Mixed hydrogen-deuterium plasmas on JET ILW

Abstract

A study of mixed hydrogen-deuterium H-mode plasmas has been carried out in JET-ILW to strengthen the physics basis for extrapolations to JET D-T operation and to support the development of strategies for isotope ratio control in future experiments. Variations of input power, gas fuelling and isotopic mixture were performed in H-mode plasmas of the same magnetic field, plasma current and divertor configuration. The analysis of the energy confinement as a function of isotope mixture reveals that the biggest change is seen in plasmas with small fractions of H or D, in particular when including pure isotope plasmas. To interpret the results correctly, the dependence of the power threshold for access to type-I ELMing H-modes on the isotope mixture must be taken into account. For plasmas with effective mass between 1.2 and 1.8 the plasma thermal stored energy () scales as , which is weaker than that in the ITER physics basis, IPB98 scaling. At fixed stored energy, deuterium-rich plasmas feature higher density pedestals, while the temperature at the pedestal top is lower, showing that at the same gas fuelling rate and power level, the pedestal pressure remains constant with an exchange of density and temperature as the isotope ratio is varied. Isotope control was successfully tested in JET-ILW by changing the isotope ratio throughout a discharge, switching from D to H gas puffing. Several energy confinement times (300 ms) are needed to fully change the isotope ratio during a discharge.

Isotope dependence of energy, momentum and particle confinement in tokamaks

“…(for i = 1, 2) from 1 to 2 (King et al 2017(King et al , 2020. The isotope composition was inferred from the line intensity ratios of the Balmer-α lines of the two species, as measured from high resolution divertor spectroscopy (Neverov et al 2017).…”

“…Experiments at I p = 1.4 MA and B T = 1.7 T with 8–10 MW of NBH in mixed hydrogen/deuterium plasmas have allowed to scan the effective mass (for i = 1, 2) from 1 to 2 (King et al . 2017, 2020). The isotope composition was inferred from the line intensity ratios of the Balmer- α lines of the two species, as measured from high resolution divertor spectroscopy (Neverov et al .…”

“…The thermal stored energy in these discharges does not rise linearly with A eff , instead exhibiting a plateau with near constant stored energy for 1.2 ≤ A eff ≤ 1.8, seen in figure 15 for 8 ≤ P NBH ≤ 10 MW (King et al . 2020). While the plateau stored energy is constant, however, the pedestal density rises from near 2.3 × 10 19 m −3 for A eff < 1.5 to ~3 × 10 19 m −3 for A eff = 1.85, while at the same time the pedestal temperature drops from ~0.6 keV to 0.5 keV (King et al .…”

“…While the plateau stored energy is constant, however, the pedestal density rises from near 2.3 × 10 19 m −3 for A eff < 1.5 to ~3 × 10 19 m −3 for A eff = 1.85, while at the same time the pedestal temperature drops from ~0.6 keV to 0.5 keV (King et al . 2020). The beam species for the mixed plasmas were also mixed, but consisted mostly of hydrogen and where therefore not matched to the bulk plasma composition established by adjusting the gas puff rates for the two species.…”

Overview of JET results for optimising ITER operation

“…Mixed isotope (H-D) experiments [60] were extended to 2.3 MA and 2.3 T to include vertical kicks for the control of f ELM and thus ensure similar pedestal conditions. At constant gas and power, the thermal stored energy depends linearly on the effective mass across the probed effective mass (m EFF ) from 1.0 to 2.0, in contrast to earlier results at 1.4 MA and 1.7 T [60] which showed a dependence on the m EFF only at the extremes of the scan, i.e. close to full purity of H and D, respectively.…”