February-March 2006
The following are the results on several major topics during this period.
(1) Sustainment of high βN with high confinement
In January, it was found that only wall pumping was effective to keep quite a low recycling level in a low divertor pressure regime for this experiment. However, it was difficult to keep effective wall pumping throughout the discharge with high heating power required to maintain high βN at 1 MA. So, the plasma current Ip was reduced to 0.9 MA to minimize the required heating power to sustain a given βN>2.0. Since we were able to dispense with a half unit of peripheral counter NB for 0.9 MA plasmas, a large plasma rotation in the co-direction was obtained together with a peaked pressure profile at smaller heating power. As a result, we achieved βN>2.3 maintained for 28.6 s. A high confinement factor of HH ~1 was also sustained for 23.1 s.
(2) Sustainment of high bootstrap current fraction
Plasmas with a high bootstrap current fraction fBS of 70-80% were optimized towards long sustainment. The reference value of the plasma stored energy for feedback control and the toroidal momentum input from tangential NBs were adjusted in order to suppress mini collapses with a wide ITB radius maintained. New feedback control algorithm based on the real time measurement of the safety factor profile q(r) using motional Stark effect (MSE) diagnostic was installed to improve the reproducibility of suppression of mini collapses. In this algorithm, tangential-NBs are stopped for several hundred milliseconds to control the pressure gradient at the ITB through modification of the toroidal rotation profile when the minimum value of q approaches an integer. As a result, duration of high fBS up to 8 s was obtained.
(3) Current profile control using lower-hybrid current drive (LHCD)
Using off-axis LHCD, the minimum value of the safety factor (qmin) was controlled in a high βp-mode plasma (Ip = 0.8 MA, Bt = 2.4 T, βN = 1.7, βp = 1.5). The reference value of qmin was set to 1.7. LH power controls the amount of the off-axis LHCD current and qmin near the plasma center changes according to the change in the Ohmic current. An MHD instability was stabilized and the plasma stored energy increased, when the qmin was raised above 2 by the control. After the qmin overshot the reference value, the control system decreased the LH power, and the qmin approached the reference value (1.7).
(4) Off-axis neutral beam current drive (NBCD)
The driven current profile by the off-axis neutral beam injected in the co-direction to the plasma current was measured using MSE diagnostic. The driven current profile was spatially localized at r/a = 0.6-0.8, which was consistent with the change in the chord-integrated neutron production rate profile during and after the NBCD. An area-integrated NBCD current density (total NBCD current of 0.15 MA) was also consistent with that estimated from the decrease in the surface loop voltage during the NBCD (0.16 MA).
(5) Slow Ip start-up by ECRF without a central solenoid
Ip start-up without applying a one-turn voltage from any coils but by ECRF alone was attempted. The vertical field coils and the inner coil loops of the triangularity coil were not used, in order to make the region with a positive decay index as wide as possible inside the vacuum vessel. During the ECRF injection, the vertical field was increased from 20 to 35 G by the outer coil loops of the triangularity coil. The plasma current was generated and increased to reach 18 kA.
(6) Dynamic transport analysis
Dynamic transport analysis for Te and Ti was done for the reversed shear ITB discharges with EC injection. Spontaneous transition from a weak ITB to a strong, narrow ITB was observed during the phase of constant heating power. The χe values were almost identical both at ρ = 0.45 and 0.65 in the wide weak ITB phase. However, the χe at ρ = 0.45 in the core region increased (the Te gradient decreased) in association with the decrease of χe at ρ=0.65 at the highest gradient region. Similar behavior was observed in the time evolution of Ti gradient measured with the newly installed fast CXRS. This behavior also indicates that there are multi meta-stable states in the transport and the spontaneous transition between these meta-stable states occurs on a transport time scale.
(7) High βN exceeding the no-wall stability limit
Achievement of high beta was attempted in high βp H-mode plasmas with Bt = 1.57 T and Ip = 0.9 MA. The highest βN of 3.83 was obtained with the plasma rotation in the direction of the plasma current (co-rotation). On the other hand, mini collapses were observed at lower βN with a faster growth of magnetic fluctuations. These results suggest the stabilization of resistive wall mode with the plasma rotation.