Detecting alpha-particle heating in JET-ILW DT Hybrid discharges by the delayed electron temperature response to ICRH modulation

In the recent DTE2 campaign of the tokamak JET-ILW, a new method to detect the alpha-particle heating by the delayed electron temperature Te response to the modulation of the Ion Cyclotron Resonance Heating (ICRH) has been tested. To better bring out the temperature modulation, JET-ILW hybrid scenarios have been used to avoid sawtooth activity and to minimise the core ion temperature stiffness level. Particularly, this work is focused on the JET T-rich hybrid plasma pulse #99965 with 15% - 85% D-T mix, operating at 3.86 T and 2.5 MA, heated up by ~ 29MW of D Neutral Beam Injection (NBI) power and by 3-4 MW of ICRH square-wave modulated power (1 Hz with 50 % duty-cycle) used in n = 1 D scheme. Since this ICRH cycle leads to a direct ion temperature Ti modulation and to a modulation of the NBI fast ions, the alpha-particle production from thermal-thermal and beam-thermal fusion reactions is also modulated. Hence the electron heating due to the alpha-particles modulates Te, but with a certain delay due to the long alpha-particle slowing down time. Analysing the time evolutions of Te and Ti, respectively measured by ECE radiometer and active charge exchange spectroscopy, modulation amplitudes of ~ 10% with centrally peaked profiles were found and the central phase delay with respect to ICRH power resulted much larger for the Te (~ 105deg) than for the Ti (~ 50deg). Such large delay could only be due to alphas since the NBI fast ion, even when accelerated by ICRH, are predicted to have energies ~ 250keV, with much shorter slowing down time than alphas. An extensive modelling work of the discharge #99965 using different transport codes and ICRH and NBI codes has been carried out to support this conclusion.