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Abstract
MULTIGAP PSEUDOSPARK SWITCH FOR FAIR
K. Frank, I. Petzenhauser*, U. Blell*, B.J. Lee + , and J. Jacoby +
Texas Tech University
Center for Pulsed Power and Power Electronics
Box 43102, Lubbock, Tx, 79409-3102, United States
E-mail: klaus.frank@ttu.edu
At the GSI Helmholtzzentrum fuer Schwerionenforschung
GmbH a new accelerator complex, called Facility for
Antiproton and Ion Research (FAIR), is under construction.
Its main components are the SIS100 and SIS300 heavy ion
synchrotrons. To operate their injection/extraction kicker
magnet systems, modulators with pulse-forming networks
(PFNs) are necessary. The PFNs will be charged to a high
voltage up to 70 kV and discharged via a high-voltage
switch. The switch has to handle currents up to 6 kA, pulse
durations up to 7 microseconds with an overall lifetime
exceeding 10 8 shots. The repetition rate is about 4 Hz and a
current rise rate of at least 4 . 10 10 A/s is required. The only
commercially available switch in this parameter range is
actually a multi-gap thyratron. As an alternative, a three-gap
pseudospark switch is under development at GSI. It
combines the major advantages of the thyratron with its low
stand-by power as a cold-cathode device, as well as its
insensitivity to large current reversal.
Like for the thyratron, the maximum hold-off voltage of a
single gap pseudospark switch is limited to about 35 kV.
For a reliable hold-off voltage of 70 kV, a three-gap system
was designed. Test results with a first sealed-off prototype
switch of this design are reported. The prototype has
demonstrated a voltage hold-off capability of more than 80
kV. The circuit of capacitive and resistive voltage dividers
was optimized to improve the switch control, the delay and
the jitter values.
As trigger unit, a conventional high-dielectric trigger is
used. With such a trigger unit a crucial issue for minimum
delay breakdown still remains the plasma coupling between
the different gaps by drift-spaces. Those drift spaces have to
be designed carefully in order to minimize the internal
delay of breakdown. An additional major issue is that the
switch suffers from losses, which principally limit the
lifetime of low-pressure gas discharge switches. At high
repetition rates a common way to minimize losses by anode
dissipation is to integrate a so-called anode inductor.
Whether this will be true, too, with a cold-cathode switch at
low repetition rates, and much larger pulse lengths,
preliminary tests with an anode inductor were performed.
I. INTRODUCTION
The "Facility for Antiproton and Ion Research" (FAIR) is a
new, international accelerator complex that will be built in
Darmstadt, Germany [1]. The project started officially in
November 2007. One of the main components of this
complex will be the SIS100/300 double ring facility with a
circumference of 1100 m. SIS indicates the type of the
machine "Schwer-Ionen-Synchrotron" which is the German
word for heavy ion synchrotron and the numbers
correspond to the maximum beam rigidity of 100 Tm and
300 Tm, respectively.
Kicker magnets are fast pulsed devices used for the
injection and ejection of particle beams of such circular
accelerators. Frequently the space available in the
accelerator lattice for these magnets is strictly limited,
resulting in difficulties in simultaneously satisfying the kick
strength and fast rise time requirements. The obvious
solution of further raising the voltage runs into technical
difficulties above 80-100 kV. Fortunately, operating kicker
magnets in the short-circuited rather than the terminated
mode generates double the kick strength from a given space
for given operating voltage, characteristic impedance and
rise time.
II. THE NEW THREE-GAP PSS
PROTOTYPE
A. The Pseudospark Discharge
Figure 1. The generalized breakdown curve from
PASCHEN and the modified one for the pseudospark
geometry [2]
*FAIR Synchrotrons, GSI Helmholtzzentrum fuer Schwerionenforschung GmbH, Planckstr.1, 64291 Darmstadt, Germany
+ Institute of Applied Physics, Goethe University, Max-von-Laue-Str. 1, 60438 Frankfurt am Main, Germany

Due to the non-planar geometry (see Figure 1.) of the
pseudospark, PASCHEN’s law is difficult to apply. This is
a result of both the non-equilibrium nature of the electron
transport and of the difficulty in defining an effective path
length. Since pseudosparks operate on the “near side” of
PASCHEN’s curve (i.e increasing hold-off voltage Ubreak,
with a decreasing product of pressure and distance, p*d),
electrons having longer paths will decrease U, due to their
having more opportunity to avalanche before reaching the
anode. Since the spatial distribution of the electric field,
both in the gap and in the hollow electrode structures,
depends on parameters such as the electrode separation,
electrode thickness, and electrode hole radii, the breakdown
voltage Ubreak, will also depend on these quantities.
