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Burning Plasma: Bringing a Star to Earth
The committee found it useful to assess the scientific readiness to undertake a burning plasma experiment in terms of the following six well-defined criteria:
Operational boundaries—plasma pressure and current,
Mitigation of abnormal events,
Maintenance of plasma purity,
Characterization techniques, and
Plasma control techniques.
It is the committee’s judgment that each of these six scientific areas must be sufficiently understood before a burning plasma experiment can be positioned for success. Each of these criteria is discussed and analyzed below. As a whole, this analysis allows for an estimate to be made of the state of readiness for undertaking a burning plasma experiment.
Reaching the burning plasma regime depends critically on the rate at which energy is lost from the plasma. This energy-loss rate can be projected on the basis of confinement scaling, scaling with similar nondimensional parameters, or models of the plasma transport averaged over magnetic-flux surfaces. Each of these methods of projecting energy-loss rates predicts that ITER will meet the goal of producing 10 times more power via fusion reactions in the plasma than the input power used to heat the plasma (i.e., Q = 10).
It is possible to predict accurately the energy-loss rate in existing tokamak experiments through confinement scaling studies that fit the observed energy confinement time τE (where τE is the reciprocal of the energy-loss rate from the tokamak global database of about a thousand discharges in eight large tokamaks) as a power law in the appropriate discharge parameters. The validity of this technique has been confirmed by results from the new-generation tokamaks. An extrapolation of the energy confinement time by a factor of approximately 3 is required to go from the best confinement time in present large tokamaks to ITER. A relevant measure of fusion performance is the “fusion triple product,” nTτE, which is roughly proportional to the fusion gain factor, Q. Figure 3.1 displays this fusion triple product for tokamak discharges as a function of the value predicted by the scaling analysis. The present database spans three orders of magnitude in nTτE. An extrapolation by an additional factor of 20 is required to reach the nominal ITER operating point corresponding to a fusion gain Q = 10.