Observation of Toroidal Flow on LHD

Transcription

1 17 th International Toki conference / 16 th International Stellarator/Heliotron Workshop 27 Observation of Toroidal Flow on LHD M. Yoshinuma, K. Ida, M. Yokoyama, K. Nagaoka, M. Osakabe and the LHD Experimental Group National Institute for Fusion Science, Toki, Japan Contents 1. Introduction 2. Charge Exchange Spectroscopy on LHD 3. Toroidal Flow driven by NBI 4. Toroidal Flow driven by Er. ECH driven Toroidal Flow driven by ECH 6. Summary 1/13

2 Introduction The stabilizing and destabilizing of the magnetic confinement plasma is considered to be sensitive to the profile of flow velocity. A moderate shear of poloidal flow can suppress turbulence and reduce the transport, although a large velocity shear causes instabilities such as Kelvin- Helmholtz instabilities. It has been pointed out that the toroidal flow contributes the stabilization of resistive wall mode in tokamaks. Therefore the spontaneous troidal flow becomes important in the next fusion device such as ITER, where the toroidal flow velocity driven by external momentum is expected to be not enough to stabilize the MHD mode. The mechanism of driving the spontaneous troidal flow has a great interests in the momentum transport physics. In this presentation, we will report observations of toroidal flow and discuss the mechanisms of spontaneous flow in helical plasma. 2/13

3 Spontaneous and External Driven Toroidal Flow Radial Force Balance : E r = V t B p V p B t + 1/(Zen) dp/dr The radial electric field is determined by the non-ambipolar loss or pressure gradient and the coupling of ExB force and viscosity tensor drives spontaneous toroidal rotation. 3/13

4 Observation of Toroidal Flow on LHD LHD Control of NBI injection (external momentum input) E r control by changing collisionality Control of radial electric field by changing collisionality. Density can change edge E r Local ECH can change center E r Measurement of both Toroidal and Poloidal flow with charge exchange spectroscopy Both NBI driven toroidal flow and spontaneous toroidal flow depended on Er have been observed in LHD. 4/13

5 Line of Sights of CXS on LHD Charge exchange spectroscopy measurement can be performed both with poloidal and toroidal line of sights. Poloidal system Toloidal system Typical scheme of discharge for CXS 4keV perpendicular injection NBI with positive ion source To acquire the background signal, the p-nbi is modulated with 1msec ON and 1msec OFF. /13

6 Profiles in the case of Counter Injection B t =2.8T R ax =3.6m γ=1.24 Bq=1% 6 4 Ion temperature t=1.3s High Ion temperature (~4.keV) obtained in NBI sustained plasma with electron density n e ~1.x1 19 m -3. Strong toroidal flow is observed in the core region of the plasma and its direction is consistent with the direction of the NBI. 723 Intensity (a.u.) t=.9s t=1.3s lhdcxs3@74282 Intensity of CVI emission T i (kev) V t 3 t=.9s t=1.3s -1 t=.9s Toroidal flow /13

7 Profiles in the case of Co Injection B t =-2.769T R ax =3.7m γ=1.24 Bq=1% 6 lhdcxs3@74282 Ion temperature 4 t=1.1s T i (kev) 3 2 t=.7s High Ion temperature (~kev) obtained in NBI sustained plasma with electron density n e ~1x1 19 m -3. The CXS intensity profile shows that the profile of carbon impurity is strongly hollowed when the T i becomes high Intensity (a.u.) lhdcxs3@74282 Intensity of CVI emission t=.7s t=1.1s V t Toroidal flow lhdcxs3@ t=1.1s 3 2 t=.7s 1 Co Ctr /13

