A fast injection kicker system for the Tevatron Page: 2 of 3
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We had a thyratron enclosure from the MI project so we
made some modifications and performed several tests. The
CX1268 thyratron was recommended by Marconi Applied
Technology (nde EEV) . This is a two gap thyratron,
without a drift space, rated at 45 kV and 10 kA with a
grid structure that allows for heavy pulse triggering. The
original housing was for a 25 2 system using a CX1668
. This provided us with an opportunity to learn of the
limiting performance of this housing. It was modified so
that it could take the shorter CX1268 and run with either
4 x 50 Q cables (12.5 S2) or 2 x 50 2 cables (25 92).
The cables were connected to the anode and charged with a
DC supply. The output cables connected the cathode
through 15 m of cable to the loads. We used two spare
25 92 loads which have - 60 nH of series inductance,
~ 50 pF of parallel capacitance and a built in current
viewing resistor (CVR). Measurements were made on the
CX1268 in a 25 S2 system and then the impedance was
changed to 12.5 2. The reservoir was kept constant to
eliminate that variable. The only change was the number
of cables to and from the pulser. The same CVR and load
were used for measuring the output current in both cases.
It was somewhat suprising that there was no substantial
change in current rise time when the impedance was
changed, 17 1 ns (10%-90%). The measurements showed
little change when done at the same peak current or the
same charge voltage. EEV later found a similar result 
using a CX1268 in a 16.7 2 system but with a different
housing and with saturating ferrite cores.
There are many possible explanations for our results.
The thyratron ionizing grid in our case has a small DC
current instead of a large pulse current. The stray
capacitance of the thyratron cathode to ground might have
been the limiting factor. The thyratron was not dl/dt
limited as doubling the voltage doubled the dI/dt, but did
not change the rise time. The conclusion of EEV on their
data was that the current rise time was limited by the
housing inductance. However, the thyratron and housing
do not seem series inductance limited in our case. If that
were the case, the rise time should have changed
substantially with an impedance change and it did not.
-40 -30 -20 -10
Figure 1, Current Measured in Original Housing
Zero Time Is Set to 50% of Flattop.
One thing that did change with impedance was the
amplitude of the pre-pulse, Figure 1. It was a lower
percentage in the lower impedance systems when either
the charging voltage or the peak current was held constant.
The pre-pulse amplitude was 10% for the 25 52 system
comparedto 8% for the 12.5 52 system. This reduction of
pre-pulse was also one of the reasons for going with a
non drift space thyratron. From these test results, we
decidedto reduce the cathode housing capacitance, reduce
the series inductance and build in the possibility for
triggering both grids to reducethe rise time.
To reduce cathode capacitance and series inductance, the
design was changedfrom a coaxial housing to a re-entrant
housing. The cathode enclosure to case distance was
increasedby 50% over the original housing distance even
though the peak voltage was reduced from 65 kV to
45 kV. The thyratron was fitted with a 5" diameter current
return shield, 80% of the original housing diameter. In
this way the current path from the anode through the
thyratron to the cathode and back through the return shield
had a minimal inductance while maintaining the reduced
cathode capacitance. Electric field simulations of the
thyratron in the return shield were also done to determine
the closest spacing.
The cathode capacitance was then measured to be
- 140 pF in air ( ~ 265 pF in Fluorinert FC-40 ). The
equivalent inductance of the thyratron during conduction
was measuredby using a 2" diameterconductorin place of
the thyratron. The total inductance was measured to be
- 80 nH, of which 30 nH is estimated to be from the
input and output connectors. The inductance is now -30%
of the original housing and the capacitance is now -70%
of the original housing. A section through the enclosure
is shown in Figure 2.
FIL/RES POWER TRIG POWER
CONNECTORS THYRATRON RETURN CATHODE
(4 PLACES) CX1268 SHIELD ENCLOSURE
Figure 2, Thyratron and Final Enclosure
Z=12.5, 14.5 kV
Z=12.5, 29.0 kV
Z=25.0, 29.0 kV
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Chris C. Jensen, Robert E. Reilly and Bruce M. Hanna. A fast injection kicker system for the Tevatron, article, July 25, 2001; Batavia, Illinois. (digital.library.unt.edu/ark:/67531/metadc716795/m1/2/: accessed October 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.