Development Toward a Repetitive Compact Torus Injector (PDF ...

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Development Toward a Repetitive Compact Torus Injector T. Onchi, D. McColl, A. Rohollahi, C. Xiao, A. Hirose, M. Dreval, and S. Wolfe Abstract— A compact torus (CT) injector operated at high repetition rates has been developed. A system consisting of a stack of insulated-gate bipolar transistors and storage capacitor banks (slow banks) for repetitive operation has been installed on the circuit of both CT formation and acceleration. Following the installation of silicon-controlled rectifiers for the purpose of triggering ignitrons at CT discharge circuits, a repetition rate of 10 Hz was achieved on the University of Saskatchewan CT Injector. Index Terms— Insulated-gate bipolar transistors (IGBTs), plasma source, thyristors.

I. I NTRODUCTION

A

COMPACT torus (CT) is a high density plasmoid magnetically confined by its own magnetic field with both toroidal and poloidal components. CTs are formed and accelerated in a coaxial plasma gun with an external bias magnetic field. Due to its high injection velocity, a CT is capable of pushing through magnetic structure. Consequently, CT injection (CTI) [1]–[6] has been considered as one of the most promising methods for core fueling of large tokamak fusion reactors in the future. The characteristic advantages of CTI are as follows: 1) suitability for core fueling [5]; 2) controllability of CT penetration depth by varying the CT mass density and velocity [7]; 3) momentum transfer from CT to tokamak plasma as observed in tangential injection of CT along the toroidal direction [8]. First, CTI enables core fueling because of its robust magnetic structure and high-directed kinetic energy density. With such characteristics, CT is able to penetrate strong magnetic fields. Core fueling also evades high recycling of fuel gases, which gives CTI an advantage over gas puff or frozen pellet. This means the precious gas, tritium, can be efficiently deposited

Manuscript received January 7, 2015; revised May 20, 2015; accepted October 29, 2015. This work was supported in part by the Natural Sciences and Engineering Research Council of Canada, in part by the Sylvia Fedoruk Canadian Centre for Nuclear Innovation, and in part by the Canada Research Chair Program. T. Onchi is with the Research Institute for Applied Mechanics, Kyushu University, Fukuoka 816-8580, Japan (e-mail: [email protected]). D. McColl, A. Rohollahi, C. Xiao, and A. Hirose are with the Plasma Physics Laboratory, Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, SK S7N5E2, Canada (e-mail: [email protected]; [email protected]; [email protected]; [email protected]). M. Dreval is with the Institute of Plasma Physics, National Science Center Kharkov Institute of Physics and Technology, Kharkiv 61108, Ukraine (e-mail: [email protected]). S. Wolfe is with Plasmionique Inc., Varennes, QC J3X 1S2, Canada (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPS.2015.2499218

in a reactor core where most fusion reactions take place. Second, CTI is able to deposit fuels at a desired magnetic surface. The penetration depth of CT in plasma increases with the injection speed and the mass density. Such flexible fueling can control density profiles in a tokamak. Therefore, there exists a potential for the bootstrap current to be optimized by accurately controllable CTI operation. This is highly advantageous because a high fraction of the bootstrap current is required for efficient reactors of conventional tokamaks [9], [10] and spherical tokamaks [11]. Third, a tangentially injected CT can provide toroidal momentum into tokamak plasma. The toroidal plasma rotation in tokamaks plays as key roles in stabilizing resistive wall mode [12], [13], and CTI may provide an additional method to control plasma flow velocity. In a reactor with alpha particles, this capability to inject momentum can control instabilities and transport. If multidirectional injections with several small injectors are installed on reactors, it is possible that such a system can finely control plasma rotation velocities in both toroidal and poloidal directions. In order to fuel a tokamak reactor in a quasi-steady-state fashion, repetitive CTI (RCTI) is required. According to [14], if the operation powers of an RCTI and a neutral beam injection (NBI) are the same, the momentum injected by CTI is about ten times higher than that injected by NBI. RCTI as a core fueling method can also serve as an important external momentum source for large tokamak devices. The Compact Toroid Injection eXperiment group has accomplished stable repetitive operation in which CT formation is passively switched at 0.2 Hz by initiating breakdown with fast gas injection [15]. In the University of Saskatchewan CT Injector (USCTI) [16]–[20], RCTI was demonstrated using high-power and high-voltage (HV) power supplies [21]. The 8-kW power supply with a current of 600 mA is capable of charging capacitors quickly so that CTs are repetitively formed and accelerated with 3-s intervals (1/3 Hz). Recently, a novel circuit for RCTI has been developed [22]. The approach slowly involves charging a storage capacitor bank (slow bank) with a large capacitance and quickly charges the CT capacitor bank (fast bank) through a stack of insulated-gate bipolar transistors (IGBTs). After the fast bank is fully charged, the IGBT stack between banks is turned OFF and the CT formation sequence starts. This process is repeated by switching the IGBT stacks. For USCTI, the circuit with a slow bank and an IGBT stack is added to all CT banks including those for the solenoid, electromagnetic gas valves, formation bank, and ignitor triggering circuits. Using these changes, actively controlled repetitive CT formation was demonstrated. A repetition rate of 1.7 Hz was achieved [22].