8 Toroidal Flow driven with NBI Toroidal Flow driven by Externally Injection of Momentum Toroidal flow is observed with in the case of co-injection, counter-injection, and balanced injection to see the NBI driven component. The NBI dominantly drives the toroidal flow near the plasma center. Co-injection case (NBI2) : Toroidal flow drive inside the R of 3.9m Counter-injection case (NBI1) : Toroidal flow drive inside the R of 4.1m Balanced-injection case : Spontaneous component is observed. V t NBI2(Co) NBI1&2(Balance) The NBI driven component is small at the plasma edge. It is considered to be the result come from the strong helical ripple and small deposition of the NBI power at the plasma edge. -1 NBI1(Ctr) B t =1.T R ax =3.6m γ=1.174 Bq=1% 7488,7489,7493 8/13

9 Toroidal Flow driven by Radial Electric Field Positive and negative Er can be formed at the plasma edge by controlling the density and input power. Toroidal flow at the plasma edge changed associated with the changing the radial electric field. Vp < (Ion root) Drives Vt in co-direction. Vp > (Electron root) Drives Vt in counter-direction. 1 1 Poloidal flow velocity Electron Root.x1 19 m -3 (Co with ECH) Toroidal flow velocity Ion Root Near Zero 1.x1 19 m -3 (Ctr).4x1 19 m -3 (Co) V p - -1 Near Zero.4x1 19 m -3 (Co) Ion Root 1.x1 19 m -3 (Ctr) V t - -1 Electron Root x1 19 m -3 (Co with ECH) R(m) B t =1.T R ax =3.6m γ=1.174 Bq=1% R(m) 9/13

10 Toroidal Flow driven by Radial Electric Field Dependence of toroidal flow velocity on radial electric field R=4.4m t=1.s NBI 1 and 2 V t E r (kv/m) ΔEr=1kV/m => ΔVt=km/s Counter flow is increased associated with growing up the positive E r (electron root) The direction of the poloidal flow driven by E r x B is changed to the direction of helical pitch which has minimum gradient of magnetic field strength. 1/13

11 Toroidal Flow driven by ECH (1) ECH is injected into the NBI sustained plasma (<ne>~.4x1 19 m -3 ). The NBI is injected with balanced combination to make the NBI driven flow small. Modulated injection of NBI4 is just used for CXS measurement. T i (kev) ECH ON 1.3s - 1.8s 1.2s (without ECH) 1.4s (with ECH) R(m) T e (kev) The CERC (Core Electron Root Confinement) profile is observed in the electron temperature, while the ion temperature has no significant change with the ECH ECH ON 1.3s - 1.8s 1.s (with ECH) 1.2s (without ECH) /13

12 Toroidal Flow driven by ECH (2) V t 1 ECH ON 1.3s - 1.8s 1.4s(with ECH) s (without ECH) R(m) E r (kv/m) ECH ON 1.3s - 1.8s 1.s (with ECH) 1.2s (without ECH) R(m) Toroidal flow is driven into codirection near the plasma core (ΔVt~1 km/s) during the ECH. During ECH, the positive radial electric field appears associated with the formation of the CERC. Positive Er drives the flow in the co-direction near the plasma core. B t =1.T R ax =3.6m γ=1.174 Bq=1% # /13

13 Summary (1) The profiles of ion temperature, toroidal flow velocity and carbon impurity are measured with charge exchange spectroscopy using the charge exchange line of fully ionized carbon. (2) Toroidal flow driven by external momentum Strong toroidal flow parallel to the momentum input of NBI is observed in the core region of the plasma. (3) Spontaneous flow (3-1) At the plasma edge The relation between the spontaneous toroidal flow and radial electirc field is investigated and the positive radial electric field drives spontaneous rotation in the counter direction (anti parallel to the <ExB p >) at the plasma edge. (3-2) In the plasma core Toroidal flow driven in the co direction is observed when the ECH is injected. This observation shows the spontaneous rotation at the plasma core is opposite to that at the plasma edge (parallel to the <ExB p >). (3-3) The differences between core and edge can be explained by the differences in the ratio of ε t /ε h. 13/13