A.J.Redd, D.J.Battaglia, M.W.Bongard, R.J.Fonck, and D.J.Schlossberg

Transcription

1 A.J.Redd, D.J.Battaglia, M.W.Bongard, R.J.Fonck, and D.J.Schlossberg 51st APS-DPP Annual Meeting November 2-6, 2009 Atlanta, GA USA

2 The PEGASUS Toroidal Experiment Helicity injection in PEGASUS Testing max I p theoretical scalings Summary

3 High-stress Ohmic heating solenoid Experimental Parameters Parameter To Date A R(m) I p (MA) I N (MA/m-T) l i κ τ shot (s) β t (%) P HHFW (MW)

4 Current is injected into the existing helical magnetic field High I inj & modest B filaments merge into current sheet High I inj & low B current-driven B θ overwhelms vacuum B z Relaxation via MHD activity to tokamak-like Taylor state w/ high toroidal current multiplication Anode V bias + Molybdenum Cathode - Anode Molybdenu m Washers Molybdenum Cathode D 2 gas Boron Nitride Washers + V arc Reduced B z B T =10 mt, B z = 5 mt

5 Driven helical filaments are strongly unstable Relax into the axisymmetric tokamak-like state Tokamak-like equilibrium satisfies a set of conditions Radial force balance Helicity/power balance Kink stability: edge q > 3 Taylor relaxation current limit Max I p determined by dk/dt and Taylor relaxation limit: εa I I I f p TF inj p G 2πR w edge 1/ 2

6 Estimated plasma evolution Anode I p max Helicity limit I TF = 288 ka V bias = 1kV V ind = 1.5 V I inj = 4 ka w = d inj L-mode τ e Plasma guns Time Relaxation limit

7 Predicted relaxation limit I p scales with (I TF I bias ) 1/2 Experimental I p consistent with these scalings:!2 Note: adding solenoid induction does not allow driven I p to exceed the Taylor relaxation limit

8 Relaxation current limit scales as w -1/2 One-gun discharges had higher limits than corresponding three-gun cases, indicating the gun array was misaligned: Anode w 3 guns

9 Changing the tilt of the gun array increased the max Ip by a factor of , implying a factor-of-3 change in w. In this configuration, have achieved Ip > 170 ka. Anode 140 After alignment Before alignment 120 w ka guns m

10 PEGASUS non-solenoidal startup goal is MA Characterize confinement/dissipation in driven plasma Access high I p /I TF, high β regimes This will involve: Increase TF: increase Taylor limit, improve confinement Increase gun current: increase Taylor limit Bigger/improved plasma guns: higher helicity injection rate Test augmenting the guns with shaped electrodes Outstanding issues: What sets the bias impedance Z inj? Is the confinement/dissipation stochastic? What sets the width w of the driven region?!

11 Making progress with non-solenoidal startup I p up to 170 ka using helicity injection and outer-pf rampup Using understanding of helicity balance and relaxation current limit to guide hardware and operational changes Outstanding physics questions: λ edge, Z inj, confinement, etc. Ultimate goal is MA non-solenoidal current High-I p non-solenoidal startup enables exploration of high I N, β T operating space I p /I TF > 2, I N > 14 achieved Extend to high I p, n e for high β t Troyon scaling for conventional tokamaks and other STs with predicitions for PEGASUS PEGASUS Phase-I START Conventional Tokamaks Aux. Heating (HHFW, EBW) ustin, TX, EAU I N = I p /(ab t ) = 6 I p /I tf PEGASUS Universit