Evidence of Electron Heating by Alpha Particles in JET Deuterium-Tritium Plasmas

The fusion-born alpha particle heating in magnetically confined fusion machines is a high priority subject for studies. The self-heating of thermonuclear fusion plasma by alpha particles was observed in recent deuterium-tritium ($D\text{−}T$) experiments on the joint European torus. This observation was possible by conducting so-called “afterglow” experiments where transient high fusion yield was achieved with neutral beam injection as the only external heating source, and then termination of the heating at peak performance. This allowed the first direct evidence for electron heating of plasmas by fusion-born alphas to be obtained. Interpretive transport modeling of the relevant $D\text{−}T$ and reference deuterium discharges is consistent with the alpha particle heating observation.

Figure 1

(a) and (b) Waveforms of the $D\text{−}T$ JET pulses; (c) and (d) The deuterium JET pulses (the waveforms were shifted in time to align both NBI afterglow periods). The panels show waveforms of central electron temperatures, $T_e(0)$, and measured neutron rates, where the dashed line is marking the start of the NBI afterglow period.

Figure 2

Waveforms of the $D\text{−}T$ JET pulse No. 99801 (red lines) vs the deuterium JET pulse No. 100793 (blue lines); panels show waveforms of the NBI heating power, the central electron temperature, $T_e(0)$ and the measured neutron rate; dashed line marks the NBI power cut.

Figure 3

ECE measurements of the electron temperature profile ($T_e$) and high-resolution Thomson scattering measurements of the electron density profiles ($n_e$): red lines—smoothed profiles during NBI heating ($t\approx 8.09\mathrm{s}$); blue lines—smoothed profiles in the afterglow ($t\approx 8.15\mathrm{s}$).

Figure 4

(a) Fourier spectrogram of an in-vessel magnetic pickup coil (upper box) and FILD (bottom box) signals detected in JET discharge No. 99801; dashed line marks the NBI power cut. (b) Typical footprint of alpha particle losses recorded with FILD in the afterglow ($t=8.15–8.20\mathrm{s}$). Grid—gyroradius (cm) and pitch angle (degree); white dashed line indicates the gyroradius related to the smoothed pitch-angle distributions ($t=8.0–8.1\mathrm{s}$ and 8.15–8.20 s) presented in Fig. 4.

Figure 5

TRANSP [10] neutron rate modeling of JET $D\text{−}T$ discharge No. 99801 (a) and deuterium discharge No. 100793 (b). The thermal, beam-target, and beam-beam neutron rate components are presented as well as the total calculated rate and measured one; dashed line marks the NBI power cut.

Figure 6

TRANSP [10] analysis of electron heating in JET $D\text{−}T$ discharge No. 99801(a) and deuterium discharge No. 100793 (b). The power transferred to electrons (left scale) by alphas, NBI, and thermal ions (-Qie) are presented as well as electron temperature on axis (TE) and a difference between the ion and electron temperatures (TI-TE) in the plasma core (right scale); dashed line marks the NBI power cut.

Figure 7

TRANSP analysis related to the plasma centrer, $\rho <0.05$; (a) comparison of the thermal and alpha particle pressure in No. 99801; (b) evolution of the total energy confinement time (smoothed) in No. 99801 and No. 100793 discharges; red dashed lines denote the time slot for calculation of the averaged pressure and energy confinement time parameters (red); black dashed line marks the NBI power cut.