Note: Descriptions are shown in the official language in which they were submitted.
8 / BACKGROUND
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g This invention is directed to improvements in a
~ 10 crossed-field switch device whereby the crossed-field
11 switch device can be onswitched with voltage applied without
12 pulsing the magnetic field to.a high ~alue.
13 In recent years crossed-field switch devices
14 have been.developed into tubes which are capable of conducting
15 fairly hi~h currents and are capable of offswitchln~ a~ainst
; 16 high volta~es. Crossed-field switch devices havin~ a 10, oon
: 17 ampere DC conducting capability and an offswitching capability
18 against 100 kilovolts have been desi~ned. Such switch devices
1~ are believed to have a considerable prospect in the developing
ield of hi~h power electric transmission by means of direct
21 current links. Such crossed-field switch devices do not have
22 long term conducting capability, and thus must ~e paralleled
23 by an in-line switch during normal line operation. When it
24 is desired to open the circuit, the in-line switch is opened,
so that it shunts the current through the crossed-field swi~ch
b.s.
~ 26 device which is thereupon turned off. ~ Patent RE27557 illustrates
27 this type of circuit breaker which incorporates a crossed- :
: 28 field switch device as an off-switchin~ component.
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1 There have been a number o developments in the art
2 o the cros~ed~ield switch device which have brought it to
3 this state of utility. Among the background patents on the
4 crossed-field switch devices are U.S. Paten~s 3,638,061,
3,641,384; 3,604,977; 3,558,960, 3,678,289; 3,769,537 and
6 3,7~9,978.
7 Of course it is necessary to ~nswitch the crossed-
8 field swi~ch device when current flow therethrough is required.
9 Under the right conditions of applied voltage and magnetic
field, initial ionization can come a~out by the action of
11 cosmic rays. However, in order to reduce the statistical
12 reliance on such events, ignition devices can supply the
13 preionization, (see~patents 3,714,510 and 3,890,520). These
14 patents covering ignition equipment are useful to reduce
the ignition time delay whenever the cond~tions are within
16 ~he conductive region of the Paschen curve.
17 Sometimes it is desired to make the crossed-field
18 switch device conductive, that is onswitch the switch device,
19 when the rated line voltage is applied thereacross. This has
formerly been done by pulsing the magnetic field sufficiently
21 high that even with voltage applied, the conditions in the
22 interelectrode space move in~o the conductive region. For
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23 example seeApatents 3,678,289 and 3,604,977.
However, the firing or ignition of a crossed-field
switch device at high voltage using a high pulse magnetic
26 field has several disadvantages. Thera is a tlme delay
27 rom the trigger ~o the ignition of the crossed~iald switch
28 device in the order of 10 microseconds. Furthermore, there
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1 is significant jitter in ignition, `in the order of 1 micro-
2 second. Additionall~, ik is difficult to obtain shor~
3 duration, high level magnetic pulses in the order of
4 0.1 Tesla or 1 K Gauss in crossed-field switch devices due
to the eddy currents created in the electrodes. Also,
6 a high powered magnetic field pulser is a difficult and
7 expensive device, particularly at a high pulse repetition
8 frequency. These requirements also create a more complicated
9 and expensive tube construction which oten includes an
internal magnetic field ~oil to minimize the above listed
11 detrimental ef~ects. Thus, it is highly desirable ~or a
12 crossed-field switch device to be ignited at high ~oltage,
13 in the order of 10 to 100 kilovolts, with a relatively low
14 magnetic ~ield, in the order of 0.01 Tesla, or 100 Gauss.
16 SUMM~RY
17 In order to aid in the understanding of this
18 invention it can be stated in essentially summary form
19 that it is directed to a gridded crossed-field switch device.
The crossed-field switch device has anode, cathode and
21 control electrodes which define a main gap and an ignition
22 gap. The control electrode is connected through an Lmpedance
23 to one of the other electrodes to prev~nt hollow cathode
24 discharge in the control gap~ The control gap can be
pulsed to ignite and the plasma passes through the inter-
26 mediate electrode into the main gap to cause its ignition
27 in~o the low pressure glow mode.
