Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

R&D OF THE LONG-LIFE THYRATRON-TUBE
J. S. OH1, A), W. NAMKUNGA), H. MATSUMOTOB), M. AKEMOTOB), X. DONGB),
H. BABAC), T. SHINTAKEC)A)
PAL/POSTECH, POHANG, KYUNGBUK, 790-784, KOREA
B)
KEK, TSUKUBA, JAPANC)
RIKEN/SPING-8, MIKAZUKI, SAYO, HYOGO, 679-5143, JAPAN
Abstract

2. Potential of Thyratron Tubes

Long lifetime over 50,000 hours for the thyratron isessential requirement to provide reasonable availability of
the C-band e+e- linear collider. The lifetime andreliability of a solid-state device are not well confirmed
yet. There are some examples that show long life of athyratron. Many thyratrons were dead due to several
common causes related to circuits and operation
environment rather than intrinsic problems of a device
itself. The C-band smart modulator uses an inverter
charging method and this scheme affects so much the lifeof a thyratron. Several valuable feedback systems areeasily adopted to enhance the lifetime. There are stillunidentified questions to be verified in the thyratron.
Close collaboration between laboratories and companies
is strongly requested in order to improve the lifetime andperformance of a thyratron.

1. INTRODUCTION
A solid-state device is emerging device that seems tohave potential to provide such a long life. However, it iscatastrophic to any kind of faults. It needs rather
complicated auxiliary system that should be more robust
and reliable than the device. The lifetime and reliability ofthese devices are not well confirmed yet. The cost is very
high and difficult to reduce due to the small scale ofsemiconductor business. Solid state devices such as IGBT
or SI thyristor are might be better for the applications that
require rather low voltage, high average current, highaverage power.
The thyratron parameters are well suited to high power
pulse applications that require high peak voltage, highpeak current, high peak di/dt, and high efficiency. A
matching driver circuit of the thyratron is very simple dueto the simple structure of the thyratron tube. Many
thyratrons were dead due to several common causes thatare related to circuits and operation environment rather
than intrinsic problems of a device itself. There are someresearches to improve the performance and lifetime but
those results are not well combined and proved well.
Even though the low inverse voltage of the thyratron
anode is important, it is not tightly controlled.
The proposed C-band e+e- linear collider is consistedof about 4,000 thyratrons for smart klystron-modulators.
In order to provide reasonable availability of the linear
collider, long lifetime over 50,000 hours for the thyratronis essential requirement [1].

There are three major development series through thehistory of SLAC thyratron for about 30 years since 1964.
The Wagner model CH191 tubes had been delivered since
1964 and rebuilt as ITT F143, Omniwave 1002. From the
beginning of SLC era in 1984, ITT F241 tubes had beenused and rebuilt as ITT F310. In the 1992, EEV tubes
such as CX1836A and CX2410 had been adopted [2].
About 1600 tubes of Wagner model CH1191 had beenused in SLAC for 10 years since 1964. The operational
statistics of CH1191 thyratron tubes is shown in the figure
1. This data analysis includes only 35% tubes since 1985.
From 1964 to 1984, operational parameters of thyratrons
are 46 kV, 4.2 kA, 3.8 us, 360 pps, 5.7 A. From 1985 to
1994, operational parameters of thyratrons are 46 kV, 6.3
kA, 5.4 us, 120 pps, 4.1 A. Amazingly 20 tubes are still
active in 1994 with ages between 75 ~ 120 kHours. We
can confirm wide distribution of age and lifetime profile
from the figure.

Figure 1: Operational statistics of CH1191 thyratrontubes.

3. REVIEW OF A THYRATRON
3.1 Switching cycle
Figure 2 shows typical Paschen curve of a thyratron. In
the thyratron filled with hydrogen to a pressure of about
0.5 Torr, the small inter-electrode spacing of about 3 mm,
marked “low pd” in the diagram, provides the highvoltage hold-off. Such gaps can hold-off a voltage of up
to 40 kV. In order to minimize the trigger-voltage
requirement, the cathode/trigger-grid spacing, marked
pdmin in the diagram, is set at about 15 mm. The righthand side of the Paschen curve is applied to the design ofthe envelope of the thyratron, which is usually in air, andthe high voltage electrode must have a spacing of about
75 mm from other electrodes.
Thyratron commutation is achieved by introducing
plasma into the grid/anode region via slots in the gridE-mail: josh@postech.ac.kr
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Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

