Exploring a small sawtooth regime in Joint European Torus plasmas with counterinjected neutral beams

Abstract

During a recent reversed toroidal field ͑B T ͒ campaign at the Joint European Torus ͑JET͒, experiments were performed to investigate the effect on sawteeth of neutral beam injection ͑NBI͒-driven toroidal plasma rotation counter to the direction of the toroidal plasma current and B T . A power scan at constant density has permitted analytical continuation, into the reversed B T domain, of previous experiments with forward field and hence corotation. Earlier JET results were confirmed, indicating that counter-NBI results in sawtooth periods shorter than in the Ohmic regime. This study has demonstrated that, whereas with co-NBI the sawtooth period increases with power, with counter-NBI the sawtooth period initially decreases with power passing through a minimum at 4 MW. Clearly this trend also manifests itself in terms of the toroidal plasma rotation, for which a minimum is observed for counter-rotation frequency ϳ3 kHz. Sawteeth smaller than Ohmic sawteeth are found to be easier to obtain with perpendicular counter-NBI, for which heating penetrates deeper into the core. The sign and magnitude of the toroidal rotation, the penetration of heating to the electrons, and the peaking of the fast-ion pressure profile in the core may all play an important role in modifying the sawtooh period. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2162528͔ Developing techniques to control the m =1, n = 1 sawtooth instability as a way of controlling the onset of neoclassical tearing modes ͑NTMs͒ has been an important area of research in recent Joint European Torus 1 ͑JET͒ experimental campaigns. Reducing the sawtooth period was found to trigger the NTMs at higher values of normalized beta. 2 In addition, the importance of sawtooth control has been demonstrated in JET high-density ELMy H-mode plasmas, where sawteeth play a beneficial role in the prevention of core impurity accumulation. 3 In order to achieve a balance between beneficial and deleterious effects, it is important to understand how to maintain small, regularly occurring sawtooth crashes. Sawtooth stabilization leading to long sawtooth periods is a well-known phenomenon in auxiliary heated plasmas. Nevertheless, sawtooth periods shorter than observed in Ohmic regimes have been obtained at JET with ion-cyclotron radio-frequency heating 4 as well as with counter-neutral beam injection ͑NBI͒ heating 5 with the toroidal field B T reversed. Recently this small sawtooth regime with counter-NBI heating has been investigated in detailed experiments, the results of which are reported here.JET operation with reversed B T is performed with the direction of the plasma current I p also reversed, in order to conserve the direction of impact of the field lines on the divertor target plates. JET has two neutral beam injector boxes, each equipped with positive ion neutral injectors ͑PINIs͒, which can provide up to 20 MW of NBI power in total. 6 The PINIs are grouped in tangential and nearly perpendicular banks ͑the latter referred here as "perpendicular" NBI͒. With the usual B T p...

Influence of rotation on the (m,n) = (3, 2) neoclassical tearing mode threshold in the ASDEX Upgrade

“…A further important issue is the effect of rotation on the trigger mechanism. On one hand, the trigger instability itself can depend on plasma rotation as already found for the sawtooth instability [4] [5], on the other hand the seeding process due to magnetic coupling can be influenced by the differential rotation between the two resonant surfaces [6]. Finally, rotation can influence the island stability due to changes of the impact of error fields on the island or due to changes in the interaction between the island and the vessel wall.…”

Sawtooth control and the interaction of energetic particles

“…However, in JET, the toroidal rotation is significantly smaller, and the sawtooth behaviour can only be explained by the effects of the fast ions. The sawtooth period is observed to vary in the same way as in MAST, with lengthening period as the co-NBI power increases, and a minimum in sawtooth period in the counter-NBI regime [19]. This minimum occurs because (i) the counter-passing ions give a strongly destabilizing contribution and (ii) the flow shear in JET reduces the stabilizing effect of the trapped ions injected counter-I p [20].…”

“…For example, shorter sawtooth periods than those in Ohmically heated plasmas can be achieved in NBI-heated plasmas in JET [19,20], MAST [21] and TEXTOR [22]. Each experiment exhibits an asymmetry of sawtooth period with respect to NBI direction.…”

Recent Advances in Sawtooth Control

“…The sawtooth period decreases by a factor of 2 when the off-axis NBI is applied, even though the total NBI power and fast ion pressure increases after 20s. It is also possible to deliberately destabilise sawteeth with on-axis NBI, but with the orientation of the beams counter to the plasma current [38,39]. Such a configuration corresponds to the conditions (∂F/∂r < 0) and F (v > 0) < F v < 0) as described in the introduction.…”

“…Such a configuration corresponds to the conditions (∂F/∂r < 0) and F (v > 0) < F v < 0) as described in the introduction. In JET, these conditions were achieved during a reverse field campaign, where it was shown [38,39] that sawteeth could be destabilised, even relative to Ohmic sawteeth by the application of moderate power of the order of 4MW. The pulses shown in Fig.…”