28 I~ is thus an obJect of this invention to provide
29 an improved gridded crossed-field switch which can
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1 be onswitched while high voltage is applied without the
2 need for a large magnetic field pulse. It is another object
3 to provide a crossed-field switch device which has three
4 electrodes, which define an ignition gap and a main conducting
gap so that ignition can be accomplished within the ignition
6 gap. It is a further object to provide a connection of
7 proper impedance between the control electrode and another
8 electrode to permit pulsing of the control electrode for
9 ignition and to prevent the initiation of hollow cathode
discharge in the control gap. It is another object to
11 provide an intermediate electrode which is of gridded
12 construction with openings directly therethrough so that
13 the ignition gap and main gap are in line of sight. It
14 is a further object to provide a three electrode crossed-
field switch device in which the main gap can be ignited
16 to operate in the crossed-field discharge mode without
17 requiring a pulsed magnetic ~ield.
18 Other objects and advantages of this invention
19 will become apparent ~rom a study of the ~ollowing po~tion
of the speci~ication~ the claims and the attached drawings.
21 More particularly there is provided a crossed-field
22 switch tube comprising a tubular anode electrode and a coaxial
23 tubular cathode electrode spaced from said anode electrode so
2~ that the active areas of the two electrodes define a main
discharge gap therebetween having a continuous closed path,
26 and means for making electrical connections to said anode and
27 said cathode electrodes and means for providing a gas at sub-
28 atmospheric pressure in said main discharge gap and a magnetic
29 field above the critical value in said main discharge gap so
that a glo~ mode plasma discharge can take place between said
31 anode and said cathode electrodes for conduction of said switch
32 device, characterized in that there are provided a tubular
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1 control electrode positioned adjacent to one of said anode and
2 cathode electrodes to form a continuous closed path ignition
3 discharge gap adjacent to, substantially over the whole active
4 area of and in communication with said main discharge gap, the
tubular electrode positioned between the main gap and the
6 ignition discharge gap having openings therethrough so that
7 plasma can pass ~rom the ignition gap to the main gap, and
8 means for electrically pulsing said control electrode with
9 respect to its adjacent electrode on the other side of the
ignition discharge gap for establishing conductive conditions
11 therein so that glow mode plasma discharge begins in the
12 ignition discharge gap and enters into substantially the entire
13 active area of the main discharge gap to cause conductive
14 condition of said main discharge gap over all of its area so
that said crossed-field switch device can be turned on with
16 voltage applied thereto without magnetic field pulsing and with
17 uniform wear of said anode and cathode electrodes over their
18 entire active areas.
19 BRIEF DESCRIPTION OF THE DRAWINGS
; 20 FIG. 1 is a longitudinal section through the gridded
r 21 crossed-field switch device of ~his invention.
22 FIG. 2 is a perspective view of one of the electrodes.
23 FIG. 3 is a schematic electrical diagram showing
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23 the connections of the gridded crossed-field switch device.
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1 PIG. 4 is a crossed-field breakdown curve relating
2 the conditions in the interelectrode gaps to the conductive
3 region of the breakdown curve.
4 FIG. 5 is a series of graphs showing various parameters
in the gridded crossed~field switch device versus time.
7 DESCRIPTION OF_T~I~ PR~F~RRED R~IBODIMENT
8 The gridded crosse~-field switch device of this
g invention i5 generally indicated at 10 in FIGS. 1 and 3. In
; 10 the schematic drawing of FIG. 3, crossed-field switch device 10
11 is shown as having an anode electrode 12, a cathode electrode 14
1~ and a control electrode 16. As will be described below, the
13 physical arrangement of the electrodes may differ, with the
1~ control electrode between the cathode and anode or even inter~
~ 15 iorly of the anode, but the preferred arrangement is as shown
i 16 in ~IG. 3. These concentric electrodes define an annular space
~ 17 or gap 18 between the anode and cathode, with the annular
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1~ space 18 serving as a main ~ap in which the main glow discharge
g is formed during conduction. Cathode electrode 14 and control
electrode 16 define between them outer annular space 20 which
21 is the gap in which the ignition dischar~e is formed. Space 18
22 and space 20 are each in the order of 1 centimeter in the radial
23 direction. While the crossed-field switch device is shown as
24 being circular, because that is a convenient construction,
other geometric constructions are also feasihle, so that the
26 gap may not be a circular annulus but may be another shape
27 such as a hollow rectangle or a hollow square. Cathode
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28 electrode 14 has openings therethrough so that the plasma of
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l a glow discharge in one of the spaces can enter into the other
2 space. Since cathode electrode 14 is positioned between
3 anode electrode 12 and contxol electrode 16, it is n~cessarily
4 open so that the plasma can pass from the ignition gap to
the main gap. The intermediate electrode has openings there-
6 through which provide line of sight visibility between the
7 ignition gap and main gap.
