Field of the invention
The present invention particularly relates to the development of a linear aperture radial
multichannel Pseudospark Switch (LARM-PSS) along with its components for pulse power
applications and more particularly relates to .the generation of high current pulse using the
developed LARM-PSS.
The LARM-PSS is a cold cathode plasma switch for pulse power generation. It consists of
trigger unit, hollow cathode and hollow anode sub-assemblies placed arn~rnda central axis
of the switch. Linear apertures in the form of rectangular sheets cut around the radial
pel-iphery of the hollow cathode and hollow anode sub-assemblies have been used for the
geometrical construction. The discharge in the hollow cathode of LARM-PSS is initiated by
trigger unit -made from a high dielectric constant ferroelectric cathode -as an axially
symmetric self-sustained transient low pressure gas discharge turning into super dense
glow discharge during the high current phase. Under pulsed operation conditions at low
pressures, the achieved peak current through the dense glow is 20 kA at the applied holdoff
voltage 20 k V . The achieved switch current can be further raised much beyond this
value from the LARM-PSS, which is at present limited due external circuit arrangement.
Background of the invention
In the proposed geometry, the transition from the dense glow discharge to super dense
glow occurs, the voltage between the electrodes falls rapidly, and current begins to rise up
to several kA orders. The design of the electrodes plays a sig'nificant role in this transition
and is not limited up to the specified geometrical parameters. However, the surface heat
and the transient plasma sheath restrictions are crucial for the specified current generation
litnit. l he metal vapors are evaporated explosively during the high current phase near to
the linear aperture due to the plasma sheath contraction and high electric field in it. In the
developed LARM-PSS the length of the plasma sheath has been increased in the linear
aperture geometry 'and the round hole apertures have been elongated to take the form of
linear aperture. This led to decrease the density of metal vapor emanating from the
aperture area and everitually resulting in to the lesser erosion of the electrodes during the
high current phase.
In the present invention, apart from the LARM-PSS development, the development of high
dielectric trigger unit for providing free electrons -lo9 cm-3 in -300 nsec is also reported.
The high dielectric constant .trigger unit has a finger-contacting electrode and a high
dielectric constant disk supported by a metal electrode. Ferroelectric electron emission
(FEE) occurs by the electrical excitation of the Ferroelectric material disk when a negative
pulse voltage IS applied to the trigger cathode electrode. The emission is generated due to
reversal of spontaneous polarization from its equilibrium state and more than 100 AIcm2
current density has been achieved from the trigger cathode. The advantage of trigger unit
using rer.ruelectric cathode is that it can be turned on more instantaneously causing less
delay in switching time.
,' .
-.. In the present invention the main features and advantages of LARM-PSS are also brought
out. It finds wide scope and a.pplications in pulse generators for medical applications
(including kidney stone destruction), high-repetition rate switches for laser systems,
replacement of thyratrons and ignitro'ns in conventional applications, pulsed radar
systems, linear particle accelerators, shock wave generation, pulsed electromagnetic
fields, etc.
Overall, the description includes drawings and examples of specific embodiments to give a ' .
broad representation of the invention. While practicing the invention from the given
description, the different changes and modifications within the spirit and scope of the.
invention will become apparent. Therefore, the scope of the invention is not intended to be
limited to the particulars disclosed herewith; rather it can cover all the modifications,
equivalents, and alternatives falling within the scope.of the invention.
Prior art search in literature and patent database provided the following references.
Reference may be.made to U. S. Pat. No. 5,126,638 issued on June 30, 1992, wherein the
coaxial round hole multichannel Pseudospark discharge switch is described. The annular
Pseudospark discharge channels about the coaxial central axis of the electrode have been
formed by Creating equally. spaced radially aligned holes around the periheter of the
cathode and anode. The plurality of such boles has been stacked along the length of the
cylindrical electrode geometry to provide more discharge current path. This has limitation
that if the current handling capacity of the switch has to be increased further, the round
hole diameters of the switch have to be increased. Nevertheless, it will lead reduced hoidoff
voltage capability due to higher field penetration through the round holes. So, a tradeoff
has to be made between current switching capability and voltage holding capability.