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Fig. 1. Simplified circuit diagram for repetitive CT formation. Ignitron with formation fast bank is triggered by ignitor circuit. Fig. 2. Voltage at the fast bank and current flowing through the SCR ignitor in the circuit shown in Fig. 1.

In this research, the trigger circuit for CT discharge in USCTI was remodeled to achieve a higher repetition rate. The krytron switch used previously to trigger the ignitrons was replaced with silicon-controlled rectifiers (SCRs). Furthermore, a fast charging circuit for repetitive operation was developed and installed to the acceleration bank of USCTI. As a result of bench test of RCTI, CTs are repetitively formed and accelerated with the frequency up to 10 Hz. The remodeling of the ignitron trigger circuit and the development of circuits for CT formation and acceleration are presented in Section II. The control system of RCTI is also described. In Section III, the results of the bench test for RCTI using the formation and acceleration circuits are shown. The conclusion is given and future work is suggested in Section IV. II. D EVELOPMENT OF D ISCHARGE C IRCUITS A. Modification of the Ignitor Circuits Krytron was previously used to trigger ignitrons for the formation and acceleration banks of USCTI. Krytron is a switching device that uses compact gas to rapidly drive high current that is delivered from a charged capacitor bank charged to an HV of up to 5 kV. The devices have been used in both circuits for the formation and the acceleration in USCTI for 20 years. However, it was found that these devices had a limited repetition rate since the flowing current would not stop in under 500 ms after it was triggered. This limited the highest repetition rate of USCTI to 1.7 Hz in the previous Repetitive Compact Torus formation experiments [22]. To achieve a higher frequency than 1.7 Hz, replacement of krytron with a solid-state switching device was required. To this end, SCR was newly installed at the circuit. Fig. 1 shows a simplified circuit diagram of the CT formation discharge circuit. Charge in the formation fast bank CFF is discharged when the ignitor circuit triggers the ignitron. The current flowing through SCR from the ignitor fast bank, CIF = 1 μF, can drive the ignitor of the ignitron via an isolation transformer to initiate formation discharges. Three diodes in series are also used as a snubber circuit to protect the SCR from high resonance voltage. Voltage and current at the fast bank in the ignitor circuit are shown in Fig. 2.

