F." "Sano

National Institute for Fusion Science (NIFS-Repository)

The main purpose of the bidirectional collaborationresearch of the Helitoron J group with NIFS is to proceedthe allotted subject, “A study on the confinementoptimization and stability control of an advanced helicalsystem” in the 2nd mid-term plan of MEXT on the basis ofthe former subject, “Confinement optimization study of anadvanced helical system for a compact, high-beta,steady-state reactor”. The steady progress for these subjectshas been done in Heliotron J experiment. The collaborationresearch activity between the Helitoron J group and otherexperimental groups such as the LHD group, or otheruniversity groups must be extended to understandmachine-independent torus plasma confinement physicsthrough the systematic study.The five schemes for the collaboration research have beenselected; (1) study of confinement improvement and relatedplasma structural formation using field configuration control,(2) study of instability control using field configurationcontrol, (3) study of toroidal current control, (4) study offuelling control and heat/particle removal, (5) physical andmechanical design study of plasma fluctuation diagnosticdevices such as a beam emission spectroscopy (BES) system.Each group joined the plasma experiment and data analysisincluding the usage of fast internet for data exchange andanalysis. Electron cyclotron current drive study concerningcategory (3), fuelling method study concerning category (4),and study of plasma startup by the NBI concerning category(1) are reported below.Study of Electron Cyclotron Current Drive (ECCD)1)The effect of the magnetic field ripple on the ECCD, andthe zero toroidal current by suppression of BS current havebeen demonstrated in Heliotron J experiment. Thedependence of the 70-GHz second harmonic ECCD on //Nis investigated by changing the injection angle of thefocused EC Gaussian beam. EC current is proved to becontrollable by changing injection angle. The maximumcurrent is driven near 4.0// N . When the ECH wave isabsorbed at the top of the magnetic field strength, thetoroidal current flows in the direction due to theFisch-Boozer effect. On the contrary, when the EC wave isabsorbed at the bottom of the magnetic field strength, thecurrent hardly flows. The ECE signal becomes stronger asthe EC current increases. The high energy electrons affectECCD from the consideration of the ECE signals with theoptical thickness. The calculated results using a ray-tracingcode, TRAVIS for the dependence of the EC current on //N ,magnetic field strength and configuration agree with theexperimental results.Study of Particle Control Method for ConfinementImprovement2)The further extension of the attainable parameter regionsand improvement of confinement is one of objectives of thiscampaign after the success of the extension of the plasmaparameter region using a super-sonic molecular beaminjection (SMBI) in the last campaign. The molecularhydrogen beam speed by the SMBI is estimated to be 1-1.6km/s by TOF. The effectiveness of the SMBI is supposed tobe due to directivity of injected particles since the speed ofthe beam is not so fast compared with that of hydrogen atroom temperature. Plasma response to SMBI is investigatedusing a shutter in front of the SMBI nozzle in order toestimate the effect of the directivity of the fuelling. Thedensity and stored energy without the shutter is increasedmore and the saturated points are higher than those inshutter-closed case. The slope of the density profile at theplasma edge is steeper. It is concluded that the efficiency offuelling is improved by focusing the injected particles.Plasma Startup Using Neutral Beam Injection (NBI)Assisted by 2.45-GHz Microwaves3)Neutral Beam Injection (NBI) plasmas have beeninitiated with the assistance of 5-kW, 2.45-GHz microwavesin Heliotron J. Plasmas with a line averaged electron densityof over 11019 m-3 are generated with 1-MW NBI using amagnetic field between 0.63 and 1.25 T within 20 ms afterturning NBI on. This technique does not require the electroncyclotron layers for the plasma startup, then, the operationalspace for the magnetic field strength can be extended.Electron cyclotron emission measured with a radiometerreveals that the 2.45-GHz microwaves play an importantrole for the startup.[1] K. Nagasaki, et al., “Experimental Study of SecondHarmonic ECCD in Heliotron J”, 23rd IAEA Conference,Daejeon, Korea (2010) EXW/P7-19.[2] T. Mizuuchi, et al, “Fueling Control for ImprovingPlasma Performance in Heliotron J”, 23rd IAEA Conference,Daejeon, Korea (2010) EXC/P8-11.[3] S. Kobayashi, “NBI experiments in Heliotron J”, 37thEPS Conference on Plasma Physics, Dublin, Ireland (2010)P1.1053.493§15. A Study on the Confinement Optimization and Stability Control of an Advanced Helical SystemSano, F. (Institute of Advanced Energy, Kyoto Univ.)

A Study on the Confinement Optimization and Stability Control of an Advanced Helical System

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