Good confinement of the fusion-born alpha particles is essential to ensure adequate burning plasma performance in next-step fusion devices. Among the processes determining this confinement, instabilities triggered by energetic particles (EPs) may play a major role, and are currently being studied in various tokamaks using auxiliary power sources to sustain EP populations. Instabilities resulting from fusion-born alphas, on the other hand, can only be observed in deuterium-tritium (D-T) plasmas. Since DTE1, the D-T campaign conducted in the Joint European Torus (JET) in 1997, the device has undergone significant changes, among which the installation of a Be/W ITER-like wall (ILW) and the development of new diagnostics directly relevant to the physics of energetic ions, in particular alphas. The preparation of a new D-T campaign (DTE2) in JET [Joffrin2019] thus includes various developments relevant to burning plasmas [Sharapov2008]. As JET is currently the only tokamak in which D-T plasmas can be produced, DTE2 constitutes the only opportunity to experimentally document the physics of alphas, and validate the numerical tools used to simulate their effects before ITER comes into operation. Among the instabilities related to the presence of EPs, alpha-driven Toroidal Alfvén Eigenmodes (TAEs) have received some attention in the past. The rationale is that the features of the alpha population differ significantly from those of energetic ions created by external sources. As a result, the instability itself differs and its impact on the plasma performance remains to be evaluated. Because of the relatively low values of normalized alpha pressure (??) attained in the only two magnetic confinement fusion devices capable of D-T operation to this day, TFTR [Nazikian1997] and JET [Sharapov1999], core-localized alpha-driven TAEs have been difficult to observe unambiguously. From these experiments and from results obtained during the present effort in JET [Dumont2018], it has been established that their observation requires i) a sufficient alpha pressure, ii) an elevated safety factor (q), iii) an "afterglow phase" consisting of abruptly switching off all external EP sources and rely on the longer slowing-down of alphas compared to other ions present in the pulse to isolate their impact, including the destabilization of TAEs. The afterglow has been key to the success of the experiments performed in TFTR [Nazikian1997]. In terms of scenario, these conditions translate into i) low density to favour large electron and ion temperatures, ii) large NBI power to maximise the fusion yield, iii) no ICRH power before the afterglow phase to exclude any contribution from ICRH-driven ions to the TAE drive, iv) an elevated q-profile. In preparation for DTE2, advanced scenarios fulfilling these requirements have been under development in deuterium plasmas during the last experimental campaigns. In pulses at 3.4T/2.5MA, NBI waveforms have been fine-tuned to inject the power early in the pulse and thus obtain elevated q-profiles, while fulfilling the requirements of the ILW in terms of beam shine-through. Operating at line-integrated densities in the range 5 - 9 × 1019m-2 has allowed clear Internal Transport Barriers (ITBs) to be observed in JET-ILW.
Scenario Preparation for the Observation of Alpha-Driven Instabilities and Transport of Alpha Particles in JET DT Plasmas
Giacomelli L;
2021
Abstract
Good confinement of the fusion-born alpha particles is essential to ensure adequate burning plasma performance in next-step fusion devices. Among the processes determining this confinement, instabilities triggered by energetic particles (EPs) may play a major role, and are currently being studied in various tokamaks using auxiliary power sources to sustain EP populations. Instabilities resulting from fusion-born alphas, on the other hand, can only be observed in deuterium-tritium (D-T) plasmas. Since DTE1, the D-T campaign conducted in the Joint European Torus (JET) in 1997, the device has undergone significant changes, among which the installation of a Be/W ITER-like wall (ILW) and the development of new diagnostics directly relevant to the physics of energetic ions, in particular alphas. The preparation of a new D-T campaign (DTE2) in JET [Joffrin2019] thus includes various developments relevant to burning plasmas [Sharapov2008]. As JET is currently the only tokamak in which D-T plasmas can be produced, DTE2 constitutes the only opportunity to experimentally document the physics of alphas, and validate the numerical tools used to simulate their effects before ITER comes into operation. Among the instabilities related to the presence of EPs, alpha-driven Toroidal Alfvén Eigenmodes (TAEs) have received some attention in the past. The rationale is that the features of the alpha population differ significantly from those of energetic ions created by external sources. As a result, the instability itself differs and its impact on the plasma performance remains to be evaluated. Because of the relatively low values of normalized alpha pressure (??) attained in the only two magnetic confinement fusion devices capable of D-T operation to this day, TFTR [Nazikian1997] and JET [Sharapov1999], core-localized alpha-driven TAEs have been difficult to observe unambiguously. From these experiments and from results obtained during the present effort in JET [Dumont2018], it has been established that their observation requires i) a sufficient alpha pressure, ii) an elevated safety factor (q), iii) an "afterglow phase" consisting of abruptly switching off all external EP sources and rely on the longer slowing-down of alphas compared to other ions present in the pulse to isolate their impact, including the destabilization of TAEs. The afterglow has been key to the success of the experiments performed in TFTR [Nazikian1997]. In terms of scenario, these conditions translate into i) low density to favour large electron and ion temperatures, ii) large NBI power to maximise the fusion yield, iii) no ICRH power before the afterglow phase to exclude any contribution from ICRH-driven ions to the TAE drive, iv) an elevated q-profile. In preparation for DTE2, advanced scenarios fulfilling these requirements have been under development in deuterium plasmas during the last experimental campaigns. In pulses at 3.4T/2.5MA, NBI waveforms have been fine-tuned to inject the power early in the pulse and thus obtain elevated q-profiles, while fulfilling the requirements of the ILW in terms of beam shine-through. Operating at line-integrated densities in the range 5 - 9 × 1019m-2 has allowed clear Internal Transport Barriers (ITBs) to be observed in JET-ILW.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
