1. INTRODUCTION
Air core transformer is an attractive alternative to the Marx generator for charging the high voltage pulse forming transmission lines, such as those used with high power electron or ion beam accelerators. In general, transformer systems are more compact than Marx generators because the low-voltage, primary capacitor bank is inherently a more dense assembly, and is ordinarily not operated in a tank of insulating oil. Hence, the high voltage pulse transformer type accelerator has been widely investigated (Robwein, 1979; Blather & Ruinously, 1983) especially, the Tesla transformer type accelerator (Boscolo et al., 1997; David et al., 1966), which is a typical example. A high voltage air core spiral strip transformer was designed, constructed, and is now working in our laboratory. The transformer has an output of 500 kV on the secondary coil when the input voltage is 40 kV. It has been employed to charge the water Blumlein line (as the secondary capacitor). The electron beam was produced by discharge of the secondary capacitor on a field-emission diode. At the present time, the electron current is about 40 kA, the voltage of diode is more than 400 kV, and the pulse duration is more than 80 ns. The accelerator is compact, inexpensive and works stably and reliably.
2. DESCRIPTION OF THE SYSTEM
The construction of the accelerator is illustrated in Figure 1. It consists of the primary storage energy system of capacitor bank, the high power air core transformer, the water Blumlein line, and the field-emission diode. The water which is resistivity of more than 10 MΩ-cm, is used as the dielectric of Blumlein line. A capacitor of about 10 kJ, 40 kV is used to provide primary pulse energy for the system of accelerator. A spark-gap switch with a trigger electrode is installed between the capacitor bank and the connector of the primary coil of transformer. When the capacitor bank is charged to achieve the required voltage, the spark-gap switch is closed by supply trigger pulse voltage. The capacitor bank discharges to provide about 100 kA for the primary coil of the pulse transformer.
The diagram of air-core transformer type accelerator.
The Blumlein line which is a 10 Ω water line shown in Figure 1, is not only a storage capacitor but also a pulse form line with a pulse duration of about 80 ns. It contains an inner conductor, a middle conductor, an outer conductor, and a spark gap switch. The inner conductor is connected to an over-electrical-field breakdown gas switch, whose function is to constrain previous voltage pulse and to maintain the excellent performance of the field-emission diode. All of the components of conductors are made of stainless steel, which unlike aluminum and steel, is hard to be oxidized in water, and is less likely to suffer from surface damage in the event of a breakdown. (1) capacitor bank; (2) trigger switch; (3) are-core transformer; (4) connector; (5) divider; (6) gas tube; (7) spark gap switch; (8) Blumlein line; (9) resistance divider; (10) gas switch; (11) field-emission diode; (12) measurement current device.
The vacuum diode is a field-emission diode, which include a cathode and an anode. The cathode is a stainless steel rod with diameter of 20 mm, and the anode is a stainless steel disk with diameter of 75 mm. In order to measure the high current of the diode, a coaxial shunt for the current measurement was installed in anode seat (Liu & Tan, 1997).
As shown in Figure 2, an air core spiral strip transformer (Liu et al., 2003) with coupling coefficient of 0.78 is employed to charge the water Blumlein line. The transformer has a primary winding of 2.5-turns surrounding a 45-turns secondary winding. The width of both coils is 20 cm in which the thickness of the copper strip is 0.2 mm, and five layers of polyester film with thickness of 0.1 mm could provide electric insulation between the turns of the coils. In addition, the width of the polyester insulation is 30 cm, which leaves a 5 cm margin on each side of the copper winding. The polyester film margins are enclosed with two disks of insulation material that are placed between a concentric ring cage that is built into the core and fiberglass shell of the transformer. The function of the ring cages is to maintain the electric field distribution around the margin regions of copper strip, which make the electric field distribution as uniform as possible. In the case the equi-potential lines outside the winding are therefore prevented from bending sharply around the edges of the thin winding conductor, which is helpful for eliminating the local electric breakdown of the system. For structural reasons, the external shell of the transformer was made of reinforced fiberglass tube with a diameter of 50 cm and integral flanges. The flanges could support the acrylic end plates of the transformer and provide a rigid connection to external Blumlein line. All the components of transformer are inside the reinforced fiberglass tube, and after fully assembled, the transformer was sealed tightly so that the interior volume level could be maintained, and there is not liquid that could leak out from the transformer. The whole transformer was vacuumized and then filled slowly with oil. The transformer was placed for a period of approximately two weeks before experimental tests began. The parameters of transformer were initially calculated and later confirmed by measurements, the inductances of primary inductance, and secondary windings are 4.5 μH and 482 μH, respectively, and coupling coefficient about 0.78.
