1SEWG Fuel removal - Glow improvement for JET D. Douai22-23 July 2008 EFDA Task Glow discharge improvement for JET D. Douai 1, D. Garnier 1, S. Brémond.

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1SEWG Fuel removal - Glow improvement for JET D. Douai22-23 July 2008 EFDA Task Glow discharge improvement for JET D. Douai 1, D. Garnier 1, S. Brmond 1, C. Grisolia 1, J. Bucalossi 1, P. Shigin 2, L. Begrambekov 2 and JET EFDA contributors* 1 Institute for Magnetic Fusion Research, CEA Cadarache, 13108 Saint Paul lez Durance Cedex (France) 2 Plasma Physics Department, Moscow Engineering and Physics Institute, 115409 Moscow, Russia *See M.L.Watkins et al., Fusion Energy 2006 (Proc. 21st Int. Conf. Chengdu, 2006) IAEA, (2006) Slide 2 2SEWG Fuel removal - Glow improvement for JET D. Douai22-23 July 2008 Intro : task description Experimental set-up Results Coverage of remote areas using the heated cathode -Langmuir probe measurements -Plasma parameters Additional anode Conclusion Plan Slide 3 3SEWG Fuel removal - Glow improvement for JET D. Douai22-23 July 2008 Improve glow discharge attractiveness for wall conditioning Heavy carbon deposits / flakes containing T observed hardly reachable with GDC Need to increase GD coverage in remote areas In-vessel configuration of the JET divertor Task description Slide 4 4SEWG Fuel removal - Glow improvement for JET D. Douai22-23 July 2008 Idea : to establish the GD between heated cathode and anode both isolated from the grounded walls Expected benefits are: -Increased glow discharge coverage in remote areas -Reduced operating pressure thanks to emitted electrons -Controlled energy of impinging ions Laboratory tests are performed in a DC glow discharge at CEA/IRFM Task description Slide 5 5SEWG Fuel removal - Glow improvement for JET D. Douai22-23 July 2008 Glow Discharge conditioning Tokamak walls act as cathode Task description V plasma A K K Possibility to control the energy of impinging ions + heated cathode Slide 6 6SEWG Fuel removal - Glow improvement for JET D. Douai22-23 July 2008 Experimental set-up narrow cylindrical duct (10 cm diameter, 32 cm long) 7 cylindrical Langmuir probes + 1 at the top of chamber Digital multimeter Slide 7 7SEWG Fuel removal - Glow improvement for JET D. Douai22-23 July 2008 Test vessel 1 2 3 7 5 4 6 tube Experimental set-up Slide 8 8SEWG Fuel removal - Glow improvement for JET D. Douai22-23 July 2008 Present conditions : non-collisional sheath mean free paths >>probe dimensions 3 regions : (AB) : V probe 15SEWG Fuel removal - Glow improvement for JET D. Douai22-23 July 2008 Additional anode in the duct 3 7 1212 both the probe voltage and current decrease with U polar But : difficulty to maintain the GD the glow is pushed outside the tube for U polar >>V plasma the effect is sudden : there is no transition observed Slide 16 16SEWG Fuel removal - Glow improvement for JET D. Douai22-23 July 2008 Standard glow discharges can hardly penetrate into remote areas except for high gas pressures Using an heated cathode allows the GD to penetrate into remote areas for given filament current Langmuir probes measurements show that the plasma potential in the main chamber is reduced as the energy of the ions striking the walls Applying a positive potential with an additional anode has a negative effect Summary Slide 17 17SEWG Fuel removal - Glow improvement for JET D. Douai22-23 July 2008 Plasma density and potential Using the heated cathode Slide 18 18SEWG Fuel removal - Glow improvement for JET D. Douai22-23 July 2008 Plasma density and potential Using the heated cathode