structure.
The
plasma is created inthecathode/gridregion by a fastrising trigger pulse
applied to the grid,
which then diffuses
to the grid slots
Figure 2: Paschen curve of awhere it comesthyratron.
under the influenceof the anode field. The trigger plasma provides a copious
supply of electrons so that anode breakdown proceeds
until ionized plasma connects the cathode and anode.
When the commutation phase is complete, the thyratron isfilled with plasma. Current is carried between cathode andanode with a potential drop of the order of 100 V when
the peak current is 1000 A. This low voltage drop resultsfrom the shielding effect of the positive ions in the plasmathat allows the electron current to flow without spacecharge limitation. The thyratron requires a recovery
period with the anode at a slight negative potential toallow the plasma to decay back to neutral gas. The plasma
decay is dominated by the recombination of ions andelectrons on adjacent electrode surfaces. Diffusion
processes in the grid/anode region therefore control
thyratron recovery. Since the anode/grid gap and gridslots are relatively narrow, the plasma density drops
rapidly in this region with a time constant in the region of
2-7 us. The grid-cathode plasma decays much more
slowly because of the wider gaps involved [3].

3.2 High voltage structure
The tube is triggered at the cathode end and the highvoltage gaps go into conduction starting with the gap
nearest the cathode, followed by a delay of around 50 ns
before the next gap, and so on. The result is that the lasthigh voltage gap at the anode must withstand the fullanode voltage for a period of 50 ns without arcing.
Molybdenum tubes have been operated successfully at up
to 160 kV but copper has been observed to suffer arc
damage above 60 kV. The generally accepted maximum
field strength for D.C. voltages in electron tubes withconditioned surfaces is 10-kV/mm. With an applied anodevoltage of 50 kV, the peak electric field exceeds this levelby a factor of two for a period of 50 nsec or so duringcommutation. Although the field during commutation israther high, it is only a transient field. The limits for
acceptable peak field strength under such transientconditions are not precisely known.

3.3 Cathode temperature
The optimum cathode operating temperature is around750-780°C. It is a balance between sufficient primaryelectron emission at one extreme and minimizing thermal
evaporation at the other. A temperature difference of as
little as +25°C may double the rate of coating
evaporation. The cathode back heating by ion impact
makes a significant contribution to overall cathode power
dissipation. The average anode current of 4 A can raise

the cathode temperature about 100°C. It is perhaps
surprising that the practice of operating thyratron with
reduced heater power to compensate for back heating has
not been adopted much more widely [4].

3.4 Cooling
The dV/dt signal provides a relative measurement of
the gas pressure in the high voltage structure. The
efficient cooling of the anode stem to below 70°C isadvisable when the tubes are being used with highforward (>30 kV) and inverse voltage. The gas pressure
variation as average current is increased can be madesmall by improving the heat extraction from the surface of
the anode due to the larger diameter anode rod. It is
possible to overcool the thyratron, which may result in a
low cathode temperature and a low gas pressure. The
ceramic envelope near the cathode must not fall below
70°C when operating normally in air. In any coolingscheme, avoid excessive airflows directed at the cathodeand reservoir regions, which can reduce the envelope
temperature below optimum levels. The resulting change
in radial gas density gradient is equivalent to operating
the thyratron at low gas pressure and can reduce theperformance and operating life.

3.5 Others
In cases where the pulse current exceeds several kiloamps, it is advisable to ensure symmetrical current flowin the connections around the thyratron. An asymmetricalmagnetic field can impose forces on the internal plasmathat prevent uniform current density at the cathode and at
grid apertures with unpredictable effects on performanceand lifetime. It is important that where glass thyratrons
are used in an environment that could contain helium (e.g.
a TEA CO2 laser), they are adequately cooled.

4. CRITICAL ISSUES OF A THYRATRON
4.1 CathodeThe thermionic cathode is one of the key features thatwill determine the life. The oxide cathode is preferredover the smaller BA counterpart simply on the grounds ofcost. Operation at a reduced average anode current isbelieved to be an effective way of increasing the tube
lifetime to over 50,000 hours [5].

4.2 Reservoir and Ranging
The switching performance of the thyratron is very
sensitive to the internal gas pressure. Gas pressure ofabout 0.5 Torr is maintained by heating titanium hydride(TiH2) in porous nickel capsules (reservoir) loaded withseveral hundred tube-volumes of hydrogen. A highcapacity reservoir system minimizes gas pressureadjustments throughout tube life. Intensive checking andadjustment of the optimum pressure (ranging) is a timeconsuming part of modulator operation and maintenance.
Automatic tracking of the optimum pressure againstchanges in applied reservoir voltage and back-heating
conditions is provided in the E2V thyratron, which has a478

Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

temperature-sensitive regulation circuit (barretter) in the
tube base.

