Experiments in high-performance JET plasmas in preparation of second harmonic ICRF heating of tritium in ITER

Abstract

The reference ion cyclotron resonance frequency (ICRF) heating schemes for ITER deuterium–tritium (D-T) plasmas at the full magnetic field of 5.3 T are second harmonic heating of T and 3He minority heating. The wave-particle resonance location for these schemes coincide and are central at a wave frequency of 53 MHz at 5.3 T. Experiments have been carried out in the second major D-T campaign (DTE2) at JET, and in its prior D campaigns, to integrate these ICRF scenarios in JET high-performance plasmas and to compare their performance with the commonly used hydrogen (H) minority heating. In 50:50 D:T plasmas, up to 35% and 5% larger fusion power and diamagnetic energy content, respectively, were obtained with second harmonic heating of T as compared to H minority heating at comparable total input powers and gas injection rates. The core ion temperature was up to 30% and 20% higher with second harmonic T and 3He minority heating, respectively, with respect to H minority heating. These are favourable results for the use of these scenarios in ITER and future fusion reactors. According to modelling, adding ICRF heating to neutral beam injection using D and T beams resulted in a 10%–20% increase of on-axis bulk ion heating in the D-T plasmas due to its localisation in the plasma core. Central power deposition was confirmed with the break-in-slope and fast Fourier transform analysis of ion and electron temperature in response to ICRF modulation. The tail temperature of fast ICRF-accelerated tritons, their enhancement of the fusion yield and time behaviour as measured by an upgraded magnetic proton recoil spectrometer and neutral particle analyser were found in agreement with theoretical predictions. No losses of ICRF-accelerated ions were observed by fast ion detectors, which was as expected given the high plasma density of n e ≈ 7–8 × 1019 m−3 in the main heating phase that limited the formation of ICRF-accelerated fast ion tails. 3He was introduced in the machine by 3He gas injection, and the 3He concentration was measured by a high-resolution optical penning gauge in the sub-divertor region. The DTE2 experiments with 3He minority heating were carried with a low 3He concentration in the range of 2%–4% given the fact that the highest neutron rates with 3He minority heating in D plasmas were obtained at low 3He concentrations of ∼2%, which also coincided with the highest plasma diamagnetic energy content. In addition to 3He introduced by 3He gas injection, an intrinsic concentration of 3He of the order of 0.2%–0.4% was measured in D-T plasmas before 3He was introduced in the device, which is attributed to the radioactive decay of tritium to 3He. According to modelling, even such low intrinsic concentrations of 3He lead to significant changes in ICRF power partitioning during second harmonic heating of T due to absorption of up to 30% of the wave power by 3He.

Overview of interpretive modelling of fusion performance in JET DTE2 discharges with TRANSP

“…Additional favourable mechanisms for the decoupling between the temperatures were recognised, such as the increase in plasma rotation due to increased beam torque and low gas puffing, and its effects on ITG stabilization [74]. We note that the various RF scenarios, extensively tested during DTE2 [35], also display a relatively high ion/electron heating ratio and T i /T e consistently above unity. This can be partially ascribed to the positive effects of various RF heating schemes-in addition to direct heating of the electron population, an RF fast ion population is formed, initially transferring energy to electrons through collisions.…”

“…These are also planned to be used to support the TRANSP analyses of similarly complex DTE2 RF heating experiments, e.g. second harmonic D and T heating [35], and the three-ion T-( 9 Be)-D scheme [77,107].…”

“…Scenario aimed at facilitating conditions for measurement of alpha heating and alpha driven effects [31,32]; • RF heating (RF): mostly hybrid-like H-mode discharges at modest toroidal current I p ∼ 2.5 MA and varying toroidal field, testing different RF heating schemes in JET D-T with Be/W wall, envisaged for ITER D-T operation, e.g. second harmonic D majority, second harmonic T majority, and 3 He minority [33][34][35][36]; • Seeded: 50-50 D-T, neon-seeded ELMy H-mode performed at high triangularity with strike points on vertical target, at I p = 2.5 MA, toroidal field of B t = 2.8 T, safety factor of q 0 < 1 and q 95 = 3.2, beta values of β P < 1 and β N ∼ 1.2-2., high electron density (core line-averaged density ≳7.5 × 10 19 m −3 ), mixed D-T NBI and H RF minority heating, aimed at assessing for the first time the core-edge integration of an ELMy H-mode in D-T with a Be/W wall and a partially detached divertor [37,38]; • Calibration: 50-50 D-T H-mode discharges with mixed D-T NBI-only heating, performed at a toroidal current of I p = 2.0 MA and toroidal field of B t = 2.4 T, aimed at crosscalibrating the neutron yield monitors against foil activation measurements, performed with the neutron activation system. The neutron calibration discharges were chosen as representatives of low-power shots, typically run for specific transport and confinement physics studies, exhibiting similar operational parameters and fusion performance drivers.…”

“…For instance, extremely efficient ion heating was demonstrated in dedicated T-rich experiments, with RF power densities on bulk and fast ions being a factor of 2-4 higher than those deposited on electrons. Other DTE2 RF heating schemes, relevant for ITER D-T RF operation, include fundamental resonance on D ions, 3 He minority and second harmonic T heating [35], as well as the 3-ion T-( 9 Be)-D scheme. In the latter a resonant ion-ion hybrid layer is formed due to the presence of a minority ion species with an intermediate charge-to-mass ratio with respect to the majority D-T ions, in this case JET's intrinsic metal wall impurity 9 Be [77].…”

The JET hybrid scenario in Deuterium, Tritium and Deuterium-Tritium

“…Another reason to choose the magnetic field of 3.45 T was be to more compatible with the ICRH schemes planned for D-T operation [43,44]. The increase in q 95 from 3.9 to 4.8 for JET has no consequences in reaching high fusion performance.…”