A paper described by H. Heo et al. (Experiments With a Radial Multichannel Pseudospark
Switch for Extremely High Coulomb Transfer, IEEE Trans. on Plasma Sci., pp. 196-202,
2004) discusses about design, construction and testing of a high charge transfer PSS. The
sealed-off switch successfully conducted charge transfer of - 78 C. Though this reported
PSS has shown to outperform over other switch for extremely high charge transfer, but for
extremely high charge transfer, Sic electrodes are used which have drawback to absorb
hydrogen.gas with a rate of -0.61 Palday and acts as a weak getter in a sealed-off type
switch and hence may not be practically suited.
Reference may be made to U.S. Pat. No.5.502.356A issued on March 26; 1994 where an
arrangement for increasing and stabilizing the operating voltage of the switch has been
described. The stabilized Pseudospark switch comprises a stabilizing electrode element
for controlling spontaneous anode-cathode current and for accelerating switch
commutation. The stabilizing electrode may lead to modify the plasma sheath during the
hollow cathode phase and the plasma discharge behavior during the accelerating switch
commutation will 'become more complex and will require in-depth understanding of
synchronization, which has not been clearly investigated so far. ~he'stabilizinel~e ctrode
may further have disadvantages in terms of geometrical constraints and require more
efforts to establish the idea.
Reference may be made to U. S. Pat. No. 6,104,022 issued on August 15, 2000 where
linear discharge aperture in interfacing electrodes has been discussed to provide higher
current conduction than round hole Pseudospark switches. it is claimed that the
continuously elongated hole or slot may allow the plasma discharge to spread evenly
throughout the slot area causing lesser electrode erosion and increased life time of the
switch. The operating life would also be more due to radial distribution of linear aperture
around the electrodes. However, the proof of the basic princip.le has not been clearly
brought out. .
A reference may be made to Y. D. Korolev, et al. (High-Current Stages in a Low-Pressure
Glow Discharge With Hollow Cathode, IEEE Trans. Plasma Sci., vol. 41, no. 8, August
. .
2013) where the interpretation of processes at dense glow and super dense glow stages
of high current PSS have been discussed. The physical reasons of current quenching
effects occurring in the Pseudospark geometry are also mentioned when high current is
drawn by the external electrical circuit. The detailed studies for a basic Pseudospark
geometry are presented. More efforts are needed to see the changes in mechanism of
glow and super dense glow discharge in different high current PSS geometries
Reference may also be made to U.S. Pat. No. 5,399,941, issued on March 21, 1995 that is
related to optically-triggered pseudospark switch with the potential of improvement in
'terms of optical isolation of the trigger circuitry. In this reference the triggering of the switch
is accomplished by ultraviolet illumination of multjple cathode apertures via fiber optic
cable. However, the light required to initiate the discharge must'pass through a window or
optical fiber which is quite troublesome. While operated for long time the optical fiber or
window can become metalizedduring the current conduction by material evaporated from
the switch electrodes. This results in a reduced switch lifetime and will be a major concern .
'
in repetitive pulsed-power systems.
I
A reference can be made to H.' Gundel et al. (A low pressure hollow cathode switch
triggered by pulsed electron beam emitted from ferroelectrics, Appl. Phys. Lett. Vol. 54,
pp. 2071-2073, 1989) where a low pressure switch is triggered by electron beam that is
emitted from the surface of a ferroelectric. sample. The electron beam is first generated
within thehollow cathode and later has been ejected through a hole of arbitrary shape into
the main gap of ttie switch. This arrangement has made fast switching along with small
jitter in the switching current. The details of the trigger unit are not well described and
require more efforts for adoptable procedure to develop the ferroelectric trigger with
specified geometries especially applicable for pseudospark switches.
A reference may t?e made to K. Frank et al. (Pseudospark Switches for High Repetition
Rates and High Current Applications, IEEE Conference, pp. 958-970, 1992) where the
state-of-art of multichannel Pseudospark switch (PSS) development has been reported.
This multichannel PSS involves three geometrical configurations, i.e., linear, coaxial, and
radial arrangement of parallel discharge channels with multiple holes instead of single hole
to increase the current transfer capability. With these configurations, they reported the
evidences of magnetic pinching, forcing all of the current to flow through one hole which
increases the erosion of the switch. However, in our developed LARM-PSS, the linear
apertures on the circumference of a cylinder in radial multichannel arrangement allow the
current flow'in radial direction and reduce the magnetic field pinching effects.
