Measuring fusion gain Q > 1 in SPARC

An early-operations mission of the SPARC tokamak [Creely 2020 JPP] is to achieve a fusion plasma gain Q > 1, a measurement which will be fairly scrutinized. In this talk, we present an approach to make this claim with confidence and transparency. The numerator of Q will be the total fusion power (P_fus) from a majority DT plasma with He3 as a minority species. A suite of neutron diagnostics will make complementary and redundant (primarily DT) fusion neutron measurements. Kinetic profile information (density, temperature, and dilution) will also be used to assess P_fus, with a minimum viable subset of measurements and chords down-selected by exploring a range of physical synthetic profiles. The denominator of Q will be the sum of ohmic power (measured by standard magnetics) and radio frequency (RF) heating less the stored energy’s rate of change (dW/dt). The RF contribution will be the power launched from the antennas, not necessarily absorbed by the plasma, which is both conservative and easier to measure. A relative contribution of |dW/dt| <10% is targeted to define a "quasi-stationary" scenario. Lastly, statistical methods are explored to evaluate confidence in Q > 1 based on these diagnostics’ data and expectations for their random and systematic uncertainties.