4.3 Triggering
An additional grid between the control grid and thecathode can be adopted to generate pre-ionizing
discharges by using either a D.C. or a pulsed current. This
tetrode tube has a performance that is satisfactory for alinear collider: very low switching jitter (typically 1 nsec)
and drift, faster switching, and a smaller grid spike. The
traditional D.C. pre-ionizing current of a few hundred
milli-amperes is insufficient to ionize fully all the cathodearea. A thyratron double-trigger system could produce apulsed current of several tens of amperes to ionize fully
the cathode space.

gradient grid electrode are measured to get dV/dt, anodedelay time, and jitter. The anode temperature will bemeasured by a pyrometer to analyze heat loading of theanode. Variable anode reactor will be optimized to getminimum anode heating. D.C. and pulse trigger scheme
will be compared to find optimum trigger condition. RF
pickup coil will amplify signals that will be used as afeedback signal to control reservoir temperature. Heater
circuit has a constant resistance control mode to keep
constant cathode temperature.

4.4 Inverse anode voltage
An inverse voltage level above 5 kV causes strong ion
bombardment of the anode and liberates enough anodematerial that will be deposited on the critical insulating
surface, to affect eventually the forward voltage hold-off
adversely. Proper design of the tail clipper and the EOLC
circuit is essential to ensure low inverse. An inverter
charging power supply provides the command-charging
feature that is effective way to turn-off a thyratron without
an inverse voltage, which is highly recommended toguarantee a long life.

4.5 Saturating anode inductor
To reduce the anode heating arising from switch-on
losses, and to reduce the voltage appearing across the top
gap during thyratron commutation, it is proposed to use asaturating anode inductor with a thyratron. A saturating
anode inductor with a volt-second product of 1-mVs will
give a switch-on delay of 50 ns. It may be necessary to
trade off the effect of the added saturated inductance on
the current rise time against the protection afforded to the
top gap. Saturating anode inductors also afford some
protection against the adverse effects of inverse voltages.

R&D OF A THYRATRON SWITCH
Further deep understanding of the thyratron isnecessary to enhance the performance and lifetime. First,
the autopsy of failed tubes is necessary to understand thecauses of failures and differences of each tube. In
addition, we should study the anode circuit of the
thyratron that includes anode temperature measurement,
anode dV/dt measurement, anode reactor optimization,etc. Cathode circuit study will include the effect of D.C.
heater vs. AC heater, cathode temperature measurement,
and auto ranging of cathode heater. Grid circuit R&D isconsisted of grid temperature measurement, thecomparison of D.C. prime vs. pulse prime, trigger
optimization of double pulse trigger. Gas circuit study
will include measure of gas condition by dV/dt and/or RF
pickup, stabilization of gas condition by auto ranging.
Figure 3 shows the experimental setup for the researchof switching characteristics. The voltage at the anode andFigure 3: Experimental setup for the thyratron R&D.

5. SUMMARYLong lifetime over 50,000 hours for the thyratron isessential requirement to provide reasonable availability inthe proposed C-band e+e- linear collider. Many thyratrons
were dead due to several common causes related tocircuits and operation environment rather than intrinsic
problems of a device itself. The C-band smart modulator
uses an inverter charging method instead of a traditional
resonant charging circuit. Proper design of the tail clipper
and the EOLC circuit ensures the low inverse voltage ofthe thyratron anode. Several valuable feedback systems
are easily adopted to enhance the lifetime. There are stillunidentified questions to be verified in the thyratron.
Close collaboration between laboratories and companies
is strongly requested in order to improve the lifetime andperformance of a thyratron.

6. REFERENCES[1] T. Shintake et al., "C-band Linac RF-system for e+e- Linear
Collider," Proceedings of International Workshop on Pulsed
RF Power Sources for Linear Colliders, RF96, Hayama,
Kanagawa, Japan, April 8-12, 1996, KEK Preprint 96-27[2] David B. Ficklin Jr., “A History of Thyratron Lifetimes at
the Stanford Linear Accelerator Center,” SLAC-PUB-6543,
December 1994
[3] 2000 Marconi Applied Technologies Limited, “A1 AHydrogen Thyratrons Preamble,” Issue 2, July 2000
[4] M. Parmenter, et al., "A Thyratron for the NLC Baseline
Modulator," 2001 Klystron-Modulator Workshop, CERN,
2001[5] C. A. Pirrie et al., "Thyratron and Modulator Design
Considerations to Maximize Thyratron Life," 1998
Klystron-Modulator Workshop, SLAC, Stanford, California,
USA, June 29 - July 2, 1998

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