A reference may be made to R. P. Lamba, et al. (Design & development of high current
pseudospark switch for pu1s.e power application', Pulsed Power Conference (PPC), 201 3
19th IEEE, June 2013) where, the preliminary idea of linear ,apertures in radial discharge
configuration has been proposed. The authors have carried, out some basic simulations to
understand high current switching characteristics in the said geometry but have not proven
the design and concept 'for its high current switching experimentally. The discharge :
, characteristics has given a preliminary projection of such switch applicability up to 100 kA
switch current for a hold-off voltage -25 kV for arbitrary selection of eight linear apertures
and has been discussed in the paper: However, no actual test results have been clearly
brought out. Also, thk effect of number of linear apertures on the built-up discharge current
time without etfecting surface properties of electrodes have not been discussed.
Objective of the invention
The 'main objective of the present invention is to develop a linear aperture radial multichannel
Pseudospark switch having multiple linear apertures arrangement (length greater
than width) on the circumference of a circle around the electrodes, permitting the switch to
transfer increased current with diffuse discharge in the direction radial to the c'entral co-
'axis of the electrodes.
Another objective of the present invention is to lower the inductance and erosion of the
switch and thereby increasing the lifetime of the switch by incorporating multiple discharge
channels.
', Still another objective of the present invention is to provide a compact ferroelectric trigger
based Pseudospai-k switch utilizing a coaxial electrode structure with radial discharge.
channels that provide uniform current distribution during the discharge.
Summary of the Invention
Accordingly, the present invention provides a 20 KVI 20 KA Linear Radial Multichannel
Pseudospark Switch (LARM-PSS) which comprises of a 'cup shaped- hollow cylindrical
cathode electrode ( I ) brazed (IA) with cathode flange (3) to form cathode sub-assembly, a
cup shaped hollow cylindrical anode electrode (2) and a hollow electrode cylinder (4)
which are brazed (48) with a bottom hollow disc (5) to form anode sub-assembly, .the
anode and the cathode sub-assemblies placed radially symmetric on the common central
axis, four linear aperture slots (6) arranged around the periphery of the anode (4) and
~ cathode (1) electrodes facing each other to allow optimum plasma discharge so as to
I operate from hollow cathode (1) to hollow anode ( Z ) , a uniform gap (7) maintained between
cathode 'and anode sub-assemblies by placing a Perspex insulator (8) between the two
sub-assemblies, a high dielectric constant trigger sub-assembly (9) supported over
variable threaded rod (10) placed inside the hollow cathode ( I ) to provide the initial free
electrons to trigger the discharge in the hbllow cathode (I), the supporting rod(10) fixed
on header disc plate (11) which is placed and brazed ( I I A ) over an inverted kovar cup
(12), external electrical connections provided to the trigger unit (9) through feedthrough
(13) (with pin electrode (14)) brazed (138) over the kovar cup (12), an exhaust port (15)
inserted through kovar clip (12) and header plate (11) along with its brazing (AM),
feedthrough (13), header plate ( I I ) , kovar cup (12) and exhau,st port (15) that are
mounted inside hollow header cup (16) along with their brazing (16A) with the groove
(17) made in the base of cathode flange (3), an 800 nF capacitor bank (19) ~n series with ,
the LARM-PSS that has been used for the test, large charging resistance (20) for charging
the capacitor, the stored energy dissipated in ground load (21) by shorting the switch, and
the digital storage oscilloscope (22) for reading and recording of the signals registered.
In an embodiment of the present invention, the developed switch has conducted effective
charge up to 1.5 C with current capacity 20 kA at 20 kV hold-off voltage keeping gas
pressure of hydrogen 0.02 mbar.
In still yet another, embodiment of the present invention, the cylindrical cathode electrode
(1) has.outer diameter 58 mm and height 40 tnm.
In still yet another embodiment of the present invention, the cylindrical anode electrode (2)
has inner diameter 94 mm and height 39 mm.
In still yet another embodiment of the present invention, the hollow electrode cylinder (4)
has outer diameter 70 rnm and height 37 mm
In still yet another embodiment of the present invention, the width of the linear aperture
slots (6) is optimized as 2.5. mm.
In still yet another embodiment of the present invention, a uniform gap (7) of 3 mm is
maintained between cathode and anode sub-assemblies by placing a Perspex insulator
(8) of thickness 12 mm. between the two sub-assemblies.
In still yet another embodiment of the present invention, the trigger sub-assembly (9)
consists of a high dielectric constant ferroelectric disk (9D) made.from LiBaTi03
which is enclosed betwe& patterned molybdenum electrode (9A) containing finger
contacts (9B) and a stainless steel metal base plate (9C) placed over a ceramic made
metal holder (9E). '
In still yet another embodiment of the present invention, the transmission line comprises of
a low inductance flat transmission 'line.
