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SSX summary: helicity balance and Ohms law. Workshop on Magnetic Self-Organization NSF Center meeting, Aug 4-6, 2004 Michael Brown C. D. Cothran, J. Fung, A. O Murchadha, Z. Michielli, M. Chang Swarthmore College Collaborators: M. Schaffer (GA), W. Matthaeus (Bartol), - PowerPoint PPT Presentation
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Workshop on Magnetic Self-OrganizationNSF Center meeting, Aug 4-6, 2004
Michael BrownC. D. Cothran, J. Fung, A. O Murchadha, Z. Michielli, M. Chang
Swarthmore College
Collaborators: M. Schaffer (GA), W. Matthaeus (Bartol),D. Cohen (Swarthmore), E. Belova (PPPL)
Research supported by US DOE grants ER54604 and ER54490
SSX summary: helicity balance and Ohms law
Outline
A brief tour of the Swarthmore Spheromak Experiment (SSX)Device, diagnostics, plasma parameters
Full merging and self-organization to large scale(magnetic helicity conservation, FRC, doublet CT)
Local 3D magnetic reconnection studies(generalized Ohms law, Hall terms, energetic ions)
Full merging: FRC formation
Right-handedSpheromak
Left-handedspheromak Large scale structure (FRC)
Magnetic structure consistent with FRC/doublet-CT
full data
•m=0 dominates•Other modes are present
Magnetic reconnection in three dimensions
Reconnection in SSX-FRC
Ensemble average of 36 identical shots
PART 1
Helicity balance
Spheromak formation
Complete merging: FRC formation
Right-handedSpheromak
Left-handedspheromak
FRC
Helicity conservation leads to a null helicity structure
The SSX Laboratory
10kV/100kAPulsed power
Cylindrical flux conservers andvacuum chamber (=0.40m, L=0.65m)
Coaxial magnetized plasmaguns on each end (1 mWb)
Diagnostics at SSX
600 channel 1.25 MHz data acquisition system
Magnetic probe arrays
Langmuir triple probe
He-Ne quadrature interferometer
0.2 m VUV monochrometer
Bolometer
Retarding Grid Energy Analyzers (RGEA)
Soft x-ray photodiodes (SXR)
Directional (Gundestrup) Mach probe
Distributed probe array
12 probe stalks: 4 toroidally at three axial positions
Magnetic structure consistent with FRC/doublet-CT
m=0 (toroidal mode) component
•Reversed field•Very little midplane toroidal field•Axially antisymmetric B•70 G RCC field (on axis)
Magnetic structure consistent with FRC/doublet-CT
full data
•m=0 dominates•Other modes are present
Peak poloidal flux and radial flux profile
•Ends reach 3-4 mWb immediately (3-4 amplification)•Midplane flux grows to match ends•Reconnection rate ≈ 0.04•No private flux after 50s, but toroidal fields remain•Midplane flux profile consistent with RS/√2: high FRC
70 G RCC field (on axis)
Axisymmetric helicity estimate
• Poloidal flux = 3 mWb (east and west)
• Toroidal flux = +/- 3 mWb (east and west)
• Helicity = 2x10 mWb^2 east – 2x10 mWb^2 west = zero total
• Rate = 2(1 kV)(1 mWb) x 10 s = 20 mWb^2
m=1 component late in time: tilted CT
Geometric axis of CT is perpendicular to the flux conserver axis
Elena Belova 2D simulation
3D simulation showing tilt instability
Full data (70 G on axis)
QuickTime™ and aTIFF decompressor
are needed to see this picture.
PART 2
Generalized Ohm’s Lawand Energetic Ions
3D magnetic reconnection experiments
Brown et al Astrophys. J. Lett. (9/02)Brown et al Phys. Plasmas 9, 2077 (2002)Brown et al Phys. Plasmas 6, 1717 (1999)Kornack et al Phys. Rev. E 58, R36 (1998)
Magnetic probe array
RGEAs
Large slots cut into FC rear walls define the reconnection region
3D magnetic propertiesEnergetic particles
3D magnetic probe array
600 coils, 558 array
~2 cm spacing
25 three channel 8:1multiplexer/integratorboards
10 eight channel 8-bitCAMAC digitizers
Full probe readoutevery 0.8 s
Reconnection in SSX-FRC
Catch reconnection early (< 32 s) then FRC forms
Generalized Ohms Law and Curl
• E + vxB = ηJ + (JxB – grad P)/ne + ∂J/∂t
• Curl (vxB + div P) =
∂B/∂t + Curl ηJ + Curl (JxB)/ne + Curl (∂J/∂t)
Hall term dominates electric field during shot
Ensemble average of 36 identical shots
Terms in curl of Ohms law (single shot)
Generalized Ohm’s Law magnitudes
• E + vxB = ηJ + (JxB – grad P)/ne + ∂J/∂t
• Ohmic and electron inertia terms are small
• From near pressure balance and unity , we know that JxB and grad P are comparable
• Only grad P can contribute at the neutral line
In plane magnetic field (ala min variance)
Out of plane magnetic field
Merger of left and right handed tori
Side view
Cross section
In plane JxB force (ala min variance)
Out of plane JxB force (slingshot)
Current channel formation correlates with RGEA activity
RGEA raw signals
Average peak signal for the out-of-plane RGEA
Fit to a thermal distribution with drift:T=33±11eV and V=86±20eV
SummarySpheromak merging in SSX forms large scale,
self-organized structure•Reconnection is fully 3D
•Merging results in self-organized structure •Helicity conservation implies null helicity
•Hall terms dominate electric field in Ohms law
•Study dynamics of doublet-FRC •Study flow with Mach probe, ion doppler•Need computational/theoretical support
Local SSX reconnection is fully 3D, generates energetic particles, flow, and heat
Plans for the near future
• Implement IDS at midplane of SSX-FRC (use with Mach probe)
• Compare flow results with Belova code
• Helium glow discharge cleaning for density control (lower density, larger c/pi)