1. 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 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)

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

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

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

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

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

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

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 <<0, I probe =-I ions (CD) : I probe =I e (BC) : both type reach the probe In particular at C, |I e |=|I ions | V probe = V plasma electron current for V probe V plasma : with : A is the probe area n 0 is the charged particles density k B the Boltzmann constant T e the electron temperature, e=1,6.10 -19 C A D C B Langmuir probe measurements 3 7 1212 probe#1, Ik=0

9. 9SEWG Fuel removal - Glow improvement for JET D. Douai22-23 July 2008 Penetration of the glow discharge into the duct No GD in the duct under standard conditions Penetration into the tube if pressure high enough or heated cathode used Coverage by the GD 3 7 1212

10. 10SEWG Fuel removal - Glow improvement for JET D. Douai22-23 July 2008 GD penetrates into the tube above a pressure threshold Increasing gas pressure pressure threshold such as e-n =1/N e-n 1/p Hysterisis: the GD remains in the duct when decreasing p But: too high gas consumption, risk of arcing 3 7 1212

11. 11SEWG Fuel removal - Glow improvement for JET D. Douai22-23 July 2008 V plasma drops down when using the heated cathode Using the heated cathode the potential drop between the GD and the wall decreases the energy of ions bombarding the walls is reduced

12. 12SEWG Fuel removal - Glow improvement for JET D. Douai22-23 July 2008 Using the heated cathode Probe#6 (In the tube) vs. cathode current Ik The glow GD penetrates into the tube for Ik ~23A Peak of current and voltage and N e at Ik~25 A Langmuir Probe can not give V plasma below Ik~23A. Peak for V plasma ? Current and voltage at top probe decrease above Ik~23A 3 7 1212

13. 13SEWG Fuel removal - Glow improvement for JET D. Douai22-23 July 2008 Using the heated cathode In the main chamber, plasma pot. V plasma drops down as Ik is increased N e increases with Ik, peak at 25 A ? Current and voltage at top probe decrease Behaviour vs. cathode current Ik - Probe#1 Same behaviour as before Peak of current is observed for the same Ik ~23A 3 7 1212

14. 14SEWG Fuel removal - Glow improvement for JET D. Douai22-23 July 2008 Additional anode in the duct annular anode instead of probe#4 in the cylinder both the heated cathode and the additional anode used simultaneously

15. 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

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

17. 17SEWG Fuel removal - Glow improvement for JET D. Douai22-23 July 2008 Plasma density and potential Using the heated cathode

18. 18SEWG Fuel removal - Glow improvement for JET D. Douai22-23 July 2008 Plasma density and potential Using the heated cathode