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Quasi-cw and Modulated Beams
Published in Chunlei Guo, Subhash Chandra Singh, Handbook of Laser Technology and Applications, 2021
The traditional way of producing high-power high-voltage current pulses was to use a pulse-forming network (PFN), consisting of capacitors and inductors in a network configuration. As the flashlamp impedance is not constant, perfect matching between the PFN and the flashlamp is only possible at one operating voltage. It is very difficult to obtain efficient operation for pulse lengths of more than 10 ms. PFNs are very limited in their range of pulse duration adjustment. With the emergence of high-power solid-state switching devices such as gate-turn-off thyristors, insulated-gate bipolar transistors and power MOSFETs, PFNs have almost been replaced by such high-power solid-state switching devices. There are a number of advantages in utilizing a transistorized power supply with feedback control of the flashlamp current in an Nd:YAG laser for material processing. These include the ability to tailor the pulse shape and modify the pulse parameter (height, width and rate) on a pulse-to-pulse basis. The current is controlled within a feedback control loop, which minimizes the output variation as a result of changes in the supply-line voltage or flashlamp impedance [2]. Various pulses, such as flat-top rectangular, ramp-up or ramp-down ones, with pulse durations in the range 0.1–20 ms are widely used for laser welding. Figure 17.1 shows a schematic diagram of a power supply with a lamp discharge current controller.
Applying Vacuum Tube Devices
Published in Jerry C. Whitaker, Power Vacuum Tubes, 2017
A radar system RF amplifier usually centers on one of two microwave devices: a crossed-field tube or linear-beam tube. Both are capable of high peak output power at microwave frequencies. To obtain high efficiency from a pulsed radar transmitter, it is necessary to cut off the current in the output tube between pulses. The modulator performs this function. Some RF tubes include control electrodes or grids to achieve the same result. Three common types of modulators are used in radar equipment: Line-type modulator (Figure 5.27). This common radar modulator is used most often to pulse a magnetron. Between pulses, a charge is stored in a pulse-forming network (PFN). A trigger signal fires a thyratron tube, short-circuiting the input to the PFN, which causes a voltage pulse to appear at the primary of transformer T1. The PFN components are chosen to produce a rectangular pulse at the magnetron cathode, with the proper voltage and current to excite the magnetron to oscillation. An advantage of this design is its simplicity. A drawback is the inability to electronically change the width of the transmitted pulse. Active-switch modulator (Figure 5.28). This system permits pulse width variation, within the limitations of the energy stored in the high-voltage power supply. A switch tube controls the generation of RF by completing the circuit path from the output tube to the power supply, or by causing stored energy to be dumped to the output device. The figure shows the basic design of an active-switch modulator and three variations on the scheme. The circuits differ in the method of coupling power supply energy to the output tube (capacitor-coupled, transformer-coupled, or a combination of the two methods).Magnetic modulator (Figure 5.29). This design is the simplest of the three modulators discussed. No thyratron or switching device is used. Operation of the modulator is based on the saturation characteristics of inductors L1, L2, and L3. A long-duration low-amplitude pulse is applied to L1, which charges C1. As C1 approaches its fully charged state, L2 saturates and the energy in C1 is transferred in a resonant fashion to C2. This process continues to the next stage (L3 and C3). The transfer time is set by selection of the components to be about one-tenth that of the previous stage. At the end of the chain, a short-duration high-amplitude pulse is generated, exciting the RF output tube.
Design of the Toroidal Field Coil System for the TPM-1U Tokamak
Published in Fusion Science and Technology, 2019
D. Hernández-Arriaga, D. M. Ventura-Ovalle, M. Nieto-Pérez
In most small tokamaks, the current required to generate the toroidal magnetic field is achieved by the simple discharge of a capacitor bank.12 Although simple, this strategy does not make optimal use of the energy storage infrastructure. Therefore, a different approach for current pulse shaping is suggested here: the use of a pulse-forming network (PFN). PFNs based on passive elements, named line-type PFNs, were key in the early days of radio, television, and radar13 since they provided a robust method for generating high-current and high-voltage pulses. The generic diagram of a PFN is shown in Fig. 3, which shows a network of impedances in series parallel. Each of the branches on this array can be used to simulate a term in a Fourier series.
Comparison of an Electrothermal Plasma Source to a Light Gas Gun for Launching Large Cryogenic Pellets for Tokamak Disruption Mitigation
Published in Fusion Science and Technology, 2018
T. E. Gebhart, S. K. Combs, L. R. Baylor
Pellets are loaded into place by manually pushing them through the exit of the barrel with a flexible rod. The pellets fit tightly within the barrel to minimize the amount of gas that blows by the pellet. Once a pellet is loaded, the chamber is closed and pumped down to ~10 mTorr. The discharge circuit can produce a variety of pulse lengths based on the installed pulse forming network (PFN). This work examined cases with an ~1-ms pulse length and an ~150-µs pulse length. The 1-ms pulse is created with a PFN that consists of a 100-µH inductor and 1-Ω resistor. The 150-µs pulse is the nominal pulse width of the system with no pulse shaping. Along with varying the electrical characteristics of the system, various liner materials were also tested. The main outputs from a pellet shot are source current, source voltage, high-speed imaging of the pellet leaving the barrel, and the signal received from a shock accelerometer attached to a flange downstream of the barrel exit. These diagnostic outputs provide data to calculate input power to the source and the speed of the pellet. The distance between the end of the barrel and the downstream flange was 0.27 m.