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Graduate students gather on the landing of the Alcator C-Mod experiment. Recent Research Activities Research on C-Mod continued during the past two years in the topical science areas of transport, wave-plasma interactions, pedestal physics, boundary physics and magneto-hydrodynamic stability, as well as in the integrated thrust areas of H-Mode Inductive Scenarios and Alternate Tokamak Scenarios. A key challenge in fusion energy is to confine the input heat long enough for the hot ionized hydrogen fuel, or plasma, to fuse and produce net energy. Over 25 years ago, the spontaneous formation of an edge transport barrier was discovered in Germany on the ASDEX tokamak, which roughly doubled the energy confinement. This “high confinement” (or H-mode) regime is obtained routinely in most tokamaks, and is expected to be the fundamental mode of operation in the international ITER project. However, these edge transport barriers also improve confinement of plasma particles, including unwanted impurities and spent fuel, which could contaminate and dilute the deuterium plasma, and prevent or even extinguish the fusion reaction. Some means of expelling the particles is thus needed. This can be accomplished by naturally occurring bursts of plasma blobs from the edge (ELMS, or edge localized modes), but they are of concern since they may erode the material surfaces of the wall of the tokamak. On Alcator C-Mod re searchers are studying a new regime that has an energy transport barrier similar to that in H- mode, but without the unwanted increase in particle confinement, leading to ELMS. This leads to steady, readily controllable densities and low radiated power, in most cases without any large-scale bursts. The Alcator C-Mod edge and core temperatures often increase dramatically, up to 5 keV (55 millionºC) in the core, and energy confinement reaches or exceeds the H-mode scalings on which the ITER design is based. A general, gradual decrease in the broadband edge turbulence is seen as the barrier is formed. A “weakly coherent” mode appears at higher frequencies, which may be responsible for regulating particle transport in such favorable confinement regimes, named the I-Mode. Such a regime has been obtained using RF heating and maintained in steady state for times greatly exceeding the energy confinement time scale. It has been studied over a wide range of plasma parameters, with toroidal magnetic fields up to 6 T and plasma currents up to 1.3 million amperes, and its The C-Mod divertor is armored with high heat-flux molybdenum tiles. PSFC Progress Report 09–11 9
access is favored by certain plasma shapes and magnetic topologies. Current C-Mod experiments are revealing the physics underlying this attractive regime, in particular the favorable decoupling of heat and particle transport. An assessment of its prospects for burning plasma experiments in ITER is underway. Developing magnetic confinement fusion into a practical energy source will require creating plasmas sufficiently hot and dense for fusion reactions, simultaneous with sustainable power fluxes to the reactor walls. Attempts to meet these requirements have typically resulted in plasmas that would produce abundant fusion power when scaled to a reactor, but would lead to local overheating of the components protecting its wall; or to the reverse—plasmas whose power outflow could be handled, but which would produce too little fusion power. The critical component, called a divertor, is located at the area of the vessel wall where the highest power fluxes are expected. New experiments in Alcator C-Mod with high levels of radio-frequency heating power have injected impurities to decrease local deposition of the exhaust plasma power. These experiments demonstrated that the requirements for high performance and acceptable power exhaust could be met in a fusion reactor, such as ITER. The Al- Two dipole radio-frequency antennas on the outer wall of the C-Mod vacuum vessel. Each antenna is used to couple up to 2 million watts of radio wave power into the plasma. The waves have a frequency of 80 million cycles per second, and after the waves damp on the hydrogen ions (protons) in the plasma, collisions of these fast protons with the colder background electrons and deuterons heat the plasma to bulk temperatures of up to 100 million degrees Kelvin. cator C-Mod experiments confirm that the key parameter governing fusion plasma performance is the power that exhausts through the edge of the confined plasma, which needs to be above a critical value, the so-called high confinement power threshold. Research on Alcator C-Mod has also shown that this minimum exhausted power for good confinement can be safely distributed by enhanced radiation from impurity ions in the exterior layers of the plasma, thereby avoiding both local wall overheating or deteriorating fusion performance. The key new feature of the Alcator C-Mod experiments is that the ratio of the plasma heating power to the minimum exhausted power for good confinement is large, allowing the requirements for good plasma confinement and large levels of electromagnetic radiation by impurities to be met for the first time. While the results from Alcator C-Mod are already significant for ITER, they are even more so for a follow-up fusion demonstration reactor (DEMO), which will demonstrate net electricity production to the grid. Such reactors are expected to have a similar requirement with regards to the minimum exhausted power and for good confinement as in ITER, but with fusion energy production about 5 times larger, posing a much greater challenge for local wall heating. The Alcator C-Mod results show that in these future reactors, redistributing the exhaust power by impurity radiation at the plasma edge and the exterior layers of the plasma is a viable option. One potential problem with the dissipation of heat load on the walls is that heat that leaks out the “magnetic bottle” may become focused into narrow channels as it streams along magnetic field lines in adjoining boundary layers. This produces narrow “footprints” on wall surfaces. The smaller the footprint, the more intense the heat flux becomes, and the intensity may exceed the power handling ability of present wall-protection materials. But physicists have answers for this as well, at least in part: shape wall surfaces and operate in regimes that promote radiative losses. Yet, no one knows if these schemes will be adequate for a power-producing fusion reactor because a fundamental question remains: What underlying physics sets the size of the footprints seen in fusion plasmas? Recent experiments performed in 10 PSFC Progress Report 09–11
Page 1 and 2: Plasma Science & Fusion Center Prog
Page 4 and 5: Contents From the Director.........
Page 6 and 7: to be repeated approximately 10 tim
Page 8 and 9: PSFC Progress Report 09-11 5
Page 10 and 11: Background The Alcator C-Mod tokama
Page 14 and 15: current to toroidal magnetic field
Page 16 and 17: perpendicular to the total edge mag
Page 18 and 19: Background The Physics Research Div
Page 20 and 21: Paul Bonoli is the Principal Invest
Page 22 and 23: experiments when funding of the pro
Page 24 and 25: spatial variations. Similarly, in t
Page 28 and 29: Background The High-Energy-Density
Page 30 and 31: Two proton spectrometers measured t
Page 32 and 33: Background The Waves and Beams Divi
Page 36 and 37: Background The Fusion Technology an
Page 38 and 39: developed a new cabling approach to
Page 40 and 41: Background The Plasma Science and F
Page 42 and 43: plasma science. Like Dr. Temkin, Pa
Page 44 and 45: during the crucial early career yea
Page 46 and 47: The Plasma Science and Fusion Cente

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