Pulsed Electrical Power
100 Megawatt Levels
STACKED BLUMLEIN PULSERS
In recent years photoconductive semiconductor switches have gained much attention and have become competitors to the conventional high-power switches for certain applications. These new devices operate jitter-free with optical isolation of the trigger. They have switching speeds which either match or greatly exceed the risetimes of the the optical pulses triggering them. Such photoconductive switches are described as LINEAR or AVALANCHE, respectively.
The application of linear switches has been limited by the relatively high optical power required to obtain their closure. In the avalanche type switches, the electron-hole pair produced by each trigger photon is multiplied through an avalanche process, thus reducing the optical energy levels necessary for initiation of the switch closure. The nonlinearities of the multiplication accelerates the pace of commutation so that the switch closes faster than the trigger power rises. It is interesting to note that Blumleins may be the most appropriate pulse-forming line for use with photoconductive switches. They provide faster output pulse risetimes and reduce the percentage of stored energy deposited in the switch.
Recent efforts in the Center for Quantum Electronics at UTD have been directed toward commuting Blumlein pulsers with GaAs switches in the avalanche mode. Adaptation of the design and fast charging schemes have enabled the stacked Blumleins to produce extremely high-power nanosecond pulses of electrical energy with sub-nanosecond risetimes.
A 2-line stacked Blumlein pulser shown in the photograph was designed and constructed for commutation by photoconductive switches. A low-profile switching assembly was constructed to distribute the switching current to each of the two Blumleins with line impedances of about 100 ohms each and line lengths of 15 cm.
During operation, the pulser was resonantly pulse charged using the charging circuit seen in the figure to voltages in the range of 30-60 kV and repetition rates of 1 to 10 Hz.
The Charging Pulse Compression (CPC) module shown in detail in the figure was resonantly charged by the slower pulse charge supply after which its conventional thyratron was triggered to generate shorter charging waveforms for the main stacked Blumlein pulser. About 80 nsec. later, when the main pulser was fully charged, the laser system produced a short burst of photons for commutation of the GaAs switch in the avalanche mode causing a rapid discharge of the Blumleins. In this way output pulses were produced with very fast risetimes and peak powers approaching 100 Megawatts.
Typical output waveforms obtained from this pulser using either a Nd:YAG laser or a low-power laser diode are seen in the figure reproducing typical data. These measurements were obtained using an SCD 5000 oscilloscope capable of recording single electrical transients with risetimes of under 100 psec. The particular data shown corresponded to the launch of a pulse carrying 70 Megawatts peak power from the system in the photograph that is as small as hand luggage.
These results prove that with small photoconductive switches, stacked Blumleins can provide nanosecond electrical pulses at powers approaching 100 Megawatts with risetimes faster than 300 picoseconds.