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Alpha storage regime in high temperature sub-ignited D-T tokamaks

Alpha particle parameters in sub-ignited D-T tokamaks such as TFTR can be optimized in a high temperature ‘alpha storage regime’ in which the alpha particle thermalization time τα is long (≈1.0 s) and in which the alpha particle source rate Sα is enhanced because of beam-target and beam-beam reactions (by a factor of ≈2-4). Near reactor-level alpha instability parameters, βα(O)≈ nα(0)/ne(0) ≈ 1%, are predicted by simulation codes when Q ≳ 0.5; present TFTR ‘supershots’ already have βα(O) ≈ nα(0)/ne(0)≈ 0.1-0.2%. This alpha storage regime can be used for the first time to test theories of collective alpha instabilities and can also be used to provide a strong (but transient) alpha heating pulse. An experimental scenario to exploit this regime is described.