Decrease of transport coefficients in the plasma core after off-axis ECRH switch-off K.A.Razumova and T-10 team

Usual decrease of T e (normalized) for different radii after ECRH switch-off. More rapid decay is seen for r=+11 – +13cm, the surface of ECRH.

The delayed decrease of T e in the plasma core (inside the magnetic surface q=1 ) after off-axis ECRH switch-off was apparently seen last years experiments in the regime: B t =2.33T; I p =180kA; n e = m -3 ; P ECR =400– 420kW.

P ECR (r) and the heat diffusivity  e for different times of the process: 1)OH (  e OH ); 2)at the end of ECRH (  e EC ); 3)during the transient improvement of confinement (  e ), calculated by code COBRA.

Next hypothesis was suggested to explain this effect: The enhanced confinement zone appears when dq/dr=0 in the vicinity of the rational surface. But  T e increases during the local confinement enhancement. This, due to the current density redistribution, leads to dq/dt increase. So, using the Ohmic current we always have a positive dq/dr. ECRH switch-off leads to the current density redistribution in the ITB region and transient appearance of dq/dr=0, which means the transport decreases in this region.

Results of calculation by ASTRA code of j(r) and q(r) for the end of ECRH and during existence of constant temperature after ECRH switch-off. Calculations have been performed with T e (r) and Z eff, taken from the experimental measurements, and with the neoclassical resistivity.

The following type of regime was investigated in the last experimental T-10 campaign. #35355, B t =2.31T; I p =185kA; n e = cm -3. Delay of T e (0) decrease (  del =25 ms) always accompanied by n e (r) peaking.

The density profile always peaks after ECRH switch-off

Using T e (r) and n e (r) we can build the plasma pressure profile, p e (r). The plasma pressure increases inside the zone of enhancement confinement, which exists after ECRH switch-off.

However, if we increase P ECR (4 gyrotrons instead of 2), the effect of T e (0) conservation has not presented, when only ½ part of P ECR was switched-off, but it exists, when the last part of the power was switched-off.

Sawteeth were suppressed in both cases. The difference was in the ECR power only. For 2 gyrotrons, sawteeth has been just stabilized and core q was near the unity. In the case of 4 gyrotrons, q was distinctly higher than unity. If this is the reason for the confinement difference, then: 1) Under the short operation of 2 from 4 gyrotrons, when j(r) has not enough time to redistribute and to increase q core, the effect of T e (0) delayed decrease has to be exist. 2) We can find another regime, with higher I p, where this effect should be seen under 4 gyrotrons heating.

The item 1) was confirmed: When 2 from 4 gyrotrons were switched-off 40 ms after start of ECRH, T e (0) continues to increase (  del =9ms). After disconnection of the last two gyrotrons,  del was 16 ms, but T e (r) slightly decreased.

When we disconnect 2 from 4 of operating gyrotrons, then  del depends on the duration of their operation: longer the time of 4 gyrotrons operation, i.e. closer is the j(r) profile to the stationary one, shorter is  del. However, the  del value is not well defined in the cases, when T e is slightly decreasing.

The check-up of the item 2) gave the next result: To obtain the same delayed decrease of T e (0) under 4 gyrotron heating, we must increase I p up to the value, when 4 gyrotrons will stabilize sawteeth, and slightly decrease B t to deposit the ECR power outside the phase inversion radius. In the regime: B t =2.3T; I p =225kA; n e = cm -3,  del =28ms (4 gyrotrons switch-off), but again we see a slow decrease of T e (0,t) (blue).

The pressure in the plasma core remains to be constant.

The value of  del is very sensitive to the ECR position in relation to q=1 magnetic surface.

We tried to receive the effect in the regime with 4 gyrotons and with the low current I p =180kA using preliminary on-axis heating (the gyrotron with F=130 GHz). On-axis heating prohibited the sawteeth stabilization, but we may receive the desirable profile using the high power off-axis heating by 4 gyrotrons. It turned out, that even ¼ of the on-axis gyrotron power is too much for the sawteeth stabilization by 4 gyrotrons with F=140 GHz. Nevertheless, we succeeded to have  del =15ms under not totally suppressed sawteeth. This makes clear that the fact of sawteeth stabilization itself does not important for  del existence. It is important to have well-aligned q(r) profile.

Scheme of experiment with off-axis heating (P=0.9MW) of the preliminary on-axis heated plasma (P=0.5 – 0.12 MW)

In spite of sawteeth existence, one can see  del =15 – 16ms (red). #35677; B=2.33T; I=185kA; n=1.4; 4gyr.140GHz against a background of 1gyr. 130GHz with diminished power

Why in some experiments T e is constant during  del and sometimes it slowly decreases? Let us compare two shots with the same initial plasma parameters (B t =2.33T; I p =185kA; n e = cm -3 ), but with different ECRH power : 2 and 4 gyrotrons.

The picture looks like the superfluous energy has to be lost and then the core confinement improves. Comparison of regimes with heating by the total power of 2 gyrotrons (black) and one-half of this power (red)

After 4 gyrotrons switch-off, T e (0) decreases till its value reach the same value, which was under 2 gyrotron heating and then remains constant during 15 ms. In the case of 2 gyrotrons, T e (0) is constant during 25 ms. Note, that not only the T e (0) value, but also T e (r) profiles are the same in both cases during the period of T e (0,t)=const. Experiment shows: more difference between preliminary ECRH power and the optimal one – more steep T e (0,t) decrease after the power switch-off.

CONCLUSIONS: q(r) profile near the rational magnetic surface play the especially important role in the ITB formation process and it determines the ITB quality. The range of dq/dr (near dq/dr=0) exists, in which the transport is minimized. The range of q deviation from the rational value exists, inside which the barrier formation is effective. Using these rules we can stimulate the ITB formation in different conditions (at least electron ITB).

Profiles of electron temperature T e before and after the current ramp-up.

Current ramp up (from 150 to 140kA) leads both to the plasma core inward shift and T e core increase Calculated equilibrium contours of magnetic surfaces before and after the current ramp-up. After ramp-up the plasma column is shifted inward.