As is seen in FIG. 3 the positive side of generator 22
9 is connected by line 24 to anode 12, while the negative side
of generator 22 is connected through load 26 by line 28
ll to cathode 14. Thus~ the turning on and off of a low
12 pressure glow discharge in the main discharge gap 18 turns
13 on and off current fxom the generator through the load.
14 O course FIG. 3 is highly schematic, and the generator 22
represents any source of direc~ current. Furthermore,
16 the switchin~ on and off of the current through the load
17 is not usually accomplished only by the gridded crossed-
18 field switch device lO but also by parallel in-line de~ice
19 through which the cuxrent passes during long running periods.
Control circuit 30 has line 32 connected to cathode
21 electrode 14 and line 34 connected to the control electrode 16.
22 Resistor 36 is connected between lines 32 and 34 so that unless
23 an additional voltage is supplied, controi electrode 16 is at
24 the same potential as cathode 14, by the leakage through the
resistor 361 When resistor 36 is of low value, the electrodes
26 can carry substantially the same potential and a hollow
27 cathode discharge will continue in gap 20 so device lO
~8 canno~ be turned off. When resistor 36 is of too high a
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1 value the potential of the control electrode 16 can not be
2 properly established prior to the initial start of the
3 magnetic field pulse and spurious turn on may result~
4 A resistance value between 1 kilohm and 1 megohm typically
satisfies both of these requirements~ Control circuit 30
6 permits voltage pulsing of control electrode 16 with respect
7 to cathode 14. As an example, capacitor 38, capacitor
discharge rate limiting resistor 40 and switch 42 are
g serially connected between lines 34 and 32. Another type
of pulse supply can be substituted. A p~llse on the order
11 o 1 kilovolt is typical but voltages as low as 300V are
12 feasible.
13 FI~,. 1 shows in more detail the structure of the
14 gridded crossed-field switch device 10. Outer vessel 44 is
the structural enclosure, tank or envelope which encloses the
16 electrodes and their insulated supports. Vessel 44 is vacuum
17 tight with connection 46 permittin~ drawing of a vacuum on
18 the vessel and maintenance of a proper gas supply therein at
19 a proper pressure. Usually helium at about 50 millitorr
is a suitable atmosphere for the low pressure glow crossed-
21 ield discharge. Vessel 44 is at cathode potential. It
22 supports cathode 14 on support ring 48 downwaxd from top
23 cover S0. Cathode electrode 14 is formed as a gr~d or
24 perforated electrode. FIG. 2 shows cathode 14 as being
formed of an upper ring Sl, lower ring 52 and bars 54
26 connected therebetween to form an electrode in the form
27 of a squirrel cage. The squirrel cage intermediate electrode
28 provides a line of sight between the two gaps so that plasma
29 from the ignition gap can easily pass into the main gap.
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1 Anode 12 is downwardly sup~orted from upper ring 50
2 on insulator 56 and is exteriorly connected by lead 58 which
3 is connected to line 24. Insulator 56 is the major insulator
4 which holds off the main anode to cathode voltage. Insulator
56 is supplied with field shaping electrodes 60 and 62 to
6 prevent glow discharge breakdown in that region.
7 Control electrode 16 is supported beneath
8 cathode 14 on insulators such as the ones shown at 64 and
9 66. Control electrode 16 is connected by lead 68 which
passes through a vacuum tight insulator 70 with respect to
11 vessel 44.
12 Ionizer 72 is mounted to inject electrons into the
13 outer annular space 20 and its lead comes out through the
14 base by means of a vacuum tight insulator 74. Ionizer 72
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can be of the type shown in patent 3,890,52Q.
16 In accordance with the physical mechanism by which
17 the discharge operates, an axial magnetic field is provided
18 by solenoid 76. The magnetic field has a value of Bo as shown
19 in FIGs. 4 and 5, which is on the order of 100 ~auss for an
interelectrode spacing of 1-2 centimeters and with a helium
21 ~as filling at S0 millitorr. The magnetic field can be
22 pulsed below the critical value Bc, (see FIGs. 4 and 5)
23 which is the leftward end of the toe of the crossed-field
24 breakdown curve 78 which separates the conductive region
80 from the nonconductive region 82.
26 The function of this equipment is best described
27 by going through an on-off cycle. Referring to FIG. 5, at
28 time to~ 9 kilovolts is first applied to the main gap 18
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1 and a magnetic field of strength Bo is then applied to
2 both gaps. This places the operating condition of the
3 main gap at a point 84 in FIG. 4. This is i.n the nonconductive
4 region and therefore no current is being conducted by the
gridded crossed-field switch device 10. Since control
6 electrode 16 is connected to cathode electrode 14 