Design of Coaxial Helicity Injection on SUNIST Li ZENG Department of Engineering Physics, Tsinghua University Supervised by Professor Yexi HE SUNIST

Outline Configuration of SUNIST (Sino UNIted Spherical Tokamak ) Some important issues of CHI system design Coaxial helicity injector on SUNIST Conclusion SUNIST

1 Configuration of SUNIST Main parameters: R~ 30 cm, r~ 23cm, natural elongation ~ 1.6 ， B0~ 0.15T, Ip~ 50kA Main aims ： basic characters of plasmas at low-aspect ratio; non-inductive heating and current startup Recent experiment plan ： routine ohmic inductive discharge SUNIST

2 Some issues of CHI system design 2.1 Theory (I) Magnetic helicity ： A measurement of the topological properties of the configuration ； more fundament than any specific geometrical property. Definition: K=∫ v A B dv Where B =   A SUNIST

2 Some issues of CHI system design 2.1 Theory (II) Helicity in open and multiply connected volumes: relative helicity ： SUNIST

2 Some issues of CHI system design 2.1 Theory (III) In order to conserve the helicity in the volume, external helicity must be compensate the helicity dissipation. The rate of helicity injection in open and multiply connected volumes: SUNIST

2 Some issues of CHI system design 2.2 Conditions of injection effectively Hypothesis: helicity balance and enery conservation; Condtions （ shown on right ） SUNIST

2Some issues of CHI system design 2.3 Typical CHI system in STs Components of CHI system ： bias field coil; insulated gaps ； injector ； absorber ； DC power supply ； fast valves ； control system SUNIST

3 Coaxial helicity injector on SUNIST 3.1 Impedance model of injector Hypothesis: ideal conducting wall, force-free conduction, helicty balance Mode: SUNIST

3 Coaxial helicity injector on SUNIST 3.2 Supported experiment of impedance mode SUNIST

3 Coaxial helicity injector on SUNIST 3.3 Impedance calculations Range of parameters: resistivity Magnetic energy ： kJ Suitable impedance; m ohm SUNIST

3 Coaxial helicity injector on SUNIST 3.4 Injector on SUNIST Injector flux:1.2 mWb; Electrode material : spray-coat plasma-facing surfaces with a thin layer of tungsten. Electrode distance:4 cm electrodes:cone-shaped Intrinsic impedance ： m ohm ； Peaked current:>10 kA; Threshold current of formation closed lines: ? kA SUNIST

3 Coaxial helicity injector on SUNIST 3.3 Others Rebuild of central conductor Pre-ionization ? Fast puff valves SUNIST

4 Conclusion Impedance model of coaxial helicity injector ： Injector on SUNIST: ECR pre-ionization assisted CHI non- inductive plasma initiation ： SUNIST

References [1] Sykes A, High beta produced by neutral beam injection in the START (Small Tight Aspect Ratio Tokamak) spherical tokamak, Physics of Plasmas 4, 1997: [2] Sykes A, Overview of recent spherical tokamak results, Plasma physics and controlled fusion 43, 2001: A127-A139 [3] Akers RJ, Ahn JW, Appel LC, et al., H-mode access and performance in the Mega-Amp Spherical Tokamak, Physics of plasmas 9, 2002: [4] Gates DA., Bell MG. et al. Recent Progress on the National Spherical Torus Experiment (NSTX), 29 th EPS Conference on Plasma Physics and Controlled. Fusion, Montreux, ECA Vol. 26B, 2001 [5] Jarboe TR, Gu P, Izzo VA et al., Current drive experiments in the HIT-II spherical tokamak, Nuclear fusion 41, 2001: [6] Taylor JB, Relaxation and magnetic reconnection in plasmas, Reviews of modern physics 58, 1986: [7] McCollam KJ, Jarboe JR, Magnetic relaxation in coaxial helicity injection, Plamsa physics and controlled fusion 44, 2002: [8] Cheng Zhang, Deng Zhou, Sizheng Zhu, Analysis of plasma current profile driven by coaxial helicity injection in a low aspect ratio tokamak, Nuclear fusion 41, 2001: [9] Mitchell A Berger, Introduction to magnetic helciity,Plasma physics and controlled fusion 41, 2001:B167-B175 SUNIST

Thanks! SUNIST