Measurement of Interaction of MeV Energy Protons with Lower Hybrid Waves in JET

摘要

NPA measurements of perpendicular distribution function of ICRF-heated protons in the energy range 0.3 ≤ E (MeV) ≤ 1.1 during Lower Hybrid (LH) Current Drive experiments in JET showed efficient coupling of LH power to the high energy protons. The coupling was deduced for driven as well for slowing-down proton populations. The significance of this measurement is that such interaction represents large parasitic dissipation of LH power, intended for electron current drive, by a fast ion population, with implications for efficiency of LH current drive in fusion plasmas. Perpendicular Landau damping enables LH waves to couple energy to ions when the resonance condition v_⊥ > v_(RES) = c_(⊥MAX) is satisfied, where n_(⊥MAX) is the maximum perpendicular refractive index in the LH wave spectrum. The observed interaction is contrary to expectations in two respects: 1. according to prevailing views of LH wave dispersion in JET plasmas the cut-off energy for interaction, E_(RES) = m_p v~2_(RES)/2, should lie in the MeV energy range, corresponding to n_(⊥MAX) ≈20. Experimentally, coupling of LH power to protons was observed in the whole measurement range, suggesting that n_(⊥MAX) ≥ 40; 2. from the measurements, the fraction of LH power coupled to fast protons was deduced to be ≈ 30%, much exceeding the expectation of ≈ 5% -10%, based on n_(⊥MAX) ≈ 20.