Equilibrium Evolution in the ZaP Flow Z-Pinch

Transcription

1 Equilibrium Evolution in the ZaP Flow Z-Pinch U. Shumlak, B.A. Nelson, C.S. Adams, D.J. Den Hartog, R.P. Golingo, S. L. Jackson, S.D. Knecht, J. Pasko, and D.T. Schmuland University of Washington, Seattle Innovative Confinement Concepts Workshop February 2006

2 ZaP Personnel Graduate Students Colin Adams Stuart Jackson Sean Knecht Keith Munson Undergraduate Students Jim Pasko Josh Proctor Derek Schmuland Collaborators Daniel Den Hartog (U Wisc) Charles Hartman (LLNL) Faculty Raymond Golingo Brian A. Nelson (Co-PI) Uri Shumlak (PI) Staff Ed Crawford Matthew Fishburn Dan Lotz James Newman Dennis Peterson Dzung Tran

3 Presentation Outline Pure Z-Pinch and Conventional Stabilization Techniques Shear Flow Stabilization of the Z-Pinch The Concept & Experimental Design Experimental Data Showing a Period of Low MHD Mode Activity Density Profiles throughout Quiescent Period and Equilibrium Analysis Suggesting a Hot Plasma Core Internal Magnetic Field Measurements Showing a Confined Plasma with High Current Conclusions

4 Conventional techniques to provide stability have drawbacks. Profile Control Stabilizes the sausage mode, but not the kink. Close-Fitting Wall Must be very close, r a< 1.2. Z-Pinch Equilibrium & Conventional Stabilization The pure Z-pinch (no applied axial fields) is described by o ( ) Bθ d rbθ dp + = µ r dr dr is classically unstable to m = 0 sausage and m = 1 kink modes. Axial Magnetic Field Limits the plasma current (and pressure) according to Kruskal-Shafranov limit and opens field lines. Theoretical analysis suggests that a sheared flow can stabilize the modes in a pure Z-pinch provided v > 0.1 k V A.* *Shumlak & Hartman, Phys. Rev. Lett. 75, 3285 (1995) 0 wall

5 A Z-Pinch with an Embedded Axial Flow To generate a Z-pinch configuration with an embedded axial flow the ZaP experiment couples a coaxial accelerator with a pinch assembly region.

6 The Concept t 1 Gas is injected and a capacitor bank is discharged across the electrodes. t 4 t 2 The plasma accelerates t 5 down the coaxial accelerator until it assembles into a Z-pinch plasma along the axis. t 3 Inertia and gun currents maintain the flow until the accelerator plasma empties.

7 The Experiment The experiment investigates the concept of using flows to stabilize an otherwise unstable plasma configuration.

8 Operating Parameters Entity Value Capacitor Bank Energy 72 kj (max) Charge Voltage 10 kv (max) Peak Current 300 ka (max) Plasma Pinch Radius cm Plasma Pinch Length 100 cm Density cm -3 Plasma Temperature (T e + T i ) ev

9 Diagnostics To Measure Plasma Flow & Stability The ZaP diagnostics measure plasma parameters (equilibrium), plasma flow, and magnetic mode activity (stability). Surface-mounted magnetic field probes Fast framing camera with optical filters Two chord, visible HeNe interferometer Holographic interferometer using a pulsed ruby laser density profiles & thermal conduction analysis to get equilibrium profiles 0.5 m imaging spectrometer with 20 input chords and an intensified CCD detector Zeeman splitting for internal magnetic fields

10 Magnetic Fluctuations Diminish after Pinch Forms Fluctuations of the magnetic modes are significantly reduced for 37 µs after pinch forms. Mode activity increases again after this quiescent period.

11 Holographic Interferometer for Density Profiles A holographic interferometer has been installed to measure chordintegrated density profiles. The system uses a pulsed ruby laser in a double-pass or single-pass configuration. The holograms are reconstructed using a He-Ne laser. The chordintegrated data are deconvolved to determine the density profile.

