Ves Fulp (Crowbar (left), surge resistor (right)):
The high level transmitter's (HLT) electrical surge arrestor, is commonly called the "CROWBAR". Its primary purpose is to protect the klystron and other critical components from being destroyed by such events as arcing inside the klystron. Numerous fault protection circuits are incorporated to sense abnormalities throughout the HLT system, which in turn initiate the action that triggers the crowbar causing it to fire.
Typically, during normal operation, both klystrons are operated in parallel (combined power out) and should a major fault occur with one of the two klystrons, fault detection circuits will prevent damage to that particular klystron by firing the crowbar. Immediately, a unique array of waveguide switches will automatically reconfigure the switches to take the failed klystron off line and place the remaining klystron's output in the correct configuration for radiating solo out the antenna face. The waveguide switches operate in less than 100 milliseconds and during this time, the RF excitation to the good klystron is inhibited.
That little cabinet (in front of the tower) is a pulse generator assembly and contains the circuitry which initiates the firing of the crowbar when it receives certain fault generated signals. Spark gap break down (arcing across the gap) is a factor of the spark gap spacing. With the series of gaps, the accumulative gap spacing is such that they will not break down at the normal operating voltage of -150KV DC; however, in event of a critical fault, the pulse generator triggers action that causes the gap breakdown voltage to be exceeded. When this happens, the total energy stored in all three capacitor banks is discharged through this series of spark gaps to ground. The potential at the top of the crowbar is at -150KV DC and the base is at ground potential. Typically, an arc occurring inside the klystron would be detected by a current transformer monitoring the -150KV DC cable just prior to its entering the modulator tank. This would instantly activate a major fault resulting in the firing of the crowbar, which in turn discharges the full energy of the cap banks to ground in less than 5 microseconds. Without this protection, the klystron would be totally destroyed.
Side notes about the crowbar. Its noise when it fires is equal to the noise of a 5 inch cannon on a ship I was on. Once the crowbar is fired, the high voltage drops to zero, and can be brought back up to operating voltage within approximately 5 seconds. A favorite prank of ours during the many VIP tours was to manually test fire the crowbar as a tour group was leaving the transmitter control room. As they left the transmitter control room (which had fairly good sound proofing), they had to exit through the high voltage control room which had no soundproofing from the crowbar tower. There was of course a heavy duty safety wire fence. The base commanding general and other officers frequently brought tours through the HLT area. The general in particular, would often give me a nod to fire the crowbar as his tour group exited through the high voltage power supply control room. Of course, if we were supporting a mission, I could not do this.
During early testing, we were having trouble involving the high voltage run up and pulsing the klystron, and the quickest way to go from full load to no load was to fire the crowbar. This was so easy, we got carried away and were firing the crowbar in pretty rapid succession. At that time (during very early test phase), we were operating on motor generators using commercial power. In very short order, our going from full load to no load was too much for the power companies and we actually caused a total shut down of power for most of ND and Alberta, Canada. Their switch gear would not handle the abrupt changes in load and did cause damage at the power stations. To them, it was kind of like a rapid series of lightening strikes.
A little trivia:
The crowbar tower weighed 5115 pounds, was 7'8" x 6'10" x 14'5" high in size
The pulse generator assembly weighed 506 pounds, was 5' x 3' x4' high in size
For more information, see "Final Power Amplifier Overview" on the
MSR High Level Transmitter (HLT) Detail page.