Charged voltage at CIF was 3.75 kV. Current through SCR starts flowing at t = 0 and reaches ISCR = 600 A quickly within 1 μs. Then, the current increases relatively slowly and achieves a peak value of 800 A within 1 μs. The duration of the current pulse is roughly 5 μs and is found to stop after 20 μs, thus allowing a maximum repetition rate of up to 50 kHz. This exceeds, by several orders of magnitude, the expected fueling rate of about 100 Hz for fueling International Thermonuclear Experimental Reactor (ITER). 800 A is sufficient to drive the ignitron via the isolation transformer. The SCR ignitor circuits are used for both the formation and acceleration circuits of USCTI as shown in Fig. 1. The charging voltages for the ignitor slow banks are typically VISB = 4 kV. The charge in the slow banks is transferred to the fast banks via the IGBT stacks. The current flowing through SCR and the isolation transformer is generated when the optical signal triggers the gate driver to switch SCR ON. This cycle of charge and discharge is repeated to trigger ignitrons in a sequence of RCTI. B. CT Formation Bank The circuit for repetitive CT formation has been developed. As shown in Fig. 1, the storage capacitor bank and IGBT stack in series have been added to CT discharge circuit. The IGBT stack works as a switch between the fast and slow banks. Since the voltage (−25 kV at maximum) between the two banks is applied to the IGBT stack, the withstand voltage must be sufficiently high to allow a safety margin. The fast bank consists of four capacitors (CFF = 5 μF each), and each capacitor has its own ignitron and a RFF = 50 m resistor in series for short circuit protection. The charge stored in the slow bank is transferred to the CT formation fast bank through a 1.2-k resistor and 10-k resistors. When the voltages on the slow bank and the fast bank are nearly equalized and the charging current decreases to a small value, the IGBT stack is turned OFF. After the IGBT stack is turned OFF, CT formation discharge starts. In order to inject several CTs in the repetitive operation, the capacitance of the

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Fig. 3. Circuit diagram of the formation banks. Abbreviations are to show formation fast bank (FF), formation slow bank (FS), formation current limiter (FC), formation dummy load (FD), IGBT (IG), slow bank fault (SF), and slow bank (SB).

slow bank is significantly larger than that for the fast bank to maintain similar voltages for sequential CT discharges. The diagram and picture of the circuit for the formation circuit are shown in Fig. 3. The slow bank consists of 12 capacitors (CSB = 170 μF each) arranged into four parallel branches with three capacitors in series in each branch. The total capacitance is CFS = 226.7 μF. The maximum voltage at the slow bank is limited to −25 kV, which is charged by an 8-kW HV power supply (Lambda 802). A series of resistors with resistance of RSB = 10 M is used to equalize the voltages on each of the three capacitors in series. The resistors (four in total) RSF = 5  are attached to each of the series’ capacitors, serving as the protective components to limit the current in the event of a short circuit. Ten diodes (ST, STTH1512PI) rectify the current from the slow bank through IGBTs. The stack of ten IGBTs (IXYS, IXBH32N300, 3 kV, and 80 A) in series is used to switch a charge transfer from the slow bank to the fast bank. IGBTs are biased by a voltage divider consisting of the resistors RIG = 1 M. The resistors evenly share applied voltage between the two banks. Each resistor RIG is connected in parallel to each IGBT in order to establish a cascade trigger circuit with a single gate. The

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Fig. 4. Circuit diagram of the acceleration banks. Abbreviations are to show acceleration fast bank (AF), acceleration slow bank (AS), acceleration current limiter (AC), acceleration dummy load (AD), IGBT (IG), and slow bank fault on acceleration (SA).

stack is turned ON when the gate voltage is applied. The gate is turned ON by an optical signal from the sequencer and a gate driver. A fiber optic receiver (Avago, HFBR-2521) and an IGBT gate driver (IGD) (ON semiconductor, MC33153) are used in the driver circuit for the IGBT stack. This gate driver circuit powered by a battery is installed at the gate of the first IGBT of the stack on the slow bank side for a common voltage up to −30 kV. The withstand voltage of the IGBT stack is 30 kV. The resistors RFC0 and RFC1 limit a charging current lower than 10 A. Inductors L V and metal–oxide varistors are able to protect IGBTs from surge voltages. In the previous development phase, this charge transfer system for repetitive CT formation has been successfully operated with the ignitor circuit with krytron installed to the formation circuit as shown in Fig. 1. C. Charging System for the Acceleration Circuit A charging system similar to the formation bank was also developed for the acceleration bank. The circuit diagram and picture are presented in Fig. 4. Charging voltage at the acceleration bank is also limited to −25 kV. The circuit itself is floating and the common is connected to the elec-

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Fig. 5. Voltages at the fast bank (black line) and the slow bank (gray line) with 5 Hz.