12 Holography Measures a Discrete Plasma Pinch During the quiescent period, density profile shows a discrete plasma pinch - density reduces to a small value at the edge. Pinch radius 8 mm. Peak density cm -3. Hologram obtained during the middle of the quiescent period (τ = 0.65) in He plasma generated with 58 kj bank energy. Density profile is determined through a deconvolution method.* *Jackson & Shumlak, Rev. Sci. Instrum. (in preparation)

13 Holography Measures a Discrete Plasma Pinch Toward the end of the quiescent period, density profile still shows a discrete plasma pinch but the density is lower everywhere. Pinch radius 6 mm. Peak density cm -3. Hologram obtained towards the end of the quiescent period (τ = 0.73) in He plasma generated with 58 kj bank energy. Density profile is determined through a deconvolution method.

14 Holography Measures a Low, Uniform Plasma After the quiescent period, density profile no longer shows a discrete plasma pinch. Instead a mostly low, uniform background density is measured Maximum density cm -3. Hologram after the quiescent period (τ = 1.13) in He plasma generated with 58 kj bank energy. Density profile is determined through a deconvolution method.

15 Analyzed Density Profiles Provide Equilibrium The density profiles are analyzed using the magnetic field measured at the outer electrode and the input power to compute equilibrium temperature and magnetic field profiles. Using radial force balance, o ( ) Bθ d rbθ d = nt e e+ nt µ r dr dr ( ) Computing the radial thermal conduction for a specified input power, 1 d q = r k ete+ k T rdr i i [ ( )] where the thermal conductivities are given by Braginskii* as k nt *Braginskii, Reviews of Plasma Physics, vol 1. (1965) i i nt 4.7 e e e = k 2 i i 2 i 2 meωceτ = e i ci i m ω τ

16 Analyzed Density Profiles Provide Equilibrium During the middle of the quiescent period, the equilibrium is computed using B 10cm = 0.16 T and P in-max = 400 MW.

17 Analyzed Density Profiles Provide Equilibrium During the middle of the quiescent period, the equilibrium is computed using B 10cm = 0.16 T and P in-max = 400 MW. Temperature profile:

18 Analyzed Density Profiles Provide Equilibrium During the middle of the quiescent period, the equilibrium is computed using B 10cm = 0.16 T and P in-max = 400 MW. Magnetic field profile:

19 Analyzed Density Profiles Provide Equilibrium Towards the end of the quiescent period, the equilibrium is computed using B 10cm = 0.16 T and P in-max = 400 MW.

20 Analyzed Density Profiles Provide Equilibrium Towards the end of the quiescent period, the equilibrium is computed using B 10cm = 0.16 T and P in-max = 400 MW. Temperature profile:

21 Analyzed Density Profiles Provide Equilibrium Towards the end of the quiescent period, the equilibrium is computed using B 10cm = 0.16 T and P in-max = 400 MW. Magnetic field profile:

22 Zeeman Splitting Measures Internal Magnetic Field Zeeman splitting measurements have been made to determine the internal magnetic field of the plasma pinch. Impurity emission of the C IV doublet at & nm is collected perpendicular to the plasma. Circularly polarized light is collected along 10 parallel chords through the pinch. Methane plasma with 35 kj bank energy.

23 Zeeman Splitting Measures Internal Magnetic Field When viewed parallel to the magnetic field, the Zeeman effect causes RHP components to shift down in wavelength and LHP components to shift up in wavelength. The chord-integrated data are deconvolved to give radial profiles. An inverse radius envelope (blue dash) contains the B values (points).

24 Zeeman Splitting Measures Internal Magnetic Field Deconvolved magnetic field values are compiled for many pulses to provide an average magnetic field profile. Characteristic pinch radius is 10 mm. The magnetic field peaks at 0.8 T and then decays as inverse radius to the value measured at the outer electrode. Structure is similar to that analyzed from the density profiles.

25 Summary & Conclusions The ZaP project is producing Z-pinch plasmas that exhibit gross stability during a quiescent period that is about 2000 times the growth times for static pinches - consistent with flow stabilization theory. The Z-pinch plasmas show evidence of confinement and heating. Analyzed density profiles show high temperature and magnetic field profiles during the quiescent period. Internal magnetic field measurements are consistent with the analyzed profiles. A flow-stabilized Z-pinch has important implications for a simple reactor design, space propulsion, other magnetic confinement configurations, and astrophysics. Future Work: Add Thomson scattering to directly measure internal T e. Replace inner electrode to increase plasma temperature.