The configuration of air-core transformer.
3. SIMULATION AND EXPERIMENTAL RESULTS
Pspice Circuit analysis was used to model the spiral strip transformer type accelerator. Figure 3 shows schematically the circuit model.
Pspice Circuit Schematic used to Modal the accelerator.
As shown in Figure 3, the spiral strip transformer is modeled and indicated by Xform2 in the circuit with primary winding inductance of 4.5 μH and secondary winding inductance of 482 μH, for a coupling coefficient of about 0.78. C12 is the primary energy-storage capacitance, and R34 and L16 are the resistance and inductance of primary winding circuit. The two gap switches in the accelerator are modeled and indicated by Gap-main and GAP6 in the circuit, respectively. T-mid-inner and T-mid-outer are connected in parallel to simulate the Blumlein line. The Diode is modeled and indicated by vacuum diode load in the circuit. The whole circuit is connected with components built in Pspice component storage or modeled by the authors according to the performance of the device.
Figure 4 shows the Pspice simulation results of diode voltage, diode current, and output voltage of the transformer. As can be seen from Figures 4a, 4b, and 4c, the peak output voltage of transformer is about 445 kV, and the peak voltage of the diode is about 610 kV, and the peak current of the diode is 37 kA.
Simulation result of voltage and current signals. (a) Simulation result of charging voltage of the Blumlein line; (b) Simulation result of Diode voltage; (c) Simulation result of Diode current.
We also tested the accelerator experimentally; the voltage wave form of the secondary winding was measured with the aid of water resistive divider. As shown in Figure 5, the voltage of the Blumlein line rises up to 450 kV when the primary winding circuit was charged at 40 kV, and the rise time is about 4 μs. The experimental results are in good agreement with the simulation result. The experimental results of the voltage and current of the diode are shown in Figure 6. The voltage of field-emission diode was measured by water resistive divider, and the current of field-emission diode was measured by a 2.6 mΩ coaxial shunt resistor which is assembled in the anode seat shown in Figure 1. The peak voltage and peak current of diode are 420 kV and 40 kA, respectively, from the experiment, and the pulse duration is more than 80 ns which can be observed from the Figure 6. According to experimental results, we know that the impedance of diode is about 10 Ω which agrees well with the designed value.
The waveform of charging voltage of the Blumlein line.
The waveform of field-emission diode. Upper: Voltage waveform. Lower: current waveform.
4. CONCLUSIONS
The transformer system was operated in an off-resonance single swing changing mode. For the experiments, the capacitor bank was directly coupled to the transformer primary winding with a minimum inductance configuration, and the transformer output could feed to the 12 nF load capacitor which is a Blumlein line of water dielectric with resistivity of more than 10 MΩ-cm. The charging transfer time of Blumlein line was about 4 μs as shown by the wave form in Figure 5. In the field-emission diode, the relativistic electron beam of 450 kV voltage and 40 kA current with pulse duration exceeding 80 ns has been measured. The energy source of primary winding, which is a capacitor, could be replaced by the explosive-drive flux compression devices (Chen et al., 2000) which could provide a pulse at the primary winding of the transformer with current of about 80 kA and voltage of 50 kV. We could obtain the same experimental results at the field-emission diode using the explosive-drive flux compression device. In the experiment, in order to guarantee the water Blumlein line works at negative pulse voltage, the gas pressure of main spark gap switch is very significant. If the gas pressure of the switch does not maintain an appropriate value, the charging system will bring positive pulse voltage to the water Blumlein line and result in breakdown in the water.
ACKNOWLEDGMENTS
The author wishes to thank B.L. Qian, H.Y. Yang, and C.H. Liu for their encouragement and valuable suggestion. The assistance of J.H. Feng, Y.Z. Liu, Q.M. Tan, and L.Y. Xu in the assembly and testing of transformer is also gratefully acknowledged.