Deviations from PASCHEN’s law resulting in lower holdoff
voltages are caused by penetration of the anode potential
into the hollow cathode making a larger effective value of
p * d. Penetration increases with increasing cathode hole
radius and decreasing cathode thickness, and so the hold-off
voltage decreases. Ionizing collisions occur predominantly
near the central hole by electrons initially emitted from the
inside of the cathode. This behavior is a consequence of the
electrons in the hollow cathode spending a longer time in
the moderate electric fields resulting from potential
penetration into the cathode. They also have a longer
effective path length to the anode resulting in a large p*d. In
the hollow anode case, the majority of electron impact
ionization occurs in the cathode hole and near gap. Once the
electrons get into the high-field region in the gap and
accelerate to energies greater than the peak in the ionization
cross section, the rate of ionization decreases dramatically.
Those electrons become so-called “run-away” electrons,
which start to form an energetic e-beam. This is an
undesired process within low-pressure plasma switches like
thyratrons and pseudosparks. The overwhelming part of the
commutation loss is related to this process, as well as major
electrode erosion. That is why the limit in hold-off voltage
is at about 30 kV for a two-electrode, low-pressure gap; for
higher hold-of voltages a stacking of two or more gaps is
mandatory. For a reliable hold-off voltage of 70 kV during
an extended lifetime, a number of at least three gaps are
necessary.
B. Comparison Of Switch Types Considered For
Application With FAIR
The following table summarizes the different types of high
voltage switches that have been discussed. When the project
started 5 years ago, solid-state switches were far from being
an useful alternative. Meanwhile the technology of solidstate
switches is so mature that even for high voltages
beyond 50 kV those are serious alternatives to gas-filled
devices. But these switches still suffer from an insufficient
current rise rate.
On the other hand high-voltage, multigap thyratrons are still
in wide-spread use in high energy particle accelerators and
are still considered for future applications.
Table 1: Comparison of different switch types related to the
requirements
Parameter Quantity Thyratron Pseudo- Solid- Spark
spark State Gap
Voltage ≤ 70 kV yes yes yes yes
Current ≤ 6 kA yes yes yes yes
Repetition
rate
≤ 4 Hz yes yes yes yes
Pulse
duration
≤ 7 µs yes yes yes yes
Current rise
time
< 150 ns yes yes yes yes
Current rise >4*10
rate
10 A/s yes yes no yes
Energy /
pulse
~ 1,5 kJ yes yes yes yes
Peak power ~ 210 MW yes yes yes yes
Lifetime >10 8 shots yes yes? yes no
The PSS is comparable in many aspects to the thyratron and
it possesses additional advantages, but it is not yet
commercially available. Spark gaps can not fulfill the
lifetime requirements.
C. Pseudospark Development For FAIR
Figure 2 reflects the progress in brazing technology by the
comparison of the former two-gap prototype (left) [3,4,5]
with the actual three-gap pseudospark switch (PSS). A
sketch of the drift space lay-out is shown in the middle.
Figure 2. Former two-gap prototype (left), drift space
geometry (middle), actual three-gap prototype (right)
The external circuitry, that divides the voltage between the
three gaps is of big importance for delay and jitter values of
the triggered PSS breakdown. Principally it is important to
provide by an auxiliary glow discharge a fast pre-ionization
of the drift space in order to initiate the next gap’s
breakdown. As important as efficient drift-space preionization
is the primary trigger plasma, which initiates the
breakdown process of the switch. It is planned to replace
the actual trigger device, the dielectric surface flashover
trigger, by new unit, which has an additional extraction grid
(see Figure 3). This extraction grid, one hand, protects the
device from the impact of the main discharge, on the other
hand, it allows to apply a pre-trigger pulse to improve the

timing, and the extraction of a higher amount of plasma
electrons.
(a)
(b)
Figure 3. Improved trigger unit with parts (a) and crosssection
(b)
The three-gap PSS was tested up to a hold-off voltage of
80 kV. A further increase was prevented by external
flashover at the auxiliary circuit. The switched current itself
was below 1 kA with the risk of quenching, which was
observed at low gas pressure (as a function of the reservoir
heater current (see Figure 4)).
Figure 4. Quench phenomenon with the three-gap PSS at
different reservoir heater currents, and the influence of
adding a heavy noble gas (argon).
It is well-known from former experiments with one-gap
pseudospark switches that at discharge currents below 1 kA
the quenching [ ] occurs, and, that quenching can be
suppressed by adding a small amount of a heavy noble gas
to the working gas (usually deuterium). This could be
confirmed by experiments with the three-gap PSS (see
Figure 7).
A. Basics
II. SATURABLE INDUCTORS
One of the challenges in all low pressure gas discharge
switches is the reduction of the lifetime due to commutation
losses resulting in anode heating, especially with high
repetition rates. The commutation loss is mainly a result of
the finite switching time of every switch. After the trigger
pulse arrives at the pseudospark switch (or thyratron), the
gas discharge needs a certain time to fully develop and to
reach a low ohmic resistance. During that commutation
phase, a significant current is already flowing in the switch.
The still high voltage and the current multiply to a huge
power loss (Multiflying the still high voltage by the current
results in a huge power loss). This problem can be solved
by using saturable inductors. Basically these inductors,
connected in series to a switch, reduce the current flow to a
negligible value. When the inductors are saturated, the
current can flow freely. Therefore the current is delayed and
the gas discharge inside the switch can fully develop before
the current arrives. When the current flows, the voltage
across the switch is already low, therefore the losses can be
significantly reduced.
For many decades [13, 14, 15, 16, 17, 18], this method has
been successfully used for thyratrons. In contrast to
thyratrons a PSS is based on a cold cathode electrode. To
reach the low-resistive phase in a PSS, a significant current
flow is necessary to allow self-heating of the cathode. A
useful compromise has to be found to fulfill these opposing
needs.
The voltseconds were judged adequate for the reactors to
hold off the voltage during the whole of the PSS
commutation period. They are available without the need
for pulsed or d.c. bias of the cores. Commutation losses of a
PSS can be determined principally from accurate
measurement of the current and voltage drop during the
switching period. A loss coefficient can be obtained for any
given tube which allows prediction of the losses under a
wide range of operating conditions. For short pulses of up
to a few hundreds of nanoseconds the conduction losses are
negligible compared to those of the initial commutation, but
not in the case of our application.
In following next figures the data of experiments a
pseudospark switch (PSS) without and with different anode
inductors are presented. The test setup is shown in Figure 5.
Different saturating inductors are connected in series with
the PSS, on the anode side. Three different designs of the
FINEMET (FT-3H) pulsed power core types were used.
These cores use a thin ceramic insulation to provide a high

eak down voltage. The properties of the tested saturating
inductors are summarized in the next Table 2.
Table 2: Survey on parameters of different anode inductors,
based upon Metglass
Cores A B C
Material Metglass Metglass Metglass
Bmax [T] 1.2 1.23 1.23
Br[T] 0.5 0.5 0.5
∆B = Bs+Br 1.7 1.7 1.7
Hc [A/m] < 3 8 8
µi (or µ) 20000 30000 50000
Core Area (cm 2 ) 0.456 12.5 12.5
OD [mm] 40 120 120
ID [mm] 32 20 20
Thickness 15 25 25
Calculated Ts
For V=10 kV and
∆B = 1.7 T (0.8 T)
62 ns
(29 ns)
213 ns
(100 ns)
213 ns
(100 ns)
The delay time TS is determined by Faraday’s law with a
voltage of 10 kV assuming that the voltage is linear with
time.
Anode
Cathode
Trigger
V
Saturable
Inductors
50
C
V
R
+ HV
Figure 5. Experimental set-up: pseudospark switch PSS
with anode inductor
B. Experimental Results
There is almost no difference in collapse time between both
figures shown in Figure.6. The significant difference is the
temporarily almost constant voltage during the
commutation phase before it turns negative. Could this be
explained by the pseudospark breakdown phase transition
“superemissive” to “arc” or by “hollow cathode phase” to
“superemissive phase”? This is an open question. During
commutation dissipation of energy in the PSS is reduced as
well as the total energy transfer for 5 µs (the energy is
mainly dissipated in the saturating inductors by eddy
currents). In addition, as mentioned before, the different
discharge phases with anode inductor indicate, that the
original pseudospark discharge evolution, based upon the
the superemissive mode theory [??? ] is not anymore valid.
CM
Rch
This indicates that a new mechanism leads to pseudospark
breakdown.
Voltage (kV)
Voltage (kV)
10
5
0
-5
-10
10
5
0
-5
Trigger Voltage
0
Voltage
Trigger Voltage
0
Voltage
Current
Power
Current
0.5
Time (µs)
(a)
0.5
Time (µs)
(b)
Power
Figure 6. Temporal behavior of the voltage, current and
power without a saturable inductor (a) and with a saturable
inductor (b)
Since the impedance of the inducting cores is still large,
after commutation of the pseudospark switch, most of the
voltage is present across the core until it becomes saturated.
As the core becomes saturated, its impedance decreases
drastically, allowing the discharging current to rise steeply.
In general, by the delayed main current flow, the losses in a
low-pressure gas switch are minimized. As can be seen in
Table 2, there is a big difference between the maximum and
the minimum calculated delay. Principally this would be
unacceptable in most applications. The reason for the
different values is that the cores were in an undefined state
before the measurements depending on the previous
experiments. To handle this problem, a reset system is
needed, which pre-magnetizes the cores to a negative flux
density before the pulse. Such a system was not used in the
presented measurements.
The delay of the current pulses and the decrease in
amplitude, the increase in pulse width, respectively are
1.0
1.0
0.5
0.4
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
0.5
0.4
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
Current (kA)
Current (kA)
2.0
1.5
1.0
0.5
0
2.0
1.5
1.0
0.5
0
Power (MW)
Power (MW)

caused by the higher inductance of the saturating inductors.
The delay time is mainly defined by magnetic swing (∆B)
and cross section area of the inductor. The delay time is
calculated by Faraday’s law using the numbers from the
data sheet.
The decrease in amplitude indicates the slow current rise
rate. Without saturating inductor the rise time is controlled
by the resistive fall time of the PSS and the residual circuit
inductance. When the inductors are added, at first, the
current will be delayed. When the inductor saturates, the
PSS has is already fully conductive, and consequently the
rise time is steeper. However a steeper rise time is observed
only if the circuit is not dominated by the inductance of the
saturable inductor. After saturating, the inductor acts like a
normal inductor. As a result the rise time decreases and the
pulse width becomes wider.
As shown in Figure 7 the commutation losses are reduced
by about a factor six by the right choice of saturating
inductors.
Energy (mJ)
50
45
40
35
30
25
20
15
10
5
0
Without A B C ABC
Saturable inductor types
Figure 7. Energy deposited during commutation by
different types of inductors
C. Summary
In order to improve the lifetime of the switch by reducing
commutation losses of a low pressure gas discharge switch,
saturating inducting cores have been series integrated to
pseudospark switch circuit. Two important results are
observed. One is that the saturating inductor can be useful
for the PSS in the same way it is useful for the thyratron
switch. The other one is that in our setup the transition
behaviour between the pseudospark discharge phases differs
from its typical behavior published previously [19]. Further
more detailed experiments with time-resolved optical
spectroscopy and precise forward voltage drop
measurements may deliver data for a better understanding
of the physical processes, which are on the bottom of the
pseudospark discharge. To investigate those physical
phenomena more carefully, with fast shutter photography
and line emission spectroscopy will be required.
III. OUTLOOK
The recent experiments with a three-gap pseudospark
switch confirmed that it is a promising alternative to
comparative thyratrons. It combines comparable or even
better switching characteristics with a cost-effective option
for extended applications, too.
To further reduce the quenching of the switch different
mixtures of deuterium and argon will be tested. An increase
of the initial trigger plasma density may contribute to a
reduction of quenching probability, too. It is planned to
replace the actual simple design by a pulsed dielectric
trigger unit with an additional grid for enhanced plasma
extraction and pre-pulsing, respectively. In order to get a
better understanding of the loss mechanisms, the
commutation and conduction losses have to be determined
individually for all gaps. Another option to study the loss
mechanisms is to detect time-resolved the X-ray emission
with and without anode inductor. In addition, further
detailed studies of magnetic materials for the saturable
inductor are necessary. Based upon those data an
experimental design with minimization of the stray
capacitance of the inductor and its housing will be made. Of
more scientific interest is the problem whether a saturable
inductor acts similar with a cold-cathode device as with a
hot cathode switch. Time-resolved optical spectroscopy
with and without the saturable inductor are planned to study
the breakdown processes during commutation. In a near–
future experiment the actual prototype has to be operated
repetitively with fully applied voltage and current to get an
estimation of the overall power dissipation, to derive the
expected lifetime of the switch, including the trigger and the
reservoir units. Finally the recovery process of the switch
after conduction has to be studied in order to determine the
feedback.
APPENDIX
Acknowledgements
The authors would like to thank the GSI Helmholtzzentrum
fuer Schwerionenforschung GmbH for the support. We
would like to thank especially the people at the workshops
at GSI and the university of Frankfurt who helped realise
the switch.
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