Transcript VTF

Magnetic Reconnection in Plasmas;
a Celestial Phenomenon in the Laboratory
J Egedal, W Fox, N Katz,
A Le, M Porkolab,
MIT, PSFC, Cambridge, MA
Outline
• The problem of magnetic reconnection
• Reconnection in the Versatile Toroidal Facility
– Experimental setup
– Experimental observation
– Electron kinetic effects
• Wind satellite data from the deep magnetotail
– Kinetic effects
• The new closed configuration in VTF
• Conclusions
The Versatile Toroidal Facility (VTF)
3.5 m
The Versatile Toroidal Facility (VTF)
The Versatile Toroidal Facility (VTF)
Two different magnetic configurations
A open cusp magnetic field.
Fast reconnection by trapped
electrons. Wind observation
A new closed cusp by internal
coil. Passing electrons &
spontaneous reconnection events.
Both configurations have Bguide and toroidal symmetry 2d
VTF open configuration plasmas
have a trapping potential
Open field lines intersect the
vessel wall.
Electrons stream faster than ions,
so plasma charges positive
Thermal electrons are
electrostatically trapped
Typical Parameters:
ne ~ 2-3 1016 m-3
Te ~ 12 eV
Ti ~ 1 eV
Bt ~ 80 mT (800 G)
Bc ~ 0-10 mT
Reconnection drive
– Electric field induced by
a central solenoid
– The solenoid is driven
by an LC circuit
– Vloop ~ 100 V
Plasma response to driven reconnection
The electrostatic potential

+70 V
Experimental
potential, 
-70 V
Electron flow:
v E B 
-   B g
B2
The electrostatic potential

EB
Ideal Plasma:
EB  Epol  Bcusp  0
Frozen in law is
broken where EB0
The electrostatic potential
EB

Ideal Plasma:
EB  Epol  Bcusp  0
δ
c
The size of the electron
diffusion region is
 c  lo  e
δ (cm)
Frozen in law is
broken where EB0
J Egedal et al., PRL 90, (2003)
ρcusp
lo  Bg / Bc
Kinetic modeling(1)
•
•
•
•
Why is the experimental current density so small?
Liouville/Vlasov’s equation: df/dt=0
For a given (x0,v0), follow the orbit back in time to x1
Particle orbits calculated using electrostatic
and magnetic fields consistent
with the experiment.
• Massively parallel code
evaluates f(x0,v0) = f(|v1|).
Computer Physics Communications ,
(2004)
Kinetic modeling(2)
• The current is calculated
3
j

v
f
d
v
as ||  ||
0 – 12 kA/m2
• Theory consistent with
measurements
(B-probe resolution: 1.5cm)
Theory
Experiment
Outline
• The problem of magnetic reconnection
• Reconnection in the Versatile Toroidal Facility
– Experimental setup
– Experimental observation
– Kinetic effects
• Wind satellite data from the deep magnetotail
– Kinetic effects
• The new closed configuration in VTF
• Conclusions
Wind satellite observations in
distant magnetotail, 60RE
• Measurements within the ion
diffusion region reveal:
Strong anisotropy in fe.
M. Øieroset et al. Nature 412, (2001)
M. Øieroset et al. PRL 89, (2002)
Wind satellite observations in
distant magnetotail, 60RE
• Measurements within the ion
diffusion region reveal:
Strong anisotropy in fe.
Log(f)
M. Øieroset et al. Nature 412, (2001)
M. Øieroset et al. PRL 89, (2002)
A trapped electron in the magnetotail
The magnetic moment:
mv 

2B
2
2

m( v 2 - v|| )
2B
Drift kinetic modeling of Wind data
• From Vlasov’s equation df/dt=0  f(x0,v0) = f(Eexit )
• Two types of orbits:
Passing:
No cooling
Trapped : =mv2/(2B)+… is constant
Cooling
 v|| 
   cosc   1  Bmin / Bmax
 v c
Drift kinetic modeling of Wind data
• Applying f(x0,v0) = f(|v1|) to an X-line geometry consistent
with the Wind measurements
• A potential, needed for trapping at low energies
• Ion outflow: 400 km/s, consistent with acceleration in 
Theory
Wind
~~-1150V
-300V
-800V
Phys. Rev. Lett. 94, (2005) 025006
Drift kinetic modeling of Wind data
• Applying f(x0,v0) = f(|v1|) to an X-line geometry consistent
with the Wind measurements
• A potential, needed for trapping at low energies
• Ion outflow: 400 km/s, consistent with acceleration in 
~ -1150V
Theory
Wind
f(x0,v0) = f(E0-q0), passing
= f(B), trapped
Cluster Obs.
Phys. Rev. Lett. 94, (2005) 025006
Outline
• The problem of magnetic reconnection
• Reconnection in the Versatile Toroidal Facility
– Experimental setup
– Experimental observation
– Kinetic effects
• Wind satellite data from the deep magnetotail
– Kinetic effects
• The new closed configuration in VTF
• Conclusions
New closed magnetic configurations
A new reconnection drive scenario
Spontaneous reconnection
Phys. Rev. Lett. 98, (2007) 015003
Sweet-Parker is out, E ≠ *j !
Magnetic energy released
Bz
R
Current channel expelled, J
4
-4
c/pi ~ 1m, s ~ 0.12m
vA ~ 10 km/s
What Triggers Reconnection?
R
R [m]
d/dt [V]
d/dt [V]
t [µs]
t [µs]
Mode at f=50 kHz
Plasma outflows
Rough energy balance
• Magnetic energy released ~ 0.5 × 6 10-6 H × (500A)2
• Electron energization
• Ion flows:
~ 500 A × 80V × 2 10-5s
~ 24 eV × 21018m-3 ×0.06m3
Strong energizations of the ions
~
0.8 J
~
0.8 J
~
0.48 J
Electrostatic (and magnetic) fluctuations
observed during reconnection events
800
Plasma Current (A)
0
Loop voltage (V)
Fluctuation
> 10 MHz (au)
(off scale)
fpe ~ 10 GHz
fce ~ 1 GHz
f (MHz)
Spectrogram
fpi ~ 30 MHz
fLH ~ 10 MHz
[Mar 22 shot 405,
0
HPF 80kHz, scope B/W 150 MHz]
t (ms)
1
Conclusions
– Fast, collisionless driven reconnection observed in the
open cusp configuration
– Classical Coulomb collisions are not important
– The width of the diffusion region scales with cusp
– Solving Vlasov’s equation (using the measured profiles of E and B)
provides current profiles consistent with the VTF measurements; the
current is limited by electron trapping.
– Wind observations consistent with fast reconnection
mediated by trapped electrons
– New closed configuration in VTF provides exciting
new parameter regimes and boundaries for future study
of collisionless magnetic reconnection & the trigger
problem.
Thank you for your attention
Future studies with the new configuration
• Fast, bursty reconnection with closed boundaries and in the
presence of guide magnetic field can be studied (for the first time)
• What controls the rate of reconnection?
• How is reconnection “triggered”
• Huge parameter regime available: Scans possible in Bcusp,
Bguide, Te, Ne, Erec.
• Spans collisional to collisionless regimes: e = 0.1 – 103 m
• High plasma pressure (compared to magnetic field):  ~ 1
• Warm and magnetized ions.
• 3D magnetic geometries are easily implemented
Upgrade of open Cusp
Existing configuration
Fields of new in-vessel coils
Upgrade of open Cusp
New total field
Ionization region
Reconnection Experiments
with a Guide Magnetic Field
J Egedal, W Fox, N Katz,
A Le and M Porkolab
MIT, PSFC, Cambridge, MA