|Solid state modulators (SSM)
PHPMS envisaged basic research and development of the solid-state modulator technology to meet future challenges of proton linear accelerator technology. Solid-state modulators are to replace traditional modulators which are based on vacuum tube and PFN. The major switching elements in solid state modulators are usually high power capacity semiconductor switches like IGBTs, IGCTs and Pulse Power Thyristors (PPT) etc. These new modulators promise to be far more compact, efficient and reliable. Development of modulators based on the new state of the art SSM topologies like HV Converter modulator (HVCM), Single HV switch modulator and Marx modulator have been started. The SSM have several advantages over the traditional counterpart such as high average power capability, variable pulse width operation and very low stored energy for crowbar less operation.
Single HV Switch Modulators:
The basic scheme used in this Modulator is Hard switched type Modulator. Since the output characteristics of this scheme has drooping behaviour so a bouncer compensating scheme has been adopted. The schematic of bouncer compensated all solid state klystron modulator is shown in fig.(1). As shown in the schematic, the droop compensation is obtained by means of a series resonant LC circuit made of bouncer inductor and capacitor. The Main Capacitor bank is charged to ~10% higher voltage and the bouncer capacitor CB is charged to ~10% of the required output voltage. During the generation of output pulse, the bouncer circuit is triggered ON to start its cycle before the start of the main pulse of the modulator and the pulse transformer sees the difference of main capacitor and bouncer voltage at its primary. As the two voltages fall at the same rate due to proper designing and setting, the output voltage is constant during the pulse with < 1% droop. To start and stop the output pulse a high voltage solid state switch using IGBT modules in series has been developed. This switch is controlled by Optical fibre signals. A crowbar network using SCR based solid state switch is also connected across the Main capacitor bank to protect the Klystron tube during arcing.
The use of Capacitor switching with droop compensation overcomes the difficulties as normally encountered with PFN topologies and also reduces the size of energy storage device. The low loss LC bouncer circuit is chosen for better efficiency over other droop compensation techniques. The use of solid state devices for main and crowbar switching has made the design simple and also improved the reliability and lifetime.
PHOTOGRAPHS OF MODULATOR AND COMPONENTS
A solid state Marx modulator is used to generate high voltage pulse with relatively low voltage power supply. The block diagram of the Marx Modulator is shown below. It consists of DC power supply, Marx cells, control and trigger circuitry.
In this type of modulator, a stack of capacitors are charged in parallel with relatively low voltage power supply and then discharged in series to generate a high voltage pulse. In this modulator, to reduce oversizing factor of capacitors, 10-20% droop is allowed in pulse and then this droop is compensated by a droop compensation circuit. Marx modulator has several advantages over other conventional type of modulator. In this modulator, no oil is required if the peak output voltage has sufficient clearance to ground. Pulse width and rise/fall time can be adjusted from the low voltage electronic circuitry. The modular nature of the Marx bank simplifies diagnosis and component replacement, reducing mean repair time. It can operate around failed cells, significantly reducing the risk of single point failure. Prototype of lower voltage version (25 kV, ~1 ms pulse width) of the modulator is under development.
High Voltage Converter Modulator:
The block diagram of the HVCM is shown below. High voltage converter type modulators consists a regulated input power supply, IGBT based high frequency (HF) DC-AC converter modules operating at 25 kHz switching frequency , HF high voltage diode rectifier, controller and filter.
The converter modules are phase shifted and phase shifting results into the higher ripple frequency at the output. The higher ripple frequency reduces the requirement of filter component values, it not only increases the bandwidth but also reduces the stored charge. This results into faster rise and fall time response output pulse and crowbar less operation of the modulator. The parallel loaded (LsCp) resonant switching topology provides good voltage gain, thus specially suited for high voltage output application. Resonant switching ensures soft switching of IGBTs in the DC-AC converter, thus provides very high efficiency. The width of the output pulse is controlled by modulating the gate drives signals of the converter modules. Prototype of lower voltage version (33 kV, ~1 ms pulse width) of the modulator is under development.