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1
Sustained Compact Toroids in MRX
S.P. Gerhardt, M. Yamada, H. Ji
M. Inomoto1, E. Belova, R. Maqueda2 Y. Ren, B. McGeehan, & C. Jacobsen
A First Look At Our Recently Completed Ohmic Campaign
1: Osaka University 2:Nova Photonics
2
Ohmic Sustainment a Step Toward the “SPIRIT” Oblate FRC Concept
4 main components of the SPIRIT concept.– Spheromak merging to form large-flux FRCs.– Ohmic system to heat the plasma and further increase the
flux for energetic ion confinement.– Conducting shells to stabilize n=1 modes.– NBI to sustain the plasma and stabilize low-n co-
interchange modes via FLR effects.
“Self-Organized Plasma With Induction, Reconnection, and Injection Techniques”
Developing Ohmic system is an important step towards realizing this concept in an MRX-scale device.
Three month campaign of sustained CTs……machine now again devoted to basic reconnection science.
3
This talk…
• The MRX facility modified for Ohmic sustainment of Compact Toroids (CTs).
• Ohmic sustainment of FRCs– FRC equilibrium maintained for >300 s.– Pressure profile peaks to maintain equilibrium– Only Argon plasmas provide sufficient stability in the
present experiments.
• Ohmic sustainment of spheromaks– Light gasses (D2, He, Ne)n=1 tilt or n=2 kink typically
terminate the configuration.– Argon, no tilt or kink is observed, and the spheromak
plasma transitions to an FRC equilibrium during Ohmic.
4
MRX Modified for CT Sustainment Campaign
• 68 turn Ohmic solenoid, Inconel liner• Three capacitor banks for 4 coils (TF, PF, SF, Ohmic)….
…reduced shape control• New shaping coils with reduced field errors.• No nearby passive stabilizers.
New 2D Probe Array7x6 array of Coil Triplets
€
ψ R,Z( ) = 2π RBZdR∫
€
φ ψ( ) = BT∫∫ dA
€
p R,Z( ) = jZBT − jTBZ( )∫ dR
6 Flux Loops on SolenoidFlux penetration through liner
Spoke Probes• Triplets at 5 radial locations• Probes at 8 toroidal angles
• Midplane magnetic perturbations in BR and BZ
Triple ProbeFast Camera
5
FRC Capabilities Recently Upgraded, Including Ohmic Solenoid
6
FRC Sustainment
Merging Spheromaks Have Oppositely Directed Toroidal Fields
7
Ohmic Sustainment for ~300s Demonstrated
No Ohmic With Ohmic
Flux Plots From Magnetic Probe Array
8
QuickTime™ and a decompressor
are needed to see this picture.
Sustainment Visible in Fast Camera Images
Fast Camera Images, Argon, White Light
9
Ohmic Sustainment for ~300s Demonstrated
275 s 325 s 375 s 450 s 550 s
10
Peaked Pressure Profile Evolves to Sustain FRC Equilibrium
275 s 325 s 375 s 450 s 550 s
Electron Pressure Triple Langmuir Probe Radial Scan
Red: SustainedBlue: Decaying
11
Increased Ohmic Voltage Leads to More Flux, Longer Sustainment
Capacitance and Inductance Fixed for Longest Ohmic Waveform
Ohmic Voltages 5kV-9kV
Input Powers: 300-800kW
Tra
pp
ed
Flu
x(m
Wb
)
Su
rfa
ce V
olta
ge
(V)
Cu
rre
nt
De
nsi
ty(A
/m2)
So
len
oid
Cu
rre
nt
(kA
)
Maximum Ohmic voltage limited by null radius expansion, not instability.
12
Lighter Gasses Demonstrate Rapid Instability
Helium Example
Shot 65788
13
Lighter Gasses Demonstrate Co-Interchange Instabilities
Helium Example
Shot 65788
Tra
pped
Flu
x(m
Wb
)
BZ BZ BZ
BR BR BR
14
Lighter Gasses Demonstrate Co-Interchange Instabilities
• Co-Interchange: pressure driven modes.• ~1• Bad curvature everywhere •MHD predicts instability…correctly
• Many toroidal mode numbers simultaneously unstable.
• Configurations have been identified with stability to all co-interchange modes via conducting shells and NBI (SPIRIT).1
• SSX experiment to test oblate FRC with complete set of nearby conductors.
Tra
pped
Flu
x(m
Wb
)
BZ BZ BZ
BR BR BR
1Belova et al, Phys Plasmas 2006; M. Yamada et al, Plasma and Fusion Research 2007..