trode of the formation fast bank, which could be a potentially floated HV when the formation discharge is initiated. The voltage from ground to the acceleration electrodes can reach up to −50 kV if the voltage on the formation capacitor bank is −25 kV. Therefore, the circuit components need to be able to high withstand voltages against the ground of the acceleration bank (−25 kV) and against the true ground of the USCTI system (up to −50 kV) in order to protect system. The slow bank consists of two capacitors (General Atomics, 25 kV) of 79.6 and 81.1 μF, respectively. They are connected in parallel, and the total capacitance is 160.7 μF. Forty diodes in series work as a rectifier between the slow bank and the IGBT stack. The IGBT stack for acceleration consists of 16 IGBTs in series. To dissipate heat effectively, each IGBT is mounted on a larger heat sink relative to that in the formation circuit. Bench test of the acceleration bank was accomplished independently. In actual CT operations, the common reference potential of the acceleration bank is the floating HV of the formation fast bank. In the bench test, however, the common is grounded to measure the capacitor bank voltages. Repetition frequency is about 5 Hz so that the duration for both the charging and the interval is 100 ms. The charging voltage on the slow bank is VAC = −20 kV. Fig. 5 shows the voltages at the fast and the slow bank. Before beginning a sequence at t = 0.0 s, the fast bank voltage is VAF ≈ −3.8 kV due to current through the resistors RIG in series. A sequence starts at t = 0.0 s, and the fast bank is charged up to VAF ≈ −16.0 kV in about 0.1 s. The slow bank voltage falls to VAS ≈ −18.0 kV. At t ≈ 0.1 s, the ignitrons are triggered and the current flows through a dummy load of RAD = 2.5  (four 10- resistors in parallel). The fast bank is discharged and the voltage dropped to zero. In the second cycle, VAF ≈ −14.0 kV and VAS ≈ −16.0 kV at t ≈ 0.3 s. In the third discharge, VAF ≈ −13.5 kV and VAS ≈ −15.0 kV at t ≈ 0.5 s. D. Charge and Discharge Control System for RCTI For an RCTI operation, it is necessary to control all IGBTs to charge the fast banks. The time scale of the charging phase is in the order of 100 ms. After the charge phase and an appropriate trigger delay, all fast banks are triggered in a preset sequence. This time scale of the discharge phase is on the order of 1 μs. A sequencer for RCTI requires a system to control two different time scales. For that reason, a repetitive operation sequencer (ROS) consisting of a timer IC (TS556, ST) and

Fig. 6. Flowchart of the RCTI operation. All gates of IGBTs are triggered by fiber optical signals through IGDs.

an optical transmitter (HFBR1521, Avago) was developed. The duration of the charge phase and the trigger delay can be set by manipulating the trimmers on TS556. The number of cycles can be set using a dip switch and binary ripple counter (HEF4040B, NXP). Fig. 6 shows a flowchart of the RCTI operation. In the charge phase drawn by thin lines, the optical signal is transmitted from ROS to a receiver. The transmitter (i) sends several identical signals to six circuits for CT formation, formation ignitor, acceleration, acceleration ignitor, bias magnetic field, and gas injection to activate or deactivate the IGBTs between the slow and fast banks. IGDs are added for reliable control of the IGBTs. The IGBT stacks (A)–(D) consist of a series of IXBH32N300, whereas (E) and (F) are single IGBTs of CM600HA-24H (Powerex). All IGBTs are simultaneously switched ON for the same period of time even though the necessary time to charge a particular fast bank is different. The discharge phase drawn by solid thick lines in Fig. 6, includes the time sequence generator (i) starts the time sequence generator (ii) and the optical transmitter (ii). This control system can also be used for a single shot operation. It triggers the bias field first, with gas injection triggered 1.83 ms later. For the next 200 μs, the CT formation fast bank is discharged. The acceleration fast bank is usually discharged 5 μs after the formation discharge. These charge and discharge phases are repeated in a sequence. There is a rest phase between the end of the discharge phase and the beginning of the next charging/discharging cycle. III. B ENCH T EST OF R EPETITIVE CTI O PERATION For a bench test of RCTI, all six systems (CT formation, CT acceleration, two ignitors, bias field, and gas injection) need to be synchronized. For the test shots presented below, the repetition rate was set to 10 Hz (charging/discharging time and the rest time are 50 ms). The number of CT pulses were set to three. The charging voltages at the slow banks were as follows: 1) Vfs = −14 kV for formation; 2) Vas = −12 kV for acceleration; 3) Vscrs = 4 kV for ignitor; 4) Vsols = 380 V for solenoid; and 5) Vgps = 900 V for gas valves. Fig. 7 shows the voltage measured at each fast bank. The voltage at

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Fig. 8. Discharge current at electrodes on CT (a) formation and (b) acceleration.

