MASSIVE GAS INJECTION SYSTEMS FOR DISRUPTION MITIGATION ON THE DIII-D TOKAMAK

Transcription

1 MASSIVE GAS INJECTION SYSTEMS FOR DISRUPTION MITIGATION ON THE DIII-D TOKAMAK T. C. Jernigan, L. A. Baylor, and S. K. Combs Oak Ridge National Laboratory D. A. Humphreys, P. B. Parks, and J. C. Wesley General Atomics E. M. Hollmann UCSD 1

2 Abstract Injection of massive quantities of deuterium or noble gases (>10 22 molecules) has proven to be very effective at mitigating the deleterious effects of disruptions in the DIII-D tokamak. Both the heat load to the divertor and the halo current forces to the first wall were reduced by more than a factor of four. Total electron densities (free and bound) of! m -3 have been achieved, close to densities required to prevent avalanche multiplication of runaway electron currents during the fast plasma current ramp down that injection produces Two gas injection configurations used in experiments in the period will be described. Both configurations use a fast-operating (open/close! 0.5 ms) solenoid valve with an orifice diameter of 4 mm. The maximum flow rate in helium is 5x10 4 Pa m 3 /s at a reservoir pressure of 7 MPa. A new valve with an orifice diameter of 22 mm will be tested when DIII-D resumes operation in The new valve, with a flow area of 25 times the original valve, has flow capabilities that approach those required for ITER. Calculations show that a set of four such valves can reach the no-avalanche density in ITER in!0.25 t CQ where t CQ,!30 ms, is the fastest-possible plasma current quench time. While there are still-open questions about the mechanisms whereby injected neutral gas and free and bound electrons penetrate (or are mixed) into the core of a high-temperature plasma, it is clear that massive gas injection can easily and reliably provide sufficient mass flow to minimize disruption damage in large reactor-scale tokamaks. The principal open physics question is what is the mixing mechanism which transports the impurities to the plasma interior and how fast and effective will it be in a reactor-scale plasma.. 2

3 Disruptions Place Severe Limits on the Lifetime of Tokamak Reactors The large heat loads generated have the potential to erode destroy plasma-facing components by extreme localized heating. In addition, the forces generated by the halo currents when the plasma strikes the wall may cause structural damage not only to the first wall components but also to the underlying structural supports. An additional hazard facing high current devices is the generation of large runaway electron currents produced during the plasma current decay 3

4 Mitigation is a Necessity When Disruption Avoidance Fails Impurity pellets have been tested for reducing the damage from both the excess heat load and the halo current forces. However, unless very large pellets are used, it is difficult to get the density high enough to prevent the large runaway electron currents generated in the rapid current rampdown of a disruption. Such large pellets have the potential to cause first wall damage themselves if not fully ablated by the plasma. 4

5 Massive Gas Injection is a Promising Mitigation Technique Experiments have been carried in DIII-D and other tokamaks to test the mitigation capability of the injection of massive amounts of gas to mitigate both the heat loads and magnetic loads In addition, sufficient mass can be injected into the torus to prevent runaway current multiplication during the plasma current decay. 5

6 Open Configuration Massive Gas Puff Pa m 3 delivered in 10 ms pulse Divertor heat load reduced by factor of 5 to 10 Halo currents reduced by factor of 2 to 4 Not aimed at plasma center 6

7 Open Configuration Detail DIII-D Port 15R+1 6 OD Tube 16 mm ID Tube Fast Valve 510 mm 470 mm 6 Gate Valve 300 ml Reservoir Maximum flow rate in helium of 5x10 4 Pa m 3 /s at a reservoir pressure of 7 MPa. 7

8 Directed, High Intensity Gas Jet Used to Test Neutral Penetration Results from open configuration showed cooling front penetrated at close to the sound speed of the injected gas Jet is dense enough to prevent plasma electrons from penetrating into the interior of the jet New jet designed to maximize the ram pressure by putting small exit tube near plasma edge to test direct penetration of neutral gas jet 8

