| 1. |
- Chapman, I. T., et al.
(författare)
-
The physics of sawtooth stabilization
- 2007
-
Ingår i: Plasma Physics and Controlled Fusion. - 0741-3335 .- 1361-6587. ; 49:12B, s. B385-B394
-
Tidskriftsartikel (refereegranskat)abstract
- Long period sawteeth have been observed to result in low-beta triggering of neo-classical tearing modes, which can significantly degrade plasma confinement. Consequently, a detailed physical understanding of sawtooth behaviour is critical, especially for ITER where fusion-born a particles are likely to lead to very long sawtooth periods. Many techniques have been developed to control, and in particular to destabilize the sawteeth. The application of counter-current neutral beam injection (NBI) in JET has resulted in shorter sawtooth periods than in Ohmic plasmas. This result has been explained because, firstly, the counter-passing fast ions give a destabilizing contribution to the n=1 internal kink mode-which is accepted to be related to sawtooth oscillations-and secondly, the flow shear strongly influences the stabilizing trapped particles. A similar experimental result has been observed in counter-NBI heated plasmas in MAST. However, the strong toroidal flows in spherical tokamaks mean that the sawtooth behaviour is determined by the gyroscopic flow stabilization of the kink mode rather than kinetic effects. In NBI heated plasmas in smaller conventional aspect-ratio tokamaks, such as TEXTOR, the flow and kinetic effects compete to give different sawtooth behaviour. Other techniques applied to destabilize sawteeth are the application of electron cyclotron current drive (ECCD) or ion cyclotron resonance heating (ICRH). In JET, it has been observed that localized ICRH is able to destabilize sawteeth which were otherwise stabilized by a co-existing population of energetic trapped ions in the core. This is explained through the dual role of the ICRH in reducing the critical magnetic shear required to trigger a sawtooth crash, and the increase in the local magnetic shear which results from driving current near the q=1 rational surface. Sawtooth control in ITER could be provided by a combination of ECCD and co-passing off-axis negative-NBI fast ions.
|
|
| 2. |
|
|
| 3. |
- Mantsinen, M. J., et al.
(författare)
-
Modification of sawtooth oscillations with ICRF waves in the JET tokamak
- 2007
-
Ingår i: RADIO FREQUENCY POWER IN PLASMAS. - : AIP. - 9780735404441 ; , s. 35-42
-
Konferensbidrag (refereegranskat)abstract
- Methods of modifying sawtooth oscillations using waves in the ion cyclotron range of frequencies (ICRF) in the JET tokamak are presented. Examples of sawtooth stabilization by ICRF-accelerated high-energy ions are shown, including experiments with ICRF-acceleration of 4He-beam ions to simulate the effects of fusion born alpha particles. With high power ICRF heating in low-density plasmas, fast ion stabilization of sawteeth is lost and a new type of small-period and small-amplitude sawteeth appears. ICRF-induced radial pinch with toroidally asymmetric waves is found to be useful in affecting the radial profile of the ICRF-driven fast ion populations and thereby their influence on sawteeth. Ion cyclotron current drive (ICCD) applied close to the sawtooth inversion radius is effective in modifying the sawtooth period. The latest achievements include the successful application of ICCD to shorten the fast-ion-induced long-period sawteeth and thereby avoid triggering of neoclassical tearing modes (NTMs).
|
|
| 4. |
- Eriksson, L. G., et al.
(författare)
-
On ion cyclotron current drive for sawtooth control
- 2006
-
Ingår i: Nuclear Fusion. - 0029-5515 .- 1741-4326. ; 46:10, s. S951-S964
-
Tidskriftsartikel (refereegranskat)abstract
- Experiments using ion cyclotron current drive (ICCD) to control sawteeth are presented. In particular, discharges demonstrating shortening of fast ion induced long sawteeth reported in (Eriksson et al 2004 Phys. Rev. Lett. 92 235004) by ICCD have been analysed in detail. Numerical simulations of the ICCD driven currents are shown to be consistent with the experimental observations. They support the hypothesis that an increase in the magnetic shear, due to the driven current, at the surface where the safety factor is unity was the critical factor for the shortening of the sawteeth. In view of the potential utility of ICCD, the mechanisms for the current drive have been further investigated experimentally. This includes the influence of the averaged energy of the resonating ions carrying the current and the spectrum of the launched waves. The results of these experiments are discussed in the light of theoretical considerations.
|
|
| 5. |
|
|
| 6. |
|
|
| 7. |
- Graves, J. P., et al.
(författare)
-
Sawtooth control mechanism using counter current propagating ICRH in JET
- 2008
-
Ingår i: EPS Conf. Plasma Phys., EPS - Europhys. Conf. Abstr.. - 9781622763351 ; , s. 2010-2013
-
Konferensbidrag (refereegranskat)abstract
- The sawtooth control mechanism in plasmas employing off-axis ICCD is reinvestigated. In particular, for counter propagating waves on the high field side, asymmetrically distributed energetic passing ions destabilise the ideal internal kink mode when the q=1 surface resides within a narrow region centred about the shifted fundamental cyclotron resonance.
|
|
| 8. |
- Graves, J. P., et al.
(författare)
-
Sawtooth-Control Mechanism using Toroidally Propagating Ion-Cyclotron-Resonance Waves in Tokamaks
- 2009
-
Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 102:6
-
Tidskriftsartikel (refereegranskat)abstract
- The sawtooth control mechanism in plasmas employing off-axis toroidally propagating ion cyclotron resonance waves in tokamaks is reinvestigated. The radial drift excursion of energetic passing ions distributed asymmetrically in the velocity parallel to the magnetic field determines stability when the rational q=1 surface resides within a narrow region centered about the shifted fundamental cyclotron resonance.
|
|
