The Marx impulse
generator can simulate a lightning strike. As most lightning discharges are in
the order 1 to 100 kilo-amps the electric charge associated is usually 5 to 50
Coulombs. At a voltage of 50 to 500 million volts this transforms to a
considerable amount of discharged energy. Even though this is a destructive
event, the current rise is not as fast as the man-made impulse generator. The
output of the Marx is under 100 kilo-amps but its current rise is faster than a
natural lightning bolt. The rise time of the discharge is usually 1 to 10
microseconds with a fall time of 20 to 50 microseconds. (The reason for the
relatively slow rise is due to the atmosphere not approaching an ideal spark
gap switch.)
Materials
testing such as cables transformers and power systems can be verified for
integrity against lightning. Flash x-rays and the simulating of electro
magnetic pulse (EMP) from nuclear detonations are possible using this device.
Marx
generators are basically a stack of capacitors which are charged in a parallel
configuration to a voltage "E" and then discharged in series with a
voltage of "nE" where "n" in the number of capacitors
charged. Selection of the capacitors will determine the peak current and rate
of current rise during the discharge cycle. Not only the value of capacity and
voltage is selected, but the discharge loop inductance and peak current
handling is also considered. These capacitors once charged must be discharged
in a series configuration through special precisely-spaced spark gap switches
for each capacitor. Open air tungsten or molybdenum electrodes with Bruce or
Rogowski discharge surfaces now used. Switching times are fast but can be
improved in a nitrogen atmosphere or doping the electrodes with a radioactive
isotope such as cesium 137 or nickel 63.
The
initial charging of the capacitors is best done via a controlled current source
by the use of a current limited
transformer operating at line frequency or a standard high voltage transformer
with a current controlled reactor in series with the primary for higher power
units. The use of semiconductors usually requires special circuit precautions
and shielding from the EMP generated by most of the discharges.
Usually charging resistance Rs is chosen to
limit the charging current to about 50 to 100 mA while the generator capacitance
C is chosen such that the product CRs is about to 10s to 1 minute. The
discharge time constant CR1/n (for n stages) will be too small (microseconds),
compared to the charging time constant CRs which will be few seconds.
Impulse generators are nominally rated by
the total voltage (nominal), the number of stages and the gross energy stored. The
nominal output voltage is the number of stages multiplied by the charging
voltage [5]. The nominal energy stored is given by:
E=C1V2/2;
where;
V = nominal maximum voltage (n times
charging voltage)
C1= discharge capacitance
The discharge capacitance, C1 given by:
C1=C/N;
where;
C = capacitance of the generator
n = number of stage