P. 1

A repetitively driven compact toroid (CT) injector has been developed for large-sized field-reversed configuration (FRC) facility of the C-2/C-2U primarily for refueling. A CT is formed and injected by a magnetized coaxial plasma gun (MCPG) exclusively developed for the C-2/C-2U FRC. To refuel the particles of long-lived FRCs, multiple CT injection is required. Thus, a multi-stage discharge circuit has been developed for multi-pulsed CT injection. Drive frequency of this system can be adjusted up to 1 kHz and the number of CT shots per injector is 2; the system can be further upgraded for larger number of injection pulses. The developed MCPG has achieved supersonic ejection velocity in the range of ~100 km/s. Key plasma parameters of electron density, electron temperature and the number of particles are ~ 5 × 1021 m-3, ~ 30 eV, and 0.5 - 1.0 × 1019, respectively. In this project, single and double pulsed CT injection fueling have been conducted on the C-2/C-2U facility by two CT injectors. The CT injectors are mounted 1 m apart on the vicinity of midplane. To avoid disruptive perturbation on the FRC, the CT injectors have been operated at the lower limit of particle inventory. The experiments demonstrated successful fueling with significant density build-up of 20 – 30 % of the FRC particle inventory per single CT injection without any deleterious effects on the C-2/C-2U FRC.
Multiple injection of CTs
➤To refuel the particles of long-lived C-2U FRCs, multiple CT injections are required.
➤Thus, we have developed the multi-stage discharge circuit for multi-pulsed CT injection [4].
➤Drive frequency of this system can be adjusted up to 1 kHz and the number of CT shots per injector is 2; the system can be further upgraded for larger number of injection pulses.
Refueling of FRC
Time evolutions of plasma radius and line-integrated electron density for (a) shot #44446 with injection at 2.0 ms (single CT) and for (b) shot #48492 with three CTs injected at 0.5 (single) and 3.0 ms (double).
CT injector
Schematic view of the developed CT injector
➤It consists of a set of coaxial cylindrical electrodes, a bias coil and four gas injection ports which are arranged tangentially on the outer electrode.
➤The inner electrode is coated by tungsten to reduce impurity influx.
➤On the test stand, the CTs pass through the drift tube and then penetrate into transverse magnetic field.
➤The penetration depth and velocity are measured by the time-of-flight method from the magnetic signals.
➤The dashed lines tracing initial peaks of magnetic field waveform. Penetration speed is approximately 100km/s.
CT trajectory
Formation section
Upper Bottom
z = 21.1 z = 31.1 z = 41.1 z = 51.1 z = 61.1
Fast camera images of single side injection.
Penetrated CT collided with FRC.
Then hot neutral gas started rotating around the FRC.
Fast camera images of counter CT injection.
The injected CTs rotate ion diamagnetic direction.
➤In both cases with single and double CT injection, the confinement magnetic field is shaken by CT injection.
➤Global shift (Toroidal mode number n =1) motion triggers by one side injection.
➤On the other hand, the motion is suppressed by “counter” CT injection.
Time evolution of the Dα emission on the plane by the CT injector. Black line is FRC without CT injection, red is the case with CT injection and green one shows the case with PI.
30 40 50 60 70
r ~ r
s ΔΦ
200100 0 20 Bx (G)
~100 km/s
25 30 35 40 45 time (μs)
PPT like plasma injector has been equipped for pre-ionization.
have been developed. The injection velocity evaluated on the test stand keeps the range of 100km/s in a transverse magnetic field even after passing through the 1m long drift tube. This is high enough to penetrate external magnetic field of C-2U FRC. CTs injected perpendicularly to the geometrical axis of the C-2U demonstrated successful fueling with significant density build-up of 20 - 30% of total particle inventory per single CT injection without any serious deleterious effects on the C-2/C-2U FRC. Dα emission indicates possible pollution by trailing gas. However, it can be reduced drastically by PI technique on the MCPG. An effective technique of particle fueling is a common development issue in any magnetically confined fusion reactor. This work would provide an effective fueling technique for magnetically confined plasmas.
Gas puff valve Bias solenoid
Conducting shell
Etot Upeak
5 × 1021 (m -3) 30 - 40 ( eV ) 0.4 ( mWb )
100 (km/s) 0.4 - 0.8 (kJ) >20 (kJ/m 3 )
Time evolution of line integrated electron density by CO2 interferometer.
4.0 3.5
3.0 2.5 2.0 1.5 1.0 0.5
0 0.5
w/ w/o
The typical separatrix position of C-2U FRC is about 40 cm from the entrance end as depicted with pink line.
C-2/C-2U FRC
➤The CT injection experiments have been conducted on
the C-2U device at Tri
Alpha Energy (TAE).
CT Injector 2
Injection Axis
➤On the C-2U device, an advanced beam-driven field-reversed configuration (FRC) with a lifetime longer than 10 ms [3, 6].
➤The CT injectors are mounted 1 m apart in the vicinity of midplane. CT injectors are oriented at an angle with respect to z-axis and both CT injector are installed 180 degrees apart, slightly off-axis, and angled such that the injected CTs’ trajectories intersect at the center of the confinement vessel.