Fig. 7. Waveforms of voltages at each discharge circuit in the bench test of RCTI.

the fast bank for CT acceleration is not measurable using the data acquisition (DAQ) system due to difficulties in setting up DAQ on the floating HV of −14 kV. The first charging phase starts at time t = 0. Initial voltage at the formation fast bank is Vff ≈ −9 kV because the bank is charged through resistor RIG as mentioned earlier. For the same reason, other banks are also charged to different voltages by the time the sequence starts. The final voltages of the fast banks are not regulated to fixed voltages and thus voltages decrease as the charges on the slow banks decrease after each shot. The time taken to be fully charged in a sequence depends on the fast bank capacitance and charging resistor in each circuit system. At t = 0, a sequence starts and each fast bank is charged by a slow bank through each IGBT. Formation fast bank rises up to VFF ≈ −13.00 kV in 50 ms. The slow bank drops from VFS = −13.92 to −13.76 kV. This cycle caused only a 160 V decrease in the slow bank because the initial voltage at the fast bank is already as high as VFF = −9.12 kV. Around t = 50 ms, the CT discharge sequence is triggered, and the voltage falls to VFF ≈ 0 V. In the second phase, VFF rises to −10.7 kV while VFS falls to −13.0 kV. In the third cycle, VFF rises to −10.1 kV, and VFS falls to −12.5 kV. VFF in the first pulse is clearly higher than in the second and the third. This is also due to the initial voltage. To use more energy from the slow bank, faster charging is needed. It is easy to realize a faster charge transfer between the fast and the slow bank by reducing the charging resistance RFC . Although the voltages at the banks for acceleration have not been recorded, it is expected that the waveforms of the banks resemble the case of the formation bank since similar circuits are used. In the second panel, the voltage at the SCR ignitor for the formation is shown. The initial voltage is VSCRF = 1.6 kV. At t ≈ 50 ms, the voltage reaches VSCRF = 3.72 kV. It falls to zero right after CT discharge, but it increases immediately. The inductive current through the three diodes likely causes this fast charging from the snubber circuits. The fast bank capacitance is CIFF = 1 μF. The solenoid bank is charged from

the initial voltage VSOLF = 80 V and reaches VSOLF = 330 V. The solenoid bank voltages in the second and the third cycle keep VSOLF = 320 and 310 V, respectively. The charging voltage on the fast solenoid bank drops only slightly even after some cycles because the capacitance of the slow bank of the solenoid is CSOLS = 108 mF, which is much higher than the solenoid fast bank capacitance (CSOLF = 5 mF). The fast bank for gas injection is charged from the initial voltage Vgpf = 80 V and reaches Vgpf = 830 V. At the second and third cycles, Vgpf = 805 and 780 V, respectively. This significant voltage drop occurs because the slow bank capacitance Cgps = 10 mF is not much higher than that of the fast bank (Cgpf = 1.3 mF). The waveforms, shown in Fig. 7, of the fast solenoid and gas puff banks are taken from two different CT sequences and the timing does not align perfectly. It was confirmed that the voltages for those two banks are high enough to ensure proper CT formation and acceleration for all three CTs in a sequence. Without proper bias field or gas injection in a cycle, it is hard to generate multiple CTs in a sequence. When CTs are generated in an RCTI sequence, the discharge currents of the formation and the acceleration bank oscillate through an equivalent RLC load circuit. When CT is not generated, the discharge current flows through the dummy load. The current drops monotonically through an equivalent RC circuit. Therefore, the oscillation of the discharge current is an indication of CT formation. The waveforms of discharge currents through the formation and acceleration electrodes are shown in Fig. 8. During the first pulse, the discharge current through the formation electrodes reaches ICTF ≈ −100 kA, whereas charging voltage at the fast bank is VFF = −13.0 kV. This formed CT is accelerated by the discharge current through the acceleration electrodes. The current reaching ICTA > −50 kA oscillates as shown in Fig. 8(b). The second and third discharge currents of the formation and the acceleration are almost identical (ICTF ≈ −60 kA and ICTA = −35 kA). The small current oscillation (ICTF ≈ −15 kA) that can be seen at t ≈ 55 μs in Fig. 8(a) is an indication of multiple CT formation during a single discharge. Such oscillating waveforms are not observed in Fig. 8(b) for the acceleration discharge, indicating that the second CT may have quickly vanished before reaching the acceleration section. The frequencies of the oscillations in the

