Switching Properties of Liquid Nitrogen Cooled IGBTs and 24 kA Demonstration of Current Multiplier by Inductive Storage

We had been developing a current multiplier by inductive storage (CMIS). The CMIS consists of 24 storage copper coils, which soaked into the liquid nitrogen, demonstrates a 24 kA of output current and the continuous current pulses of 3 pulses per second. Switching performance of the IGBTs and diode were tested in the liquid nitrogen bath. These experimental data were used to design the mega-ampere class CMIS. The system consists of the superconductive magnet section with a temperature of 20 K and the IGBT control switch section with a temperature of 77 K.


Introduction
The critical problem in a development of the inductive pulsed-power supply is in the opening switch to commutate a large current. This opening switch is required to commutate the current of several megaamperes for the application to an electro-magnetic launcher. To reduce the duty of switch, the parallel operations of the inductive storages had been done [1,2]. However, the current magnitude for each opening switch was still large because of the parallel operations of only several inductive storages.
For above solution, we have been developing the inductive pulsed power supply consisting of many coils. This inductive pulsed-power supply is excited in the series connection of each coil and discharged in the parallel connection to obtain the pulsed large electric current. The preliminary experiment showed that this process was correct [3][4][5].
In this paper, the preliminary experiments of CMIS with 24 inductors have been done. These inductors can be cooled down by liquid nitrogen (L-N 2 ) to decrease the resistance of the coils and busbar. Switching performance of the insulated-gate bipolar transistor (IGBT) and diode were tested in the liquid nitrogen bath. The conceptual design of mega-ampere CMIS and long pulse CMIS were also describes in this paper. Figure 1 illustrates the electrical circuit of the SC CMIS. Start-up and termination of the output current are formed by the on and off control of the series IGBTs and parallel IGBTs. Energizing of the SC magnet starts by turning on all series IGBTs. When the parallel IGBTs P 1 turns on and series IGBTs   S 1 turns off coil current I 0 is commutated to the Load. When P 2 is on and S 2 is off, current of No.1 coil and current of No.2 to No. n are commutated to the Load. In case of P 1 to P n are off and S 1 to S n are on, the all of module coil current flow into the load, and load current of nI 0 can be obtained. On the other hand, S 1 to S n turn on and P 1 to P n turn off, all module coil currents are commutated to the power supply.

Experimental apparatus 24 kA CMIS
The schematic of experimental apparatus is shown in Fig. 2. It has 24 inductors which form a toroidal coil. The inductance of the toroidal coil in parallel connection is 55 H, and stored energy is about 25 kJ. An L-N 2 cooler was used to cool down the inductors until the temperature of L-N 2 . The L-N 2 cooler cools only the toroidal coil, and the lead wire connecting to inductors cannot be cooled in whole. Thus the 24 lead wires were led out the cooler, and connected to IGBTs on the bench for the control devices. For the power supplies to energize the inductors, a 16 volts dc power supply with electric double layer capacitor is used.

Characteristics of the CMIS
As shown in Fig. 3, an output current of 24 kA was obtained, when the toroidal coil of serial connection was charged up to 1 kA. Decay time constant of the output current was about 70 msec. Equivalent resistance of the output circuit including switching device and coils was about 790 . Continuous current pulse test was conducted with a time interval of 0.3 sec. Each pulse width was adjusted to 50 msec. Typical waveforms of continuous pulses are CMIS is shown in Fig. 4. Pulse heights of output current decreased about 10 % respectively. This was the cause of the large impedance of the electric double layer capacitor which was used in the power supply for charging the serial coils.   Figure 5. Test-bed for testing IGBTs.

Experimental set-up
The merit of using superconducting (SC) coil and SC bus-bar are to realize, 1) low energy consumption for long pulse operation, and 2) high current density and high magnetic field for downsizing the system. To decrease the number of the current leads and to suppress the heat load into the cryostat, the switching device should also be soaked into L-N 2 . On the basis of this idea, they were tested in the L-N 2 . Test circuit is shown in Fig. 5, and the parameters of test sample are listed in Table1. These samples were used in a prototype CMIS [4,5].

Test results
The result of the IGBT cooled by L-N 2 is shown in Fig. 6. When temperature goes down to 77 K, Voltage drop of the IGBT increases. Voltage drop at 300 K and 77 K with zero current were 0.5 V and 1.2 V, respectively. Voltage waveform was non-linear in comparison with the current waveform, but maximum voltage drop did not exceed 2.6 V. When Joule heat occurs at the junction plane, temperature its will go up from 77 K. Increase of the temperature may leads to decrease of the voltage drop, even if the rated current is 100 A.
The result of diode test cooled by L-N 2 is shown in Fig. 6. Voltage drop at 300 K and 77 K with zero current were 0.5 V and 1.1 V, respectively. Non-linearity between voltage drop and current were smaller than that of IGBT. The maximum voltage drop was kept to around 1.5 V.      Figure 8 shows a schematic drawing of the SC CMIS [5]. It consists of the SC module coils and the IGBT control switch. An SC coil is made of multi-filamentary BISCCO tape. The 360 module coils generate the current pulse of 1 MA. Since a decay time constant of the load current is 8.56 s, the pulse duration that the load current decayed to 80 % is 1.7 ms. To increase the pulse duration, repetition of current regulation is applied at the current decay phase. An energy stored coil consists of 72 module coils, and turn number of each coil is 100. Thirty-six modules were used for the current rise, and other 32 modules were used to compensate the current decay. In this way, duration of current flat-top increased to more than 0.1 s. Parameters of MA CMIS and long-pulse CMIS are listed in Table 2.