Pulse generator (power engineering)

A pulse generator is an electrotechnical device which initially stores a certain amount of electrical energy over a longer period of time and can then release this amount of energy in a very short time ("suddenly").

Due to the short time of the energy release, time spans of a few nanoseconds to a few microseconds are common, very high instantaneous powers occur. With large pulse generators, peak outputs of up to one terawatt are possible. Electrical voltages of up to a few 100 kilovolts and currents of up to a few mega amperes are characteristic.

Pulse generator Shiva Star at the LANL

Technical applications of pulse generators include the supply of transmitter tubes such as the klystron or the magnetron for pulsed radar devices , for pulsed lasers ( gas lasers , pump laser diodes or flash tubes for solid-state lasers), for X-ray tubes and for industrial production methods such as magnet forming , electromagnetic pulse welding and the Hydrospark process. Military applications are, for example, the supply of a vircator for electronic warfare .

Scientific applications are, for example, nuclear fusion and the generation of strong magnetic fields.

Smaller pulse generators are used as test generators in electromagnetic tests for electromagnetic compatibility and in high-voltage laboratories, for example for tests on lightning protection .

Depending on the design and principle, pulse generators can sometimes only emit a single pulse, such as the flow compression generator , because they are destroyed when the pulse is generated .

construction

The structure of pulse generators is very different depending on the magnitude of the pulse duration and the peak power. Most devices have in common that they use concentrated components such as capacitors and / or coils or the capacitive or inductive coating of an electrical line . The modeling of the processes takes place within the framework of the line theory . There are also devices with a mechanical energy source, e.g. B. the flow compression generator and the so-called compulsator .

Another characteristic of pulse generators are fast, powerful electrical switches. These can be, for example, mechanical switches , thyristors , thyratrons , MOSFETs , electron tubes or, in particular, switching spark gaps . A typical example of the latter is the Marx generator .

A pulse shaping network (PFN) made up of capacitors and coils or a circuit is often used to shorten the pulse duration or to increase the output.

Pulse generator from line circles

The line theory provides the description of the electrical processes . For smaller powers, pulse generators can also be implemented with just a piece of coaxial cable , but striplines are typical .

Simple pulse shaper stage with one line

The simplified functional principle of a pulse shaper stage in the form of an electrical line with length D is shown in the adjacent sketch. When the switch is open, the line is initially charged to the constant DC voltage of the DC voltage source over a longer period of time via a DC voltage source with an internal resistance R S significantly greater than the line impedance Z 0 . At the time of discharge, the switch is closed and the energy stored in the line is delivered to the load resistor R L , which is matched to the line impedance in terms of impedance . The entire energy of the line is not immediately converted; the limitation of the speed of propagation results in an effect of wave propagation along the line: The direct voltage at the load resistor R L breaks to half of the direct voltage source, this follows from the fact that the Load resistance is adapted to the line impedance . This voltage drop, which can be described as a wave, is propagated at about half the speed of light in the direction of the feed point; the specific speed depends, among other things, on the design of the line and the shortening factor VKF , is reflected at the end and then runs until the line to the load resistor R L . The duration T of the pulse at the load resistor R L depends on the transit time of the wave along the line and is:

${\ displaystyle T = {\ frac {2D} {c_ {0}}} \ cdot VKF}$

with the vacuum speed of light and the shortening factor VKF . ${\ displaystyle c_ {0}}$

Principle structure of a Blumlein pulse generator

The disadvantage of this design of a pulse generator is that only half the voltage of the DC voltage source is available at the load resistor R L. The Blumlein generator , named after Alan Blumlein and as shown in simplified form in the sketch opposite, avoids this disadvantage . In the case of the Blümlein generator, the load resistance R L is in the middle of the line, the total length of the line is twice as long as that of a simple pulse generator. The load resistance R L must have twice the impedance of the line, so it is not matched to the line. As a result of this deliberate misalignment, the incoming wave, triggered by a short circuit in the DC voltage source, causes a reflection and transmission of the wave with half the amplitude. This leads to a positive and negative voltage of the same magnitude at the connections of the load resistor for the duration of the pulse. This means that full voltage is applied to the load resistance during the pulse. The disadvantage of the Blumlein pulse generator is that it is twice as expensive due to the additional cable length.

One of the world's largest pulse generators based on the Blumlein principle is the Shiva Star at Los Alamos National Laboratory . Among other things, it serves as a source of impetus for fusion experiments and military research. The energy stored in the capacitors is 10  MJ , the voltage during discharge reaches over 100  kV at the load resistor with impulse currents of around 10  MA . The power reaches around 1 TW in a few microseconds.

Other designs

Examples of pulse generators:

• With the Marx generator, capacitors are charged in parallel on a DC voltage source and suddenly connected in series through spark gaps .
• in the case of the ignition coil , the energy is stored in its magnetic field and (traditionally) released with a mechanical switch (interrupter)
• in the fence or at some high voltage ignition test generators and a capacitor is in a transformer discharged
• High-voltage pulses for operating Pockels cells are generated from a DC voltage source by series-connected MOSFETs
• Pulses for magnet transformation, the Hydrospark process and in the Dresden high-field magnet laboratory are generated by discharging high-voltage capacitors (10 ... 40 kV)

Individual evidence

1. W. König: "Manufacturing process: sheet metal working"; Springer-Verlag 2013 - 270 pages; Page 53
2. Klaus Wille: Lecture Electronics, Chapter 4: Lines. Archived from the original on January 24, 2014 ; accessed on March 5, 2015 .
3. patent GB589127 : Improvements in or relating to apparatus for generating electrical pulse. Registered October 10, 1941 , published June 12, 1947 , Applicant: Alan Dower Blumlein.
4. ^ Fritz Herlach, Noboru Miura: High Magnetic Fields, Science and Technology. Theory and Experiments II . tape 3 . World Scientific, 2006, ISBN 978-981-277-488-0 , pp. 243 .