In still yet another embodiment of the present invention, a method of generating a high
current pulse using LARM-PSS comprises of:
I. evacuating the whole assembly up to a base pressure of 10'~.~orr;
II. flushing the chamber with hydrogen or argon in controlled manner with the
help of mass flow meter;
Ill. maintaining the working gas pressure in the range 5-50 Pa for the selfbreakdown
of the gas;
IV. applying voltage in the range of 20-5 kV through a high resistance 6 Mil,
across the gap with the.help of discharge power supply in the range of 0-40
kVl3A;
V. generating trigger pulse with pulse width -500 ns and pulse voltage - -2kV by
trigger supply to release the seed electrons from the ferroelectric cathode
which causes the ionization of the gas under the effect of electric field
present in the hollow cathode and stored energy is delivered to ground as
soon as a conduction path is formed in the main gap of the switch
Brief description of the drawing
a,
FIG. 1 shows the schematic view of the complete LARM-PSS along with its various
components
FIG. 2 shows the half-sectional view of the hollow cathode and hollow anode cup
electrode structures along with the linear apertures.
FIG. 3 shows the 2-D view of the hollow cathode-anode geometry along with crosssectional
view at the linear aperture cut for the LARM-PSS.
FIG. 4 shows the schematic view of the ferroelectric trigger unit.
FIG. 5 schematically depicts the use of LARM-PSS made in accordance with the' present
invention to selectively connect a desired load to a power source by'way of a flat plate
transmission line.
FIG. 6 shows the self-breakdown characteristic of the switch at different pressures of (a)
Hydrogen and (b) Argon gases.
FIG. 7 shows the switching voltage and current waveforms from the PSS.
FIG. 8 shows the images of the electrodes after 1000 shots of operation.
Detailed description of.the invention
Referring mainly tq FIG.?, which shows the schematic view of the invented LARM-PSS,
the device comprises of a hollow cathode electrode 1 and hollow anode electrode 2
developed in a cylindrical geometry aligned on the common axis. The hollow cathode
electrode 1 in cup shape makes the inner cylinder whereas the hollow anode electrode 2
again in cup shape shows the outer cylinder in the geometry. The cathode cylinder 1 has
outer diameter 58 mm and height 40 mm and is brazed l A with a cathode flange 3 to form
cathode subassembly. The anode cylinder 2 having inner diameter 94 mm and height 39
mm and a hollow electrode cylinder 4 with outer diameter 70 mm and height 37 mm are
brazed 4A with a bottom hollow disc 5 to form anode subassembly. A uniform gap is
maintained between cathode and anode subassemblies to isolate the discharge
electrodes.. The anode and cathode sub-assemblies are radially placed sym.metric on the
common central axis. Four linear aperture slots 6 of 2.5 mm width are arranged around the
periphery of the anode 4 and cathode 1 electrodes facing each other to allow plasma
discharge to operate from hollow cathode I to hollow anode 2. A fixed gap of 3 mm is
maintained 7 between cathode and anode sub-assemblies by placing a Perspex insulator
8 of thickness 12 .mm between the two sub-assemblies. The entire electrode housing is
made leak tight through neoprene O-rings placed to make contact with the electrodes 1, 2
and Perspex insulation 8. , .
A. high dielectric constant trigger unit 9 supported over a variable threaded rod 10 is place
inside the hollow cathode cup 1 to provide the initial free electrons to trigger the discharge
in the hollow cathode cup 1. For preparing this trigger sub-assembly 9, a high dielectric
curlstant ferroeleclric disk 9D made from LiBaTi03 is enclosed between patterned
molybdenum electrode 9A containing finger contacts 9B and a stahless steel metal base
plate 9C. These details are shown in FIG 4. Due to the seed electrons and applied
potential between the hollow cathode 1 and anodes 2,4, plasma has been generated
inside the hollow cathode 1 and it further results into the discharge current conduction
through the gap 6 between the cathode and anode sub-assemblies. The supporting rod 10
is fixed on header disc plate 11 which is placed and brazed 11A over a inverted kovar cup
12. External electrical connections are provided to the trigger unit 9 through feedthrough
13 (with pin electrode 14) brazed 13A over the kovar cup 12. A exhaust port 15, inserted
.through kovar cup 12, and header plate 11 are finally brazed 15A. The developed subassembly
of feedthrough 13, header plate 11, kovar cup 12 and exhaust port 15 are
mounted inside hollow header cup 16 which is again brazed 168 with the groove 17 made
.in the base of cathode flange 3.
