Experimental and numerical analyses of electro-pulse rock-breaking drilling

In recent years, semiconductor switches have been widely used in pulse generators. In order to generate bipolar or rectangular pulses, switches in these generators operate in different time sequences. However, the dv/dt induces false triggering of the semiconductor switches that leads to local short circuits. Breakdown of the load will also produce a high short-circuit current which will affect the switches' reliability and lifetime. In this paper, a new gate drive circuit is proposed to protect switches from short-circuit currents. A 52-stage solid state Marx generator using the proposed drive circuit was tested to verify the overcurrent protection. The experimental results show that the short-circuit current is limited within a safe range and the output voltage increased significantly.

A novel drive circuit with overcurrent protection for solid state pulse generators

The linear accelerators (linacs) producing high energy and high power of electron-beam or X-ray beam have been used in medicine, industry, national security, etc. In the linac, the electrons are generated by the electron gun and accelerated in the accelerating column with the high-power RF fields. The high-voltage pulses from the pulse modulator are supplied to the RF power source and the electron gun. The pulse modulator is one of the big and expensive components in the linac. The commercial medical linacs commonly use the pulse modulator based on the thyratron-switched pulse-forming network. In order to improve the power efficiency, achieve the system compactness, and optimize the cost and space, the solid-state pulse modulator based on the Marx generator was proposed. The low-power solid-state pulse modulator was developed for the electron gun operation. The conceptual design and functional results were confirmed. In order to apply it to the RF power source, such as a magnetron or a klystron, the 6-MW pulse modulator with the same Marx scheme is proposed. It consists of 40 storageswitch stages and one high-voltage pulse transformer, producing the pulse of 50 kV and 120 A required by the magnetron in the medical linac. A storage-switch stage was designed for insulated gate bipolar transistors to switch high current of 280 A and 720 V and to use the capacitor of 25 μF which was chosen for the voltage droop of 10% with the pulsewidth of 5 μs. The prototype system with eight storage-switch stages was fabricated and tested with a load system. The performance results show that it can be extended to be the 6-MW solid-state pulse modulator. In this paper, we describe the design features, and discuss the results and also the future plan to optimize the solid-state pulse modulator in the medical linac.

Design of a 6-MW Solid-State Pulse Modulator Using Marx Generator for the Medical Linac

This article describes a modification and an experiment conducted on a solid-state pulsed power modulator (SSPPM) for medical linear particle accelerator (LINAC) applications. Depending on the operating condition of the magnetron, an existing design of an insulated-gate bipolar transistor (IGBT)-based modulator is modified, from a positive pulse generator to a negative pulse generator. The design of the gate driver circuit of the pulse discharging IGBTs is also slightly changed to secure reliability against the arc condition. The developed modulator has the following maximum output—pulse voltage of −40 kV, pulse current of −100 A, pulsewidth of 5 μ s, and pulse repetition rate of 250 Hz. By adapting the proposed design to the pulsed power modulator for driving the magnetron, high peak and average power densities of 43 kW/L and 48.9 W/L, respectively, can be achieved. The experimental results from various tests conducted using a resistor load, including a rated operation and an arc protection, verify the robustness and utility of the developed SSPPM for medical LINAC applications. An experiment using a 9.3-GHz 1.7-MW X-band magnetron is also conducted. The developed modulator achieved a successful arc protection operation for the arc generated in the waveguide during the magnetron drive test.

Solid-State Pulsed Power Modulator for 9.3 GHz 1.7 MW X-Band Magnetron

In recent years, semiconductor switches have gradually replaced traditional gas switches due to their long lifetime and smaller volume. However, the rated current of the semiconductor switches limits the applications of the all-solid-state pulsed power generators. Usually, multiple semiconductor switches or pulse generators are required in parallel to share a large current in the case of the enormous power applications. In this article, a new high-current all-solid-state pulsed power generator based on Marx generators connected in parallel was proposed. One group of semiconductor switches was used to control the charging of the Marx generators. The driving circuits can be simplified due to fewer switches. It can also improve the discharge frequency and the output voltage over a capacitive load. Six 4-stage Marx generators connected in parallel have been employed to verify the proposed topologies on a capacitive and a resistive load. From the experimental results, with 1.2 kV, 150 A, and 2.4 kV, 50 A, pulses have been got on an 8- $\Omega $ resistive load and a 440-pF capacitive load, respectively. The balanced output currents can reduce the output voltage overshoot and make the system more stable and reliable.

A High-Current All-Solid-State Pulse Generator Based on Marx Structure

A medical linac is used for the cancer treatment and consists of an accelerating column, waveguide components, a magnetron, an electron-gun, a pulse modulator, and an irradiation system. The pulse modulator based on hydrogen thyratron-switched pulse-forming network is commonly used in linac. As the improvement of the high power semiconductors in switching speed, voltage rating, and current rating, an insulated gate bipolar transistor has become the more popular device used for pulsed power systems. We propose a solid-state pulse modulator to generator high voltage by multi-stacked storage-switch stages based on the Marx generator. The advantage of our modulator comes from the use of two semiconductors to control charging and discharging of the storage capacitor at each stage and it allows to generate the pulse with various amplitudes, widths, and shapes. In addition, a gate driver for two semiconductors is designed to reduce the control channels and to protect the circuits. It is developed for providing the pulsed power to a medical linac electron-gun that requires 25 kV and 1 A as the first application. In order to improve the power efficiency and achieve the compactness modulator, a capacitor charging power supply, a Marx pulse generator, and an electron-gun heater isolated transformer are constructed and integrated. This technology is also being developed to extend the high power pulsed system with > 1 MW and also other applications such as a plasma immersed ion implantation and a micro pulse electrostatic precipitator which especially require variable pulse shape and high repetition rate > 1 kHz. The paper describes the design features and the construction of this solid-state pulse modulator. Also shown are the performance results into the linac electron-gun.

https://iopscience.iop.org/article/10.1088/1748-0221/11/04/P04003/pdf

Sung Chae Ro

Papers

Design of a 6-MW Solid-State Pulse Modulator Using Marx Generator for the Medical Linac

Solid-state pulse modulator using Marx generator for a medical linac electron-gun