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n Abstract
C-2 and C-2U experiments [1] have used magnetized coaxial plasma guns (MCPG) to inject compact toroids (CTs)
for refueling the long-lived advanced beam-driven field-reversed configuration (FRC) plasma [2]. This refueling method will also be used for the C-2W experiment. To minimize momentum transfer from the CT to the FRC two CTs are injected radially, diametrically opposed and coincident in time. To improve understanding of the CT characteristics TAE has a dedicated test bed for the development of CT injectors (CTI), where plasmoid merging experiments are performed. The test bed has two CTIs on axis with both axial and transverse magnetic fields. The ~1kG magnetic fields, intended to approximate the magnetic field strength and injection angle on C-2W, allow studies of cross-field transport and merging. Both CTIs are capable of injecting multiple CTs at up to 1kHz. The resulting merged CT lives >100μs with a radius of ~25 cm. More detailed results of CT parameters will be presented.
[1] M. Binderbauer et al., Physics of Plasmas, 22, 056110 (2015) [2] T. Matsumoto et al., Rev. Sci. Instrum. 87, 053512 (2016)
n Overview of Plasmoid Merging System using Compact Toroid (CT) Injectors
1TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610 2Nihon University, Chiyoda-ku, Tokyo 101-8308, Japan
n Measurements on Glass Tube
Ø Magnetic Probe and Flux Loop Measurement
Machine axis (cm)
-40 -20 0 20 40
40
50
60
Collided
n CT Colliding/Merging in “Axial” Magnetic Field
CT injector 1
Drift tube
Ø PMT Array at Mid-Plane Axial field direction 1.00
• •
It shows agreement between the calculated excluded-flux radius and contour plot of the optical emission, measured by the PMT array.
The CTs merged at ~42μs and expanded radially, while the
Inner Electrode
CT formation process
1. Neutralgasisinjectedbetweenelectrodesviagas
injection ports, while Bias field is applied.
2. Plasmaisgeneratedwhenthemaingunpowersupply breaks down the gas in the electrode gap.
3. Theradialcurrentbetweenelectrodesgeneratesa toroidal magnetic field which accelerates the plasma by Self-Lorentz force.
4. Theacceleratedplasmaisinterlinkedwithappliedbias field, which provides poloidal flux. Thus the spheromak- like plasmoid is formed and ejected from CTI.
Collided CT
Fig. Schematic view of Camera Location
Fig. Line out from each frame
Outer Electrode Bias Coil
25.0 20.0 15.0 10.0
5.0 40
rotated along arrow
n CT Injection in “Transverse” Magnetic Field
Ø Fast Framing Camera Pictures Frame rate: 500 kHz, exposure: .5 μs
125 μF,
-10kV GasInjectionPort
RΔΦ
70 80
t(μs)
n CT Parameters
t (μs)
t (μs)
Fig. Time evolution of gun current and PMT TOF signals.
Ne
Te
• The radial scan is taken over multiple shots, one at each radial position.
• There is good agreement between measurements.
• After merging FRC-like plasmoid remains.
Beginning of
PMT Fan Array
Glass tube
50 cm
CT injector 2
PMT1, Magnetic Probes 0.75 Transverse field direction PMT2, Triple Langmuir Probe 0.50
• •
Magnetic field measurements show CT motion along the machine axis.
The peak excluded-flux radius RΔφ is ~20 cm, where the wall radius is 30cm.
Axial Field Coil
Studies on Plasmoid Merging using Compact Toroid Injectors
I. Allfrey1, T. Matsumoto1, T. Roche1, H. Gota1, T. Edo2, T. Asai2, D. Sheftman1, D. Osin1, R. Smith1, S. Krause1, F. Tanaka2, A. Hosozawa2 and the TAE Team
Transverse Field Coil 0.25 Fig. Schematic overview of the experimental setup.
