Blumleingenerator

The Blumleingenerator , in relation to lasers and more rarely also as the Blümleingenerator and in different ways also known as the inversion circuit or LC inversion circuit , is a type of pulse generator for the generation of short electrical pulses with high instantaneous power .

Areas of application are, among other things, in the area of pulse technology for generating square pulses in order to be able to determine step responses by measurement. Further applications are the spatial and temporal compression of electromagnetic energy in fusion experiments . One of the world's largest Blumlein generators in this area of ​​application is the Shiva Star at Los Alamos National Laboratory . Smaller assemblies of Blumlein generators serve as pulse generators for pulsed lasers like the nitrogen laser .

Layout and function

Basic structure of a Blumlein generator

The Blumleingenerator consists of two electrical line sections of the length with a line wave resistance which are connected to the load resistance with a resistance value of . This is open at the right end of the line. In the case of the two electrical line sections, the capacitance per unit length , which is described within the framework of line theory, plays the dominant role. Depending on the specific embodiment, the conductor arrangement can also be supplemented by additional capacitors designed as concentrated components. ${\displaystyle D}$ ${\displaystyle Z_{0}}$ ${\displaystyle Z_{L}=2\cdot Z_{0}}$

To start the generator, the length of the line is first charged to a direct voltage . A few 10 to 100 kilovolts are typical . This process takes a few powers of ten longer than the discharge process; with larger Blumlein generators, charging can take a few seconds. There is no voltage at the load resistor during and after the charging process . ${\displaystyle 2\cdot D}$ ${\displaystyle U_{0}}$ ${\displaystyle Z_{L}}$

The actual pulse is initiated by an electrical switch , usually a spark gap , at the beginning of the line. It includes the line shortly , whereupon the short circuit as a traveling wave (voltage jump spreads) direction load resistance. There is a jump in the value of the wave resistance at the load resistance with the resistance value. This results in a reflection factor of and transmission factor of , which is why the reflected portion of the transducer wave runs back to the source with the amplitude . The continuous wave has the amplitude of and runs to the end of the line. From this moment on, the voltage is applied to the load impedance . The two traveling waves are reflected at the line ends, the traveling wave at the short-circuited switch is reflected with a change in sign of the amplitude, the continuous traveling wave is reflected back at the open line end without a change in sign, which means that they cancel each other out when they arrive at the load resistance. The voltage is present there until the two wave fronts arrive at the load resistor . The pulse duration is determined by the length of the lines and the transit time along the line, which depends on its dielectric . ${\displaystyle U_{0}\to 0}$ ${\displaystyle Z_{L}=2\cdot Z_{0}}$ ${\displaystyle Z_{L}+Z_{0}=3\cdot Z_{0}}$ ${\displaystyle 1/2}$ ${\displaystyle 3/2}$ ${\displaystyle -U_{0}/2}$ ${\displaystyle U_{0}/2}$ ${\displaystyle U_{0}}$ ${\displaystyle U_{0}}$

In practical constructions, the lines are often designed as plates; water sometimes serves as the insulating medium between the plates , because in addition to the high dielectric constant of, it also has a very high impulse withstand voltage to achieve the highest possible capacitance. ${\displaystyle \varepsilon _{r}=81}$

Application of gas laser

Construction of a self-made nitrogen laser in cross section (schematic diagram)

The Blümlein generator is used, for example, to excite pulsed gas lasers; a popular example are transversely excited TEA nitrogen lasers (TEA from Transversal Excited and Atmospheric pressure ), which use air at atmospheric pressure as the laser gas. Due to the comparatively high pressure and the "impurities" (mainly oxygen), the life of the excited states is reduced to a few nanoseconds. An electrical discharge used to pump the gas must therefore also take place within this period of time.

With a simple structure, the Blümleingenerator allows the generation of the necessary short pulses if the components and their parameters match each other (e.g. wave resistance, length, width, location of the spark gap, low-inductance coupling of the spark gap).

The supply voltage of some 10 kV (battery symbol) applied via R in the picture above charges the two plates (strip lines) 2 and 3 formed with the base 1; the charging current flows unhindered through the coil, i. H. the influence of the inductance of the coil is negligible here and the plates are at practically the same voltage during the charging process. This charging process continues until the breakdown voltage of the spark gap (black balls) is reached. The conductivity of the spark gap increases abruptly and it forms a short circuit. A wave propagates in the direction of the slot forming the laser cavity below . The inductance of the coil is so great that it prevents the wave from advancing; the laser discharge ignites (blue oval in the picture between the knife edges). The parallel voltage difference in the laser channel is ideally doubled (see above). The gas discharge that begins there, also known as secondary discharge, pumps the laser medium and creates a laser pulse that reaches several kilowatts even with small self-made arrangements. The pump discharge leads to charge equalization. It has a very low impedance and therefore the striplines must have very low line impedances - they are very wide and have a high capacitance per unit length . ${\displaystyle L}$ ${\displaystyle L}$ ${\displaystyle L}$

The plates are then charged again by the high-voltage source and the process starts again at the interval of the time constant determined by the capacity and charging resistance .

Origin of name

The origin of the name is not clearly established. In an article from 1974 in Scientific American , the author describes the pulse generator for a nitrogen laser with the designation English Blumlein line , which is said to correspond in the arrangement to that of a pulse generator with pulse shaping stage by the British electrical engineer Alan Blumlein . This name is controversial in relation to pulse generators in lasers.

Individual evidence

1. ↑ ^{a b c} Andreas Küchler: High voltage technology: Basics - Technology - Applications . 3. Edition. Springer, 2009, ISBN 978-3-540-78413-5 , pp. 135, 136 .
2. ^ 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 .
3. ↑ Light and Its Uses . In: Scientific American . 1974, ISBN 0-7167-1185-0 , Nitrogen Laser, p. 40-43 .
4. ↑ Your DiY Nitrogen Laser is NOT a Blumlein! ( Memento of the original from March 29, 2017 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.