9 High Intensity Massive Gas Puff Jet Tube 46.5 Gate Valve Ballast Volume Fast Valve High Pressure Reservoir Same flow as open configuration 100X gas density at tube exit Aimed at plasma center 9

10 High Intensity Configuration Detail DIII-D Port 15R+1 12 mm ID Tube 990 mm Pressure Transducer 142 mm Gate Valve Fast Valve 100 mm OD support tube High Pressure Reservoir (300 ml) to 9 liter Ballast Volume 10

11 Several Iterations in the High Intensity Configuration Were Tried Flow rate was similar open configuration Rise time was much slower Reductions in parasitic volumes and minimization of flow channel diameter changes improved response times. 11

12 High Intensity MGP Mk I 2/10/2004-4/15/2004 Poor Coupling to Jet Tube Large volume of 1.5" Tee (225 cc) and 1.5" TIV Large conductance into 1.5" Tee (13 mm clearance) Shots Jet Tube (12 mm dia) 1.5" Tee Adapter Tube (8mm) Fast Valve 2 To Plasma Insulator 340 mm reservoir (1000 psi) 1.5" TIV 9 liter ballast volume 12

13 High Intensity MGP Mk II 4/16/2004-9/30/2004 Small volume of Tee (14 cc) Small conductance into Tee (1 mm clearance) Shots Changed now Previously changed Minimum volume Tee Jet Tube (12 mm dia) Adapter Tube (8mm) Fast Valve 2 To Plasma Insulator reservoir (1000 psi) 1.5" TIV 9 liter ballast volume 13

14 High Intensity MGP Mk III 10/1/2004-1/6/2005 Somewhat better coupling to Jet Tube and less volume with 15 mm TIV Jet Tube (12 mm dia) Minimum volume Tee Adapter Tube (8 mm) Shots Changed now Previously changed Fast Valve 2 Insulator reservoir (1000 psi) 15 mm TIV 9 liter ballast volume 14

15 High Intensity MGP Mk IV Valve isolates ballast volume when TIV open (eliminates any flow into ballast volume) 1/7/2005-4/1/2005 Shots Changed now Previously changed Jet Tube (12 mm dia) Minimum volume Tee Adapter Tube (8mm) Fast Valve 2 Insulator 15 mm TIV reservoir (1000 psi) Ballast isolation valve (out of phase with TIV) 9 liter ballast volume 15

16 Delay/Risetime in Final Configuration Similar to Open Configuration Open Configuration Hi Intensity Mk I Hi Intensity Mk IV Argon Gas 16

17 Recent Experiments Contradict The Neutral Penetration Hypothesis High speed camera images with ArI and ArII filters indicate no neutral argon in plasma interior until after the thermal quench MHD burst may be responsible for mixing impurity ions into interior Mitigation with high intensity configuration no better than open configuration at maximum flow 17

18 New Configuration Designed to Increase Gas Input before the Thermal Quench Same basic layout as the original open configuration New 22 mm diameter orifice valve Flow rate = 3X10 5 Pa m 3 /s at 3 MPa Pressure range 0.5 MPa to 7 MPa to allow direct comparison with previous results Intent is to get same amount of gas in shorter period Ready for 2006 Operating Period 18

19 High Flow Configuration Detail 9 liter ballast volume original valve as pilot valve DIII-D Port 15R+1 22 mm ID tube High flow valve 19

20 Valve Injection Line Massive Gas Injection on ITER Four high flow valves provide enough mass input for ITER If speed is not a problem, external mounting will work Other valves may be suitable for internal mounting 20

21 Summary Direct injection of large amounts of gas has provided effective mitigation of heat and magnetic loads from disruptions Some of these experiments have injected enough gas to raise the total electron inventory to levels that are estimated to be nearly sufficient to prevent runaway electron current multiplication A set of four valves such as those being used in the high flow configuration could, if all injected electrons are assimilated, provide enough total (free + bound) electron density to prevent runaway multiplication Big question: do the injected electrons get to where they need to be to stop runaway multiplication? 21