The toroidal rotation without any external momentum sources known as an intrinsic rotation has been focused an important topic since the most promising toroidal rotation source driven externally from a neutral beam injection may not successful for the future burning devices like ITER and DEMO. The toroidal rotation in pure ohmic plasmas is self-generated and it is considered as one of the most fundamental types of intrinsic rotation for magnetic fusion researches 1. There have been reported a wide range of magnitudes, directions and abrupt reversals for intrinsic rotation studies no clear physical mechanisms are concluded to explain the complicated intrinsic rotation behaviors. Hence, investigation for the generation mechanism and characteristics for ohmic plasmas provide critical information to understand the origin of intrinsic rotation. In general, the faster intrinsic rotation is the more beneficial to suppress resistive wall modes and neoclassical tearing modes which easily lead major plasma disruptions.
All ohmic L-mode discharges more than 10 years experimental campaigns in KSTAR consistently show distinctive behaviors in the beginning of plasma current ramp-up phase that the core toroidal rotation always starts in the counter-current direction and the toroidal rotation shows dynamic changes at early plasma current ramp-up as shown in Fig. 1. The (-) sign of the toroidal rotation means a counter-current direction, which indicates in the opposite direction to the plasma current. Although the core toroidal rotation in the beginning of the plasma current ramp-up phase usually starts in the counter-current direction its magnitude and direction are dramatically changed with the electron density during plasma current flat-top phase as shown in Fig. 2. The core toroidal rotation bifurcations like rotation reversals are clearly observed to the counter to co-current and co to counter-current direction when the electron density is changed to the lower and higher density regimes, respectively. It is suggested that the physics mechanisms at early plasma current ramp-up and current flat-top phase are separately investigated since the main plasma parameters are quite different between the two phases.
The generation mechanism at early plasma current ramp-up for the counter-current rotation in ohmic discharges shown in Fig. 1 is speculated from the momentum transfer between neutrals and plasma particles. It is suggested that the momentum transfer from electrons to neutrals is larger than that of the ions to neutrals and the momentum exchange of neutrals easily go back to plasma by the plasma-neutral interactions. The calculated toroidal rotation with continuous dotted lines based on the momentum transfer mechanism is sensitive to the ratio of electron density to the main ion density (ne/nD+) and temporal behavior of toroidal rotation agrees well with the experimental measurement with red dots as shown in the bottom box of Fig. 3 (a). The core ohmic rotation during plasma current flat-top phase has been investigated from scaling studies for many ohmic L-mode discharges in KSTAR [1,2]. As a result, the core intrinsic toroidal rotation scaling for ohmic plasmas strongly depends on the ion temperature over plasma current regardless of the toroidal rotation direction. However, the toroidal rotation shown in Fig. 3 (b) keeps almost constant after passing the dynamic rotation change at early plasma current ramp-up stage even the electron density increases beyond the saturated ohmic confinement (SOC) regime. The calculated SOC density is marked with a blue asterisk in Fig. 3 (b). Unlike to the slower rotation less than 20 km/s with higher plasma current discharge shown in Fig. 2, the core toroidal rotation reversal is not observed under the fast rotating discharge with lower plasma current discharge. This experimental evidence elucidates that there is a certain threshold for the toroidal rotation reversal phenomena from ohmic L-mode discharges.
In this presentation, we will discuss the generation mechanism and detailed characteristics of intrinsic rotation including rotation reversal and its threshold, and extended rotation scaling results.
*Work supported by the Korea Ministry of Science and ICT under the KSTAR project contracts.
|Affiliation||National Fusion Research Institute|
|Country or International Organization||Korea, Republic of|