➤The “CT injector 1” has multi-pulse system that can inject two CTs with repetition rate up to 1 kHz. Three CTs can be injected in the configuration lifeime of the C-2U FRC.
CT Injector 1
Arrangement of installed CT injectors on
z = 71.1 cm 50 55
Particles ( 1019)
1.0–1.5 0.5– 1.0
Temperature Poloidal flux [eV] [mWb]
600–800 5–7 (total)
20– 30 0.4 (electron)
Energy [kJ] 5–7
0.1– 0.3
Effect on confinement magnetic field
0 180
45 225
90 270
135 315
the C-2U confinement vessel.
10 -1
CT injection on the test stand with (bottom) and without (top) transverse field.
➡ Trailing neutral gas can be reduced.
In this project, a CT injector, which fuels 0.5 – 1.0 × 1019 of particles with 1kHz of repetition frequency
Compact Toroid Injection Fueling on a Large-sized Field-Reversed Configuration
T. Asai1, T. Matsumoto1, T. Roche2, I. Allfrey2, H. Gota2, J. Sekiguchi1, T. Edo1, E. Garate2, Ts. Takahashi1, M. Binderbauer2, and T. Tajima2,3
➤A plasma ring is generated within a gap between the electrodes and is accelerated by Lorenz self-force.
➤During this acceleration process, toroidal current is induced by a poloidal flux interlinked with the plasma ring.
2.0 2.5
1) College of Science and Technology, Nihon University, Tokyo 101-8308, Japan
2) Tri Alpha Energy, Inc., California 92688, USA
3) Department of Physics and Astronomy, University of California, Irvine, California 92697, USA
Charging circuit
-10 kV
Main bank
125 μF
125 μF
CT innjector
Schematic diagram of multi-pulse discharge circuit, which includes charging circuit, main bank, snubber circuit, and CT injector.
Behaviour of CTs in the FRC
➤To avoid disruptive perturbation on the FRC, the CT injectors have been operated at the lower limit of particle inventory.
➤Approximately 30% of increment in line-integrated density was observed by single CT injection at t = 2.0 ms.
➤This is approximately 60% of particle inventory of injected CTs [5].
➤Multi-pulsed injection with frequency of 0.5kHz has also been performed with two injectors. It also demonstrated successful fueling.
➤Injected CTs have spheromak-like magnetic configuration and they temporarily tear flux surfaces when entering the FRC. Also the temperature of the injected CT is about 10% of the target plasma. This leads to some fast particle loss and cooling of plasma.
➤However, any serious effect on the FRC has not been observed.
➡Line-integrated density is increased about 10 – 20% ➡Particle inventory is increased about 15 – 20% by 2
➡CT injection increases Dα emission. It is potentially caused by the particle loss from FRC core and/or neutral trailing gas flowing from the CT injector.
FRC (#48500), FRC+CT (#48468), FRC+CT+PI(#49755)
FRC w/o CTI: 50 kHz, 20 μs
NC plane FRC
Snubber circuit
ΔBz [Gauss]
ΔBx (G)
z (cm)
Intensity (a.u.)
Inventory [×1019]
Composite pictures showing the trajectory of injected CT taken by a fast framing camera (ULTRA Cam HS-106E / nac Image Technology).
CTI w/ confinement field: 500 kHz,1.9 μs
Time [ms]
Time evolution of total particle inventory of C-2U
Pre-Ionization system
➤To reduce the adverse effect of CT injection into the target C-2U FRC, PI technique has been developed.
➤This suggests that the trailing neutral gas is successfully reduced by the PI on the MCPG.
Time [μs]
Time evolutions of the signals of the azimuthal Bz probes. The CTs are injected from
single (left) and both facing side(s).
[1] T. Matsumoto et al., “Development of a magnetized coaxial plasma for compact toroid injection into the C-2 field-reversed configuration device” , Rev. Sci. Instrum. 87, (2016) 053512.
[2] M. Binderbauer et al., “A high performance field-reversed configuration” , Physics of Plasmas 22, (2015) 056110.
[3] M. Binderbauer et al., “Recent breakthroughs on C-2U: Norman’ s legacy” , AIP Conf. Proc. 1721, (2016) 030003.
[4] I. Allfrey et al., “Development of Multi-pulse Compact Toroid Injector System for C-2U” , Bull. Am. Phys. Soc. 60, BP12.00024 (2015).
[5] T. Roche et al., “Compact toroid injection into C-2U” , Bull. Am. Phys. Soc. 60, BP12.00023 (2015).
[6]H. Gota et al., “Achievement of Field-Reversed Configuration Plasma Sustainment via 10 MW Neutral-Beam Injection on the C-2U Device” , EX/P3-41 in this conference (2016).
[7] T. Matsumoto et al., “Characterization of compact-toroid injection during formation, translation, and field penetration” , Rev. Sci. Instrum. 87, (2016) 11D406.
Time [μs]
~3.1 x 1015 cm-2
Semi-rigid coax cable
1.5 1.0 0.5
0 0
1.0 2.0 3.0 4.0 5.0 time (ms)