Before creating the plasma discharge, the developed LARM-PSS has been evacuated up
to base pressure Torr with the help of appropriate pumping system. The chamber is
flushed out with inert gases in controlled manner with the help of mass flow meter through
connecting lines. The chamber has been evacuated again up to .the base pressure which
is being monitored by the pressure gauge and the gases are controlled by the needle
valves. The hydrogen or argon gas pressure has been maintained below the self-
. breakdown condition requirements for these gases at fixed inter-electrode gap against
corresponding voltages. .With appropriate discharge power and pulsed trigger power,
plasma discharge has been created inside the hollow cathode. The discharge in the
LARM-PSS h as be en repeated for various ranges of applied voltages and operating
pressures. Typically these are 5-20 kV applied voltages and corresponding operating
pressures 0.05-0.01 millibars.
!
' Referring to FIG. 2, which shows the half-sectional view of hollow cathode 1 and hollow
anode cup 2,4 electrode structure along with the linear apertures for the geometrical
construction of the LARM-PSS. The. linear apertures . 6A and 6B are put in place in the
hollow cathode 1 and anode cylinder 4, respectively.
Referring to FIG. 3, which shows the two dimensional view of hollow cathode-anode
geometry in FIG. 3 (a) and cross-sectional view at the linear aperture cut for the LARMPSS
in FIG. 3 (b). Th e array of four linear apertures 6A and 6B in the interfacing
electrodes 1 and 4 are perfectly aligned to each other40 allow the plasma discharge to
pass from hollow cathode to hollow anode in the form of sheet of discharge current.
Referring to FIG. 4, which is the half cut view of the trigger system 9, acts as a free
electron emission source. In this configuration a high dielectric constant ferroelectric disc
9D is sandwiched between molybdenum made electrode 9A containing finger contacts 96
and another stainless steel base metal electrode 9C placed over a ceramic made metal
holder 9E. he ferroelectric disc 9D in the trigger ,system operates on the basis of field
,emission based electron generatio'n and provides seed electrons. The amount of the seed
electrons can be controlled by varying the potential difference across the negative
potential finger contacting electrode 9A and ground potential based electrode 9C.
Typically voltage has been varied between -1 kV to -5 kZ/ in the.experiment.
Referring to FIG. 5, the characterizatiori setup for the developed LARM-PSS deals with the
an.alysis of the switch in-terms of pulse waveform parameters. A 800-nf capacitor bank 19
in series with the LARMLPSS has been used for the test. The capacitor is charged through
the large charging resistance 20 and stored energy is dissipated in ground load 21 by
shorting the switch. Voltages have been measured with a 1000:l high voltage probe
(Tektronix P). The discharge current is measured using Pearson's current transformer of
200 kA peak current. The cable path length in the current discharge circuit has been used
shortest, so as to minimize the wire inductance. The breaded copper sheets'have been
used as connecting wires for this purpose., The signals are registered with digital storage
oscilloscope 22 (Tektronix.DSA). A 6 Mf2 resistance 20 is used in series with the high
voltage supply to limit the current through the switch and hence isolate the power supply ,
during the conduction. .The LARM-PSS has been initially evacuated down to 10'~m har
using turbo molecular vacuum pump. Initially the virgin switch has been conditioned up to
voltage - 25 kV for more than 24 hours before experimentation. Hydrogen and argon are
separately used as filling gas in the switch which can also be appropriately changed with
Deuterium to incre'ase further higher hold-off voltage.
Self-breakdown characteristics of the switch have been studicd to obtain the uperalirty
ranges of voltage and pressure. The self-breakdown of the switch has been induced by
slowly increasing applied voltage while maintaining the internal gas pressure. As the
applied voltage is further increased, we could observe an abrupt increase of the current
due to a hollow cathode discharge in all the discharge channels. This voltage has been
recorded as self-breakdown voltage. The results of self-breakdown voltages as a function
of internal gas pressure of hydrogen and argon are shown in FIG. 6 (a) and FIG. 6(b),
respectively.
It is seen that the self-breakdown voltage increases as the internal pressure decreases for
both the gases in the experimental range. Also, the argon shows lower self-breakdown
strength than the hydrogen. To obtain a hold-off voltage above 15 kV, the operating
pressure range of the switch can be selected from 0.02 to 0.05 mbar for hydrogen or
argon.
