Induction Linac Pulsers Page: 3 of 4
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lumped element capacitors and inductors for a 1.5 s PFN. The subsequent ETA and
ATA induction linacs at LLNL were similar to the ERA injector, but produced -70 ns
pulses at higher power levels. FXR and DARHT I are similar.
The induction linac pulsers have generally comprised a closing switch and a line type of
pulse forming network. The choice of the closing switch includes magnetic modulators of
several kinds, hydrogen thyratrons, ignitrons, FET's and other semiconductors, and spark
gaps of various kinds. For at least 40 years there have been research efforts on many light
or electron beam activated switches and amplifiers, but despite good prospects and high
expectations, including modest contributions from our group, these switches are not in
general use. A modest exception is the optically coupled SCR, but it is too slow for our
For obtaining higher voltages, the initiating pulse generator can use the Blumlein
configuration, use a "cable" or a standard output transformer, or in some cases a tapered
impedance transmission line. The ERA pulse line accelerator was designed with both a
Blumlein circuit and a tapered radial transmission line. Because of the unfavorable
scaling of breakdown voltage, there has been little incentive to develop higher voltage
modules. The Astron Injector used several parallel 50 Q transmission lines charged to
-30 kV to produce -15 kV pulses into smaller transmission lines (coax cables) going to
each core. One hydrogen thyratron drove one 12" long core of about 30" OD, 11" ID, and
altogether -600 thyratron chasses were used. Our 2000 FET pulser provided a similar
output pulse to that of one thyratron pulser and, for a developmental model, cost about
100 times more and was several times larger. The ERA Injector used 250 kV pulsers, for
shorter but higher current pulses, and was switched with a spark gap. Roughly, it was
equivalent to 16 thyratron pulsers.
The use of a 250 kV induction module switched by a spark gap compared to the "16"
thyratron switched modules resulted in a smaller and less expensive system overall. The
higher gap voltage was and still is a concern, and better insulator prototypes were made at
LBL and later at LLNL, but not used. At the end of the ERA and ATA projects a few of
the accelerator modules were disassembled for inspection, and a small fraction of those
showed well developed discharge trees going partially through the insulator. With
additional pulses these would have failed. The unused better solution was an essential
part of the LIACEP insulator: subdivision of the insulator into several shorter pieces. The
work on glass ceramics of a decade ago had promise of embedding the required metal
subdivision rings into one insulator casting at low cost.
An important difference between the various switches is the switching speed:
nanoseconds for a hard tube and a power FET, -20 ns for the spark gaps, and -50 ns for
the thyratrons. The rise time of the voltage in the accelerator gap is substantially longer
due to the "gap capacity" or the wave travel and settling times around the typically 1-2 m
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Faltens, Andris. Induction Linac Pulsers, report, January 7, 2011; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc843641/m1/3/: accessed June 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.