help me upgrade profiles fitting software. Throughout the thesis I relied heavily on discussions with many colleagues, in particular with John Rice, Paul Bonoli, Martin Greenwald, Amanda Hubbard, David Mikkelsen, and Jim Irby. Their valuable comments significantly improved my various presentations and papers, including this thesis. I am thankful to Steve Wolfe for his help running kinetic EFIT. I am grateful to many engineers and technicians who helped me with various hardware, software, and electronics issues at different times of my thesis project. Edgar Rollins was of great help at the early stage of building the core TS system. Tom Toland provided his expertise and help on numerous vacuum-related matters during my entire time at MIT. I would like to thank Jim Irby, Ed Fitzgerald, Gary Dekow, and Rui Vieira for their advice on engineering issues. Much hardware support came from Joe Bosco, Yuri Rokhman, and Bill Burke. Josh Stillerman and Tom Fredian were always there when software-related help was needed. I am thankful to Bill Parkin and Dave Bellofatto for their often sought and much needed help with solving various electronics problems. Finally, I want to express my appreciation to Jerry Hughes, with whom I worked hand in hand in the TS group for six years. I would like to thank Doug McCune and Rob Andre for the time they spent helping to get my TRANSP runs to work. I acknowledge the contribution of MIT undergraduate students, Eric Grebing and David Ely, to the development of the graphical user interface for the profiles fitting routines. David really made this project come to life. I also am grateful for the support of the Alcator HQ administrative staff: Valerie Censabella, Megan Tabak, Corrinne Fogg, Dragana Zubcevic, and Jessica Coco. I have benefited from being in the company of fellow students, some of whom have moved on: Ken Marr, Greg Wallace, John Liptac, Vincent Tang, Howard Yuh, Ishtak Karim, Jennifer Ellsworth. Alex Ince-Cushman, Matt Reinke, Rachael McDermott, Sanjay Gangadhara, Brock Bose, Arturo Dominguez, Eric Edlung, Liang Lin, Natalia Krasheninnikova, Eugenia Smirnova, Andrey Simakov, Chris Boswell, Alex Mazurenko, Bo Bai, Vadim Roytershtein, Tim Graves, Marco Ferrara, Noah Smick, Xenia Samokhvalova, Alexey Kuznetsov, Eric Nelson-Melby, Davis Lee, Jinseok Ko, Dave Schmittdiel, Alex Parisot, and Eugenio Ortiz. Extracurricular activities helped me free my mind and release stress. I thank the entire Alcator softball team for teaching me this wonderful sport and giving me a chance to play. I particularly want to recognize the team leaders who embodied team spirit: Bill Parkin and Valerie Censabella. I will never forget Bob Granetz who introduced me into scuba diving. He is the most skillful underwater navigator and 'lobstah divah' I have ever seen. I would like to thank my friends with whom I shared trips to Maine, New Hampshire, and New York, and MIT soccer club 'footie'. I am grateful to Paul Rivenberg and Mary Pat McNally for their help in organizing the PSFC Student Alumni Club and the setting up a website for that. I say many thanks to my mom Svetlana and brother Maxim Zhurovich for their love and support. My mom always wanted me to become a scientist and come to the US. So, her dream came true and I know she is very proud. Finally, I thank my long time girlfriend Lena Kadeykina for being by my side all these years. Lena gave me her love, encouragement, and support when I needed it. She even stayed long hours at the PSFC helping me do the calibrations and handle some TS components. I doubt that I would be able to endure the stress and perils of graduate student life without her.