A typical discharge waveform obtained with 0.02 mbar of hydrogen gas pressure and.20
kV charging voltage is shown in FIG.7. A 800-nF capacitor bank has been used for this
test, which has been shorted through developed LARM-PSS to ground potential. The
capacitor is charged through large charging resistance and stored energy is delivered to
ground during the commutation of the switch. The obtained peak current 'and the
conduction time of the discharge current (see, FIG. 7) are .20 kA and 80 ps, respectively.
An appropriate resistance can also be used to limit the current during the switch operation.
Also, a higher capacity energy storage unit can be used to test this developed LARM-PSS
at higher current rating and hence is not restricted to the presented current-voltage results.
The images of the cathode and the anode surfaces of the LARM-PSS taken after about
1000 shots during the high current test are shown in FIG 8 (a) and FIG. 8 (b), respectjvely. . .
Clearly the arc traces have been observed around the linear slot apertures. However, no
significant arc damage is found near the linear slots. The LARM-PSS shows more diffused .
discharge then their co-axial counterparts and is highlysuitable for high current switching
applications.
The non-obvious. inventive steps of the present invention, which enable realization of the
novelty, are as follows:
1. ~ x t e n d i ?th~e round hole apertures in the form of linear slots ( length
greater than the width in radial channel geometry of PSS) allow increased current handing
capacity without compromising the hold-off voltage of the switch and reducing the surface
erosion characteristics.
2. For spreading the discharge ove'r larger electrode surface area, the radial
multichannel geometry has been adopted. This has enabled the directed discharge current
and the resulting force on the discharge current due to electromagnetic field to interact in
such a.way as to produce the diffused discharge.
3. In terms of instant availability of seed electrons, higher life time and
reduced complexity in the hollow cathode region of LARM-PSS, a ferroelectric based
trigger has been used as trigger source instead of conventional triggers, such as, glow
discharge plasmas, optical triggers, surface flash over, etc. This has enabled to reduce the
switch delay time and also supports to operate the developed LARM-PSS at higher
repetition rate.
The new results achieved due to the inventive steps of the present invention are as
follows:
(a) By fabricating the LARM-PSS, a peak current of 20 kA has been achieved
from a PSS for hold-off voltage 20 kV. The generated current pulse has shown damping
sinusoidal oscillations which is mere due to inductance of the circuit: A proper pulse has
been clearly observed but at lower peak current.
(b) The switch has been operated at varied pressure conditions from 5 Pa to
50 Pa and found to follow well-known Paschen curve.
(c) ~ e ~ e a t eswdit ching of the developed LARM-PSS showed very low jitter of
-20 ns.
(d) Due to the low switch inductance and less delay time (because of the
instant availability of the free electrons provided by the ferroelectric trigger), this switch
also showed very high rise time in the current of - - 100 ns.
(e) After operating the switch for sufficient number of shots (i.e.,-1000 shots),
the electrodes have been dismantled to investigate their surfacc properties. No siyr~iricant
erosion took place on the electrode surfaces except minor marks present around the linear
apertures.
. The following .examples are given by way of illustration of the working of the invention in
actual practice and should not be construed to limit the scope of the present invention in
any way.
Example-I
This example illustrates the controlled parameters for conducting high current
from the developed LARM-PSS, which the other conventional Pseudospark switches and
thyratrons cannot produce due to their inability to handle high rate of current rise or the
peak current. It consists of a hollow cathode sub-assembly placed coaxially inside hollow
anode sub-assembly. A uniform gap is maintained between two sub-assemblies with the
help of Perspex insulator. The Neoprene O-rings have been used to make vacuum tight
assembly. Arrays of hear apertures have been made in the interfacing electrodes of
cathode and anode .sub-assemblies which are radially aligned to each other. The
electrostatic sjmulation using OMNI-TRACK has been carried out to optimize the designed
geometry. The initial approximation of the hold-off voltage by analyzing' the initial field . I
I
distribution in the main gap and field penetration through linear apertures has been verified '
. I I
using simulations.
Moreover, a ferroelectric cathode has been used for field discharge based electron . .
emissions that provides the seed electrons during' the discharge in the designed LARMPSS.
The amount of the seed electrons has been controlled by varying the potential
across the two terminals of.the trigger. The position of the ferroelectric cathode has also
been optimized by varying the height of supporting rod on which it is mounted. The height
has been adjusted for providing seed electrons near to the linear aperture so that the
released electrons come under the influence of penetrated field, instantaneously.