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formation and acceleration discharges are fCTF ≈ 83.3 kHz and f CTA ≈ 62.5 kHz, respectively. The amplitude decays, corresponding to a damped RLC discharge. IV. D ISCUSSION AND C ONCLUSION In this research, the system toward RCTI has been developed. Using SCR with snubber diodes instead of krytron in the ignitor circuits, repetition frequency of 10 Hz higher than the previous record was achieved even though only three CTs were shot. The difference between using SCR and krytron is that the SCR can stop the current through it in under 50 ms. Charge in slow bank is transferred to fast bank via IGBTs. This fast charging system is applied in circuits for the CT formation and acceleration. IGBTs in series between the fast and slow banks have been designed to withstand high voltages of the formation and acceleration banks. Current through IGBTs is less than 10 A in this system for CT formation and acceleration. Three pulses of CTs are generated by repetitive operation of all IGBT systems (A)–(F). To obtain the same quality of CT at each discharge, fast bank voltages at all discharge circuits should be constant. The final voltage can be set by controlling pulse width given from the gate driver. Voltage monitor can be easily installed at the CT fast bank even though the voltage divider must be able to withstand a high voltage. Measured voltage is compared with a setting voltage, and the signal from IGD is stopped when a measured voltage reaches a setting voltage that is sufficiently lower than at the slow bank. A comparator IC circuit can be added for this purpose. For more repetitions than three CTs, higher capacitance at the CT slow banks will be also needed. Such voltage control systems are required in all discharge circuits used in the fast and slow banks. All CTs in a repetitive sequence can be of a similar quality if gas loading of the vacuum system, due to accumulative gas puffing, can be eliminated by increasing the pumping speed. As the first step, the comparator circuit was tested in the gas puffing system with voltage in the order of 100 V. The constant voltage on the fast gas puffing bank for all CT discharges was achieved using a comparator circuit to turn the IGBT gate ON and OFF through feedback control. A similar circuit was also implemented for the solenoid bank. Further development is on the way to turn the IGBT stacks ON and OFF for the formation and acceleration banks. There are some limitations to achieving a higher repetition rate in the present system. In the circuits for CT formation and acceleration, each total resistance of the charging resistor is RFC = 3.7 k. If charging resistance is lower, charging speed can be higher. However, current limit and heat capacity of the IGBT stacks need to be carefully evaluated before such fast charging systems are implemented. Voltage drop at the gas injection fast bank, as shown in Fig. 7, influences the quality of CT significantly. Stable gas feeding is a key point to control the quality of multiple CTs. The gas feeder consists of four fast solenoid valves; therefore, a high current discharge circuit switched by an SCR is necessary to open the valve. Multiple CTI in a tokamak discharge is the key to steady-state operation of a practical reactor. With a