Baseline of CT trajectory
36μs
64μs
Axial field coil
Transverse field coil
Main discharge
• Charge Voltage: 8-12kV
• Current: 120-150 kA
• Rise time: ~10 μs
PMT time of flight (TOF)
Fig.TimeevolutionofneandTe onaxis.
As the CT crosses the transverse magnetic field it deflects off axis due to charge particles crossing the magnetic field.
These images are of the second pulse of a multi-pulse discharge, fired at t = 2ms. The difference in emission spectrum may be caused by the remaining neutral gas from the first injection cooling the CT. Additionally, the CT may excite the neutral gas, leading to an increase in Dα emission.
In the case of the transverse field the CT’s do not merge to form a single CT, but bounce off each other.
• •
Mounted in drift tube, 9.5 cm separation
Typical velocity ~100 km/s
v⊥ E
After collision t = 65μs
Before collision t = 49μs
r (cm)
• Lower density on axis of the CT indicates FRC-like plasmoid profile.
n Conclusion and Future Work
We successfully created an FRC-like plasma by merging two CTs, with lifetimes of ~30 μs, Te of 40 eV
Diagnostic
Rogowskicoil
B probe array (qty: 5)
PMTarray(qty:2) Triple Langmuir probe
Fast framing camera PMT array (qty: 16)
Parameters
Guncurrent B-field fluctuations
Velocity(Timeofflight)
T and density e
Global motion of CT Tomography
Value
2-5×1021 (m-3) 30 - 40 (eV) 0.4 (mWb) 50 - 150 (km/s)
0.4 - 0.8 (kJ) 200 (kJ/m3)
t(μs)
Fig. Time evolution of the PMT signals.
0 Diagnostic Suite 40
45
50
55
60
65 70
50
60
90
100
Typical Parameters
ne Te Φ
U
2.00 1.50 1.00 0.50
0
RDF • •
kin peak
rΔΦ/√2
6 8
rΔΦ 1 0
and ne of 2x1019 m-3.
Plan to measure the distribution of magnetic field inside collided/merged CTs. Add mirror field for improved confinement.
Presented at the 59th annual Meeting of the APS DPP
• •
Merged CT
Estimate of the Excluded-Flux Radius
Excluded-Flux Radius
Bz3 is near machine center line
t (μs)
Fig. Time evolution of the excluded flux radius
from B probes and flux loops. z
25 20 15 10 5
Radial expansion
49.95μs 53.3μs
Plot of the values of yellow line for each frame
• The green trace is the FWHM of each frame.
• The blue trace is the center of the FWHM.
• From emission the lifetime of the plasma is estimated to be ~60 μs, where the FRC lifetime is ~30 μs.
s
Bv p
Intensity (A.U.) Current (kA)
Density ( 1019 m-3)
Intensity (A.U.)
Temperature ( eV)
Te (eV)
Electron density (m-3)
ne (m-3)
r (cm)
FWHM (A.U.)
RΔφ (cm)
t (μs)
1 B
B , B : Magnetic probe signals
R = rw vp
of vacuum and plasma shot at the glass tube
Φv, Φp: Magnetic flux signals of vacuum and plasma shot inside glass tube
p

v
Fig. Contour plot of ND filtered radiation and RDF Ø Electron Temperature and Density Measurements
40μs 44μs 48μs
58μs
optical emission lasted ~30μs.
collision FRC lifetime: ~30μs
t(μs) •
68μs 72μs 76μs 80μs
0
Fig. Radial scan of ne and Te at the mid-plane.
2
4
1 2
1 4
4 0
3 0
2 0
1 0
0 •
Fig. Contour plot of ne scan over the radius at the mid-plane compared with the excluded-flux radius.
• •
30
Ø Fast Framing Camera Pictures
Frame Rate: 300 kHz, exposure: 1.1 μs
70 0
Fig. Contour plot of the time evolution of excluded-flux radius.
40μs
43.3μs
High Speed Camera:
Ultra Cam HS-106
(NAC Image Technology, Inc.)
46.6μs
t (μs)