Example-2
This example illustrates the generation of high current pulse through the developed LARMPSS.
Initially, the whole assembly has been evacuated up to base pressure ~ o r r .
Then the chamber is flushed repeatedly with hydrogen or argon gas in controlled manner
with the help of mass flow meter. The working gas pressure is maintained in the range 5-
.50 Pa for the self-breakdown of the gas for corresponding applied voltages 20-5 kV. The
voltage has been applied, through a high resistance 6 Mn, across the gap with the help of
discharge power supply (0-40 kV/3A) for conditioning of the switch. Initially the device is in
the open state and the capacitor bank is charged through large charging resistor 6 Mn. A
trigger pulse (with pulse width -500 ns and pulse voltage - -2kV) has been generated by
trigger supply to release the seed electrons from the ferroelectric cathode. These seed
electrons cause the ionization of the gas under the effect of electric fiel'd present in the
hollow cathode and the stored energy is delivered to ground as soon as a conduction path
is formed in the main gap of the switch. The hollow cathode effect increases the ionization
efficiency and further the discharge plasma in the hollow cathode starts communicating to
the hollow anode through the linear apertures and main gap. At this stage the voltage
across the gap start falling and current starts rising as required by the external circuit. For
very high current, which the plasma cannot communicate, the requirement is met by metal
vapors released from the cathode surface near to linear aperture where electric field
density is very high.
For very high current applications, the inductance of the circuit needs to be minimized for
getting the desired pulse parameters. A parallel combination of flat silver plates
arrangement has been developed for this .purpose. Furthermore, this developed LARMPSS
is inherently able to have low inductance due 'to multichannel and spread discharge
in the form of linear sheets.
For fast switching ,of this LARM-PSS, small recovery time is required. This is possible by
means of fast recombination of charge particles. The more surface area over the
electrodes due to radial arrangement of discharge zone in this source provides self-
I consistently base for fast recombination and hence fast recovery to the switch. . .
The self-breakdown characteristics of the switch have been studied and plotted for argon
l and hydrogen gas at different operating gas pressures. Also, the argon has shown lower . . ~ self-breakdown strength than the hydrogen at same gas pressure conditions. To obtain a
I hold-off voltage above 15 kV, the operating pressure, range of the switch has been
I selected from 0.02 to 0.05 mbar for hydrogen. A 800-nF capacitor bank has been charged
, . through large charging resistance and stored energy is delivered to ground via flat plate
transmission line offering lower inductance in the path of current buildup. During the
conduction phase a peak current of 20 kA for 80 ps has been achieved with effective
charge of approximately 1.5 C through the electrode gap. Equivalent total inductance of
the test circuit estimated from the oscillations present in the current waveform is 800 nH.
The other switching parameters like rise time, jitter and delay time have also been
measured. The observed current rise time is 110 ns with rate of current rise -1 0'' Als for
this switch, which makes it suitable for strategic pulse power generations. The observed
mean delay time from the initiation of the trigger to the breakdown of the gas in the gap is
found to be 108 ns: The jitter has been measured at repetition rate of 0.04 Hz for 25 shots
keeping same applied plasma discharge conditions. The, observed jitter in the discharge
voltage and current are 30 ns and 20 ns, respectively for full operation of the switch at the
specified operating conditions.
' .
The. main advantages of the present invention are: ,.. I
' I
1. It give's a new design of the switch for generation of high current r 10 kA pulse and I
even can go more than 100's of kA pulse i.e. quite useful for various strategic and I
I
induskial applications.
2. The presented design of electrodes provides uniform distribution of the current
around the electrodes.
3. Inspection of the electrodes after test of several hundreds of shots showed no I
serious mark.of erosion.
4. The developed LARM-PSS has high life time due,to lesser electrode erosion.
5. Because of the simultaneous discharge through multi-channels in the form of
. current sheets, this switch has showed high rise time in the current due to its low
inherent inductance.
6. There is huge interest in India in pulse power technology including accelerators,
dense plasma focus, modulators, X-ray sources, LASER sources, etc., which
require high current conduction above 10s of kA but the conventionally available
plasma switches, such as, thyratrons, round hole and co-axial Pseudospark
switches, and also s'olid state semiconductor switches are.not able to meet the
requirement of high current applications r 20 kA. However, the developed LARMPSS
has the capability of conducting high current 2 20 kA and can fulfill the need of 7
the high current switching requirement worldwide.

