grid driver circuit design, and the
performance required from the
thyratron itself. Contact the appli-
cations engineering department at
PerkinElmer to discuss the spe-
cific details of your requirement.
Conduction
Once the commutation interval
has ended, a typical hydrogen
thyratron will conduct with near-
ly constant voltage drop on the
order of 100 volts regardless of
the current through the tube.
Recovery
Thyratrons open (recover) via
diffusion of ions to the tube inner
walls and electrode surfaces,
where the ions can recombine
with electrons. This process takes
from 30 to 150 microseconds,
depending on the tube type, fill
pressure, and gas (hydrogen or
deuterium). The theoretical maxi-
mum pulse repetition rate is the
inverse of the recovery time.
Recovery can be promoted by
arranging to have a small nega-
tive DC bias voltage on the con-
trol grid when forward conduc-
tion has ceased. A bias voltage of
50 to 100 volts is usually suffi-
cient.
Recovery can also be improved
by arranging to have small nega-
tive voltage on the anode after
forward conduction has ceased.
In many radar circuits, a few-per-
cent negative mismatch between
a pulse-forming network and the
load ensures a residual negative
anode voltage. In laser circuits,
classical pulse-forming networks
are seldom used, so inverse
anode voltage may not be easily
generated. Recovery then strong-
ly depends on the characteristics
of the anode charging circuit. In
general, charging schemes
involving gently rising voltages
(i.e., resonant charging and ramp
charging) favor thyratron recov-
ery, and therefore allow higher
pulse repetition rates. Fast ramp-
ing and resistive charging put
large voltages on the anode
quickly, thus making recovery
more difficult. The ideal charging
scheme from the viewpoint of
thyratron recovery is command
charging, wherein voltage is
applied to the thyratron only an
instant before firing.
CURRENT LIMITING AND/OR
MATCHING RESISTOR
GRID SPIKE
SUPPRESSION CIRCUIT
GRID DRIVER
CIRCUIT
Figure 3. Grid Circuit
(a)
Filter
(b)
Zener
(c)
MOV
(d)
Spark Gap
Figure 4. Typical Grid Spike Suppression Circuits
PERKINELMER [ PERKINELMER OPTOELECTRONICS ]
