Design of Coaxial Helicity Injection on SUNISTLi ZENG Department of Engineering Physics, Tsinghua UniversityCl01@mails.tsinghua.edu.cnSupervised byProfessor Yexi HESUNIST

Outline

Configuration of SUNIST (Sino UNIted Spherical Tokamak )Some important issues of CHI system designCoaxial helicity injector on SUNIST ConclusionSUNIST

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

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2 Some issues of CHI system design2.1 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

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2 Some issues of CHI system design2.1 Theory (II)Helicity in open and multiply connected volumes: relative helicity

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2 Some issues of CHI system design2.1 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:

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2 Some issues of CHI system design2.2 Conditions of injection effectivelyHypothesis: helicity balance and enery conservation;

Condtionsshown on right

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Some issues of CHI system design2.3 Typical CHI system in STsComponents of CHI system bias field coil; insulated gaps injector absorber DC power supply fast valves control systemSUNIST

3 Coaxial helicity injector on SUNIST3.1 Impedance model of injectorHypothesis: ideal conducting wall, force-free conduction, helicty balanceMode:

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3 Coaxial helicity injector on SUNIST3.2 Supported experiment of impedance mode SUNIST

3 Coaxial helicity injector on SUNIST3.3 Impedance calculations

Range of parameters: resistivity Magnetic energy4-12 kJSuitable impedance; 50-100 m ohmSUNIST

3 Coaxial helicity injector on SUNIST3.4 Injector on SUNISTInjector flux:1.2 mWb;Electrode material : spray-coat plasma-facing surfaces with a thin layer of tungsten.Electrode distance:4 cmelectrodes:cone-shapedIntrinsic impedance50-100 m ohmPeaked current:>10 kA;Threshold current of formation closed lines: ? kASUNIST

3 Coaxial helicity injector on SUNIST3.3 Others Rebuild of central conductor

Pre-ionization ?

Fast puff valves

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4 ConclusionImpedance model of coaxial helicity injectorInjector on SUNIST:ECR pre-ionization assisted CHI non-inductive plasma initiation

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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, 1997:1665-1671 [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: 3919-3929[4] Gates DA., Bell MG. et al. Recent Progress on the National Spherical Torus Experiment (NSTX), 29th 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, 2001:679-685[6] Taylor JB, Relaxation and magnetic reconnection in plasmas, Reviews of modern physics 58, 1986:741-763[7] McCollam KJ, Jarboe JR, Magnetic relaxation in coaxial helicity injection, Plamsa physics and controlled fusion 44, 2002:493-516 [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, 2001:1567-1573[9] Mitchell A Berger, Introduction to magnetic helciity,Plasma physics and controlled fusion 41, 2001:B167-B175

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Thanks!SUNIST