JET D-T scenario with optimized non-thermal fusion

Abstract

In JET deuterium-tritium (D-T) plasmas, the fusion power is produced through thermonuclear reactions and reactions between thermal ions and fast particles generated by neutral beam injection (NBI) heating or accelerated by electromagnetic wave heating in the ion cyclotron range of frequencies (ICRFs). To complement the experiments with 50/50 D/T mixtures maximizing thermonuclear reactivity, a scenario with dominant non-thermal reactivity has been developed and successfully demonstrated during the second JET deuterium-tritium campaign DTE2, as it was predicted to generate the highest fusion power in JET with a Be/W wall. It was performed in a 15/85 D/T mixture with pure D-NBI heating combined with ICRF heating at the fundamental deuterium resonance. In steady plasma conditions, a record 59 MJ of fusion energy has been achieved in a single pulse, of which 50.5 MJ were produced in a 5 s time window (P fus = 10.1 MW) with average Q = 0.33, confirming predictive modelling in preparation of the experiment. The highest fusion power in these experiments, P fus = 12.5 MW with average Q = 0.38, was achieved over a shorter 2 s time window, with the period of sustainment limited by high-Z impurity accumulation. This scenario provides unique data for the validation of physics-based models used to predict D-T fusion power.

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

“…The high fusion performance branch of DTE2 experiments has confirmed these observations, albeit with the combined NBI and RF power reaching maximum values of around 35 MW [25,28,30]. In figure 3 we display how the onesecond-averaged TRANSP computed power depends on the total external heating power P heat , launched by the heating systems, for the DTE2 modelling database.…”

“…• Baseline: 50-50 D-T, ELMy (not type I) H-mode scenario generally performed at high toroidal current I p ⩾ 3.0 MA, toroidal field B t ∼ 3.3 T, safety factor of q 0 < 1 and q 95 ∼ 3, beta values of β P < 1 and β N ∼ 1.8-2.0, high electron density (core line-averaged density ≳7.5 • 10 19 m −3 ), mixed 50-50 D-T NBI and H RF minority heating, D pellet ELMpacing, aimed at maximizing thermal fusion performance [20,24,25]; • Hybrid: 50-50 D-T, Type I ELMy H-mode scenario generally performed at lower current than baseline with 2.2 ⩽ I p ⩽ 2.5 MA, toroidal field B t ∼ 3.45 T, shaped broad qprofile at q 0 ⩾ 1 and 4.5 ⩽ q 95 ⩽ 5.0, beta values of β P ⩾ 1 and β N ∼ 2.0-2.3, modest electron density (core lineaveraged density within 4.0 × 10 19 ≲ ne ≲ 5.5 × 10 19 m −3 ), mixed 50-50 D-T NBI and H RF minority heating, aimed at improved stability and reduced core transport for better confinement [26][27][28]; • T-rich: hybrid-like H-mode scenario optimized to maximize non-thermal fusion with a D-T fuel mix of ∼15%-85%, single species D NBI and fundamental D RF heating with T gas injection-achieved fusion energy record in DTE2 [29,30]; • Energetic particle afterglow (EP): 50-50 D-T H-mode discharges with mixed D-T NBI-only heating with two reference plasmas-ITB lowest density discharges for high transient fusion power, exploiting q 2 scaling in TAE stability, performed at toroidal current I p = 2.9 MA, toroidal field B t = 3.45 T, elevated q-profile with q 0 > 1.5, q 95 = 3.8 and conventional magnetic shear, with β N ∼ 1.3 and T i ≫ 2T e .…”

“…The highest-performing discharges of the campaign were the hybrid-like T-rich plasmas, in which the fuel mixture and beam settings were optimized to maximize fusion reactivity. This was achieved by developing a scenario with the beam species in deuterium-only, and the thermal ion species being tritium, in order to exploit the favourable fast deuterium → thermal T reactivity at beam injection energy of ∼110 keV, typical for JET's NBI system [30,69]. This scenario yielded the largest fusion energy output ever at 59 MJ, with high P fus sustained for 5 s. Two of the modelled EP scenario discharges, employing a hybrid-like scheme using only Left: TRANSP computed total (solid) and thermal (dashed) neutron rate density profiles rn, normalized to the total maximum value.…”

“…TRANSP and ASCOT simulations have shown that the effect of including rotation profiles on the neutron yield was up to 10% in deuterium high performance plasmas, with the addition of rotation generally resulting in a net yield decrease [75]. Further investigations into the effects of rotation in high performance D-T plasmas are presented in [30]. On the z-axis we display the ratio between the beam deposition source in the core (at ρ = 0.25) and at the edge (at ρ = 0.8), which is a measure of the beam deposition profile peakedness.…”

“…In highest-performing deuterium plasmas the direct contribution of RF fast ions to fusion performance was found to be between 10% ∼ 15%, with the beam synergy effect having a large role due to a monotonously increasing D-D cross-section [15,18,45,78]. In D-T plasmas the direct performance contribution of RF is found to be smaller, varying between 3% ∼ 5% for synergy fast ions in baseline and hybrid discharges [76,79,80], to approximately 10% for fundamental D large minority heating in the T-rich experiment, discussed in more detail in section 4 and other dedicated studies [30].…”