repetition rate higher than 100 Hz, multiple CTI in an Saskatchewan TORus-Modified (STOR-M) tokamak will be feasible since the discharge duration of the tokamak is 30– 40 ms. Therefore, two to three CTs can be injected in the current flat top phase of the STOR-M discharge. Incidentally, the required repetition rate for fuelling ITER is also about 100 Hz. CT formation and acceleration at a frequency of 100 Hz will be attempted on USCTI in the future. R EFERENCES [1] L. J. Perkins, S. K. Ho, and J. H. Hammer, “Deep penetration fuelling of reactor-grade tokamak plasmas with accelerated compact toroids,” Nucl. Fusion, vol. 28, no. 8, p. 1365, 1988. [2] P. B. Parks, “Refueling tokamaks by injection of compact toroids,” Phys. Rev. Lett., vol. 61, p. 1364, Sep. 1988. [3] J. H. Hammer, C. W. Hartman, J. L. Eddleman, and H. S. McLean, “Experimental demonstration of acceleration and focusing of magnetically confined plasma rings,” Phys. Rev. Lett., vol. 61, pp. 2843–2846, Dec. 1988. [4] J. H. Degnan et al., “Compact toroid formation, compression, and acceleration,” Phys. Fluids B, vol. 5, no. 8, p. 2938, 1993. [5] R. Raman et al., “Experimental demonstration of nondisruptive, central fueling of a tokamak by compact toroid injection,” Phys. Rev. Lett., vol. 73, pp. 3101–3104, Dec. 1994. [6] R. Raman and P. Gierszewski, “Compact toroid fuelling for ITER,” Fusion Eng. Design, vols. 39–40, pp. 977–985, Sep. 1998. [7] W. Liu, S. C. Hsu, and H. Li, “Ideal magnetohydrodynamic simulations of low beta compact toroid injection into a hot strongly magnetized plasma,” Nucl. Fusion, vol. 49, no. 9, p. 095008, 2009. [8] T. Onchi et al., “Effects of compact torus injection on toroidal flow in the STOR-M tokamak,” Plasma Phys. Controlled Fusion, vol. 55, no. 3, p. 035003, 2013. [9] R. J. Bickerton, J. W. Connor, and J. B. Taylor, “Diffusion driven plasma currents and bootstrap tokamak,” Nature Phys. Sci., vol. 229, pp. 110–112, Jan. 1971. [10] M. Kikuchi, “Steady state tokamak reactor based on the bootstrap current,” Nucl. Fusion, vol. 30, no. 2, p. 265, 1990. [11] R. Raman and V. F. Shevchenko, “Solenoid-free plasma start-up in spherical tokamaks,” Plasma Phys. Controlled Fusion, vol. 56, no. 10, p. 103001, 2014. [12] A. Bondeson and D. J. Ward, “Stabilization of external modes in tokamaks by resistive walls and plasma rotation,” Phys. Rev. Lett., vol. 72, p. 2709, Apr. 1994. [13] E. J. Strait et al., “Wall stabilization of high beta tokamak discharges in DIII-D,” Phys. Rev. Lett., vol. 74, p. 2483, Mar. 1995. [14] R. Raman, “Advanced fuelling system for ITER,” Fusion Eng. Design, vol. 83, nos. 10–12, pp. 1368–1374, 2008. [15] H. S. McLean et al., “Design and operation of a passively switched repetitive compact toroid plasma accelerator,” Fusion Sci. Technol., vol. 33, no. 3, pp. 252–272, 1998. [16] S. Sen, C. Xiao, A. Hirose, and R. A. Cairns, “Role of parallel flow in the improved mode on the STOR-M tokamak,” Phys. Rev. Lett., vol. 88, p. 185001, Apr. 2002. [17] C. Xiao, A. Hirose, and S. Sen, “Improved confinement induced by tangential injection of compact torus into the Saskatchewan torusmodified (STOR-M) tokamak,” Phys. Plasmas, vol. 11, no. 8, p. 4041, 2004. [18] A. Hirose, C. Xiao, O. Mitarai, J. Morelli, and H. M. Skarsgard, “STOR-M tokamak design and instrumentation,” Phys. Canada, vol. 62, no. 2, pp. 111–120, 2006. [19] D. Liu, C. Xiao, A. K. Singh, and A. Hirose, “Bench test and preliminary results of vertical compact torus injection experiments on the STOR-M tokamak,” Nucl. Fusion, vol. 46, no. 1, pp. 104–109, 2006. [20] T. Onchi et al., “Fuelling effect of tangential compact toroid injection in STOR-M tokamak,” in Proc. Can. Nucl. Soc. (CNS), 2012, p. 137. [21] A. Pant, C. Xiao, and A. Hirose, “Repetitive operation of the University of Saskatchewan compact torus injector,” Radiat. Effects Defects Solids, Incorporating Plasma Sci. Plasma Technol., vol. 165, no. 2, pp. 96–105, 2010. [22] T. Onchi et al., “Design and implementation of fast charging circuit for repetitive compact torus injector,” Fusion Eng. Design, vol. 89, no. 11, pp. 2559–2565, 2014.

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