Stability of alpha particle driven Alfvén eigenmodes in high performance JET DT plasmas

Abstract

The stability of alpha particle driven Alfvén eigenmodes (AEs) is analysed in high fusion power DT discharges on JET. Both hot ion H mode and shear optimized discharges are considered. Unstable AEs are not observed in hot ion H mode DT discharges even at the highest fusion power with alpha particle beta βα (0) ≈ 0.7%. Theoretical analysis shows that the AE stabilization is caused by the large plasma pressure, which prevents the existence of core localized AEs at peak fusion performance. Kinetic toroidal AEs (KTAEs), which persist at high plasma pressure, are found to be radially extended and subject to strong damping. The stability analysis based on the CASTOR-K code confirms that AEs cannot be driven unstable by alpha particles in high performance hot ion H mode discharges performed at JET. Alfvén eigenmodes in shear optimized regimes are more unstable than those in the hot ion H mode mainly due to the elevated central safety factor q, which increases the efficiency of AE interaction with energetic ions. As a consequence, AEs are observed in shear optimized DT discharges when ion cyclotron heating as low as 1 MW is applied.

Energetic-Ion-Driven Global Instabilities Observed in the Large Helical Device and Their Effects on Energetic Ion Confinement

“…This was confirmed theoretically [16]. The C-TAE with even parity was preferentially observed in tokamaks [17][18][19]. The C-TAE with odd parity was also observed later in the Joint European Torus (JET) [20].…”

Chapter 5: Burning Plasma Studies at JET

“…19!. It is important to note, however, that during the high-fusion power D-T experiments, 9 unstable AEs were not observed in hot ion H-mode even at the highest fusion power. On the other hand, in the optimized shear plasmas aiming at developing internal transport barriers 20~I TBs!, AEs were driven unstable by the ICRH-accelerated minority ions when ICRH power as low as 1 MW was applied so that possible AE instabilities driven by alpha particles could not be unambiguously identified.…”

“…[6][7][8] A population of energetic 4 He ions can be obtained in D-T fusion reactions with quite significant values of the fast ion density and energy contents. 9,10 It is also possible to accelerate a population of 4 He ions up to the MeV energy range with NBI plus ICRH technique in radiation-free helium plasma. 11 Among the techniques of plasma heating, NBI heating with a power in excess of 20 MW plays a major role in JET~Ref.…”

Review of D-T Experiments Relevant to Burning Plasma Issues

“…Neutral beam and ICRF experiments on TFTR and also on the other large facilities have shown that a large fraction of the particles can be ejected from the plasma at sufficiently high power. Compared with the high power neutral beam and ICRF experiments used to excite the toroidal Alfvén eigenmode instability, the alpha power was relatively low and this was not a violent instability in either TFTR or JET D-T experiments [11,20,24]. In particular, in the highest fusion power discharges, this instability was not driven by alpha particles and did not have a deleterious effect.…”