15
Flux & Lifetime Best for Argon
Average Flux During Ohmic
Discharge Peak Flux
Life
time
(s)
Average Flux During Ohmic (mWb)
Life
time/
Res
istiv
e T
ime
Steady SustainmentTransient
Helium4
4-106-11
Deuterium2
2-44-6
Nitrogen14
10-2015-30
Neon20
10-2020-32
Argon40
25-3540-90
Krypton84
35-5050-100
Mass
€
ρi (cm)
s)
Ohmic Waveform Limit
€
R =μ0a
2
η Sp
16
Spheromak Sustainment
Merging Spheromaks Have Toroidal Fields Which Point in the Same Direction
17
Helium and Neon Spheromaks Often Tilt
Helium Example: Pure n=1 “tilt” spectrum
Flux
BR, n=1
BR, n=2
BR, n=3
18
Helium and Neon Spheromaks Often Tilt
Helium Example
Nova Photonics Fast CameraWhite Light, 100kHz
N=1 amplitudePoloidal flux
Poloidal Field VectorsToroidal Field Colors
QuickTime™ and a decompressor
are needed to see this picture.
19
Increased EF and higher fill pressure can suppress the n=1 tilt
…but n=2 kink develops to terminate the discharge.
10 mT8.2 mT6.77 mT5.4 mT4.9 mT
time (s)
Pol
oid
al F
lux
(m
Wb
)B
R,
n=
1 (
T)
BR,
n=
2 (
T)
-q0
All Neon
20
SpheromakFRC Conversion Observed in Argon Plasmas
With Ohmic
21
Toroidal Field Decays As Poloidal Flux is Sustained
66536 & 66523
Pol
oida
l Flu
x (
mW
b)T
oroi
dal F
lux
(m
Wb)
Tay
lor
Eig
enva
lue
()
€
ψ R,Z( ) = 2π RBZdR∫
€
φ ψ( ) = BTInside Separatrix
∫∫ dA
€
"Poloidal Flux"=ψ null −ψ sep
€
∇×B = λB
⇓
λ ∝2πRBTψ
time (s)
22
“Conversion” To FRC is Robust in Argon
Transition Occurs:• in Argon and Krypton• over a wide range of
fill pressures and
Ohmic voltages. • never in He or Ne
Pol
oida
l Flu
x (
mW
b)
Tay
lor
Eig
enva
lue
Sol
enoi
d C
urre
nt (
A)
time (s)
23
Instability Suppression is Key to “transition”
• Tilt/shift instabilities can terminate plasma even before Ohmic is energized.
• Ohmic adds poloidal flux to the system, while toroidal flux decaysdrops q.
• In He and Ne, when q0<0.5, a terminal n=2 mode appears.
• Similar to previous results:– In S-1, non-uniform Te profile leads to a drop in q and nonuniform , with
n=2 mode restoring Taylor state.1
• In Ar and Kr, the kink is suppressed, the toroidal flux decays to zero, and an FRC equilibrium is formed.
1: Ono et al. Phys. Plasmas B 3, 1452 (1991); 2: Knox et al, PRL 56, 843 (1986).
24
Conclusions
• FRCs sustained for >300s using Ohmic current drive.– Evidence that an equilibrium suitable for NBI can be prepared with Ohmic.– Need larger Ohmic bank, additional EF coils to realize full potential.
• Argon utilized to stabilize both merging and sustainment phases.– Nearby passive stabilizers are essential for oblate FRCs.
– FLR stabilization by NBI will be necessary.
• Spheromaks in D2, He, and Ne show violent n=1 and n=2 instabilities with Ohmic.– Nearby passive stabilizers are essential…well known from S-1, CTX,…
• Argon Spheromaks can be driven to an FRC-equilibrium with Ohmic– Under these conditions, the FRC may be a preferred state.
…and Implications
25
• Extra stuff, for reference and for poster
26
Co-Helicity Merging Forms a Spheromak
• Initial spheromaks have the same polarity of toroidal field• Merging results in a new spheromak.
27
Sustainment Visible in Fast Camera Images
200 s 280 s 330 s 380 s 480 s
Argon Counter-Helicity Merging with Sustainment
28
Tilt Shows Field Opening
T. Hayashi, T. Sato, F. Wysocki, D.D. Meyerhofer, & M. Yamada, JPSF 54, 4172 (1985)
• Magnetics and images show clear tilt.• Field lines appear to open as in simulation by Hayashi.• Plasma cannot tilt beyond 90 degrees.
184 s 242 s 300 s 358 s 416 s
29
BZ Profile Used to Estimate Flux, Plasma Size, Taylor Eigenvalue
Separatrix Flux
Null Flux
N=0 Component from N-Probes
Inner Separatrix Radius
Outer Separatrix Radius
Null Radius
Solenoid Surface Flux
30
Lighter Gasses Demonstrate Rapid Instability
Shot 65788• Many toroidal mode numbers simultaneously unstable to co-interchange.• Illustrates the importance of nearby conducting structures.
Tra
pped
Flu
x(m
Wb
)
Helium Example
31
Increased Ohmic Voltage Leads to More Flux, Longer Sustainment
Ohmic Voltages From 5kV-9kVInput Powers: 300-600kW