Tearing mode stabilisation by ECRH and ECCD

A target plasma at Ip=400 kA, Btor=5.6 T, qa~5, ne,line 0.4 1020 m-3, POH~500 kW, showing saturated Tearing Modes (likely m=2,n=1), coupled to a more internal odd m-order mode, was used for MHD stabilization by localized ECRH (Electron Cyclotron Resonance Heating).

Two gyrotron were available for ECRH/ECCD (Electron Cyclotron Current Drive), at PECRH=430 kW each.

The position of the absorbing volume was changed by steering poloidally the beam axis from shot to shot, at fixed toroidal magnetic field and constant target plasma parameters.

The main results are summarized in Figs.1-4.

In Fig.s 1,2 the two shots #18004 and #18015 are compared. They do represent typical cases of ECRH (perpendicular beam injection) where TM stabilization is not and is achieved respectively.

Fig.1 - Top to bottom: 1) magnetic signals (Mirnov coils); 2) Te,0 (fast ECE polychromator); 3) Tq=2 (fast ECE polychromator); 4) average central line density; 5) peak ion temperature from neutron emission; 6) average stored density; 7) tE/tE,OH. Shots #18004 (MHD enhanced) and #18015 (MHD stabilized) are compared. ECRH starts at 0.5 s.

ECCD contribution is exemplified in Fig.s 2,3, where typical shots with oblique launch #18041 and #18045 are compared. In both discharges co-injection at a toroidal angle of 10o was performed. In #18041 a beam with PECRH=430 kW at perpendicular injection is switched on first at 0.5 s, then replaced a beam at oblique injection (at about the same poloidal position and having the same power) from 0.75 s up to 1.1 s. In #18045, instead, oblique injection is applied first.

Fig.2 – top: Te,0 and Te,q=2;  0.5-0.75 s;  400 kW perpendicular injection (no ECCD); 0.75-1.1 s 400 kW at toroidal angle10o

bottom: Te,0 and Te,q=2;  0.5-0.75 s ; 400 kW at toroidal angle10o; 0.75-1.1 s  400 kW perpendicular injection (no ECCD).

Fig.3 – top: Te,0 and Te,q=2; 400 kW perpendicular injection (no ECCD);

bottom: Te,0 and Te,q=2; 400 kW at toroidal angle +10o.

Preliminary analysis of the experimental data led to the following assessments:

1. If the absorption volume is inside the radius where the Te fluctuations due to mode rotation are largest, both temperature and magnetic fluctuations are maintained during ECRH. Fluctuations are even enhanced when approaching from the inside the island position.

2. If the absorption volume is located where the maximum Te fluctuations occur (likely within the magnetic island), stabilization is achieved with PECRH=430 kW. A difference of ~ 2 cm in the absorption radius rabs is enough to switch from TM enhancement to TM stabilization.

3. Stabilization is marked by a strong reduction of magnetic oscillations. Thermal oscillations are also suppressed.

4. Stabilization involves not only (2,1) tearing mode mode, but also an odd mode, not shown in the figures, coupled to (2,1).

5. If absorption radius is moved further out, stabilization may still occur, but with a significant delay (~0.1 s) (case not shown in figures).

6. If TM modes are stabilized, a net improvement of core confinement is observed.

7. The central temperature increase in case of stabilization is much higher than in the case of maintained, or even reinforced, TM oscillations (fig.4).

8. The density increases more during stabilization; Thomson Scattering measurements confirm the peaking of the electron density in the core (not shown in figures).

9. Neutron emission increases only if TM are stabilized.

10. The energy stored in the core is therefore much higher with stabilization. Also the global confinement time has a much less degradation with ECRH if TM disappear.

11. When stabilization occurs, Te peaking due to enhanced core confinement may lead to sawtooth appearance (but not necessarily, since there are cases with TM stabilization but without sawteeth appearance).

12. The main stabilizing effect comes from ECRH. ECCD apparently has only a second order effect on sawteeth (which might be suppressed also).

13. When ECCD is added (oblique injection) to ECRH, a much less stable condition is achieved.

Experimental Reports