We claim:
1. ' A 20 KVI 20 KA Linear Radial Multichannel Pseudospark Switch (LARM-PSS)
which comprises of:
A cup shaped hollow cylindrical cathode electrode (1) brazed (IA) with cathode
flange (3) to form cathode sub-assembl\/, a cup shaped hollow cylindrical anode
electrode (2) and a hollow electrode cylinder (4) which are brazed (4A) with a
bottom hollow disc ( 5 ) to form anode sub.asscmbly, the anode and the cathode
sub-assemblies placed radially symmetric on the common central axis, four linear
aperture slots (6) arranged around the periphery of the anode (4) and cathode (1)
electrodes facing each other to allow optimum plasma discharge so as to operate
from hollow cathode (1) to hollow anode (2), a uniform gap (7) maintained between
cathode and anode sub-assemblies by placing a Perspex insulator (8) between
the two sub-assemblies, a high dielectric constant trigger sub-assembly (9) ..
supported over variable threaded rod (10) placed inside the hollow cathode (1) to
provide the initial free electrons to trigger the discharge in the hollow cathode
(I.), the supporting rod (10) fixed on header disc plate (11) which is placed and
brazed (118) over an inverted kovar cup (12), external electrical connections
provided to the trigger unit (9) through feedthrough (13) (with pin electrode (14))
brazed (13A) over the kovar cup (12), an exhaust port (15) inserted through kovar
cup (12) and header plate (11) along with its brazikg (ISA), feedthrough (13),
header plate ( I I ) , 'kovar cup (12) and exhaust port (15) that are mounted inside i
hollow header cup (16) along with their brazing (16A) with the groove ('l7) made
in the base of cathode flange (3), an 800 nF capacitor bank (19) in series with the
LARM-PSS that has been used for the test, large charging resistance (20) for
charging the capacitor, the stored energy .dissipated in ground load (21) by shorting
the switch and the digital storage oscilloscope (22) for reading and recording of the
signals registered.
The LARM-PSS switch as claimed in claim 1, wherein the developed switch has
conducted effective charge up to 1.5 C with current capacity 20 kA at 20 kV hold-off
voltage keeping gas pressure of hydrogen 0.02 mbar.
3. The LARM-PSS switch as claimed in claim 1, wherein the cylindrical cathode
electrode (1) has outer diameter 58 mm and height 40 Am.
4. The LARM-PSS switch as claimed in claim I, wherein the cylindrical anode electrode
(2) has inner diameter 94 mm and height 39 mm.
5. The LARM-PSS switch as claimed in claim 1, wherein the hollow electrode cylinder
(4) has outer diameter 70 mm and height 37 mm.
6. The LARM-PSS switch as claimed in claim 1, wherein the: width of the linear
aperture slots (6) is optimized as 2.5 mm.
7. The LARM-PSS switch as claimed in claim 1, wherein a uniform gap (7) of 3 mm is
maintained between cathode and anode sub-assemblies by placing a Perspex
insulator (8) of thickness 12 mm between the two sub-assemblies.
8. The LARM-PSS switch as claimed in claim 1, wherein the trigger subassembly
(9) consists of a high dielectric constant ferroelectric disk (9D) made
from LiBaTiOs which is enclosed between molybdenum electrode ( 9 ~ )
containing finger contacts (9B) and a stainless steel metal base plate (9C) placed
over a ceramic made metal holder (9E).
9. The LARM-PSS switch as claimed in claim 1, wherein said transmission line
comprises of a low inductance flat transmission line.
10. A method of generating a high current pulse using LARM-PSS as claimed in
claim 1, comprises of:
VI. evacuating thewhole assembly up to a base pressure of 1 0 - ~ ~ o r r ;
VII. flushing the chamber with hydrogen or argon in controlled manner. with the
help of mass flow meter;
VIII. maintaining the working gas pressure in the range 5-50 Pa for the selfbreakdown.
of the gas;
IX. applying voltage in the range of 20-5 kV through a high resistance 6 Mf2,
across the gap with the help of discharge power supply in the range of 0-40
X. generating trigger pulse with pulse width -500 ns and pulse voltage - -2kV by
trigger supply to release the seed electrons from the ferroelectric cathode
which causes the ionization of the gas under the effect of electric field
present in the .hollow cathode and stored energy is delivered to ground as
soon as a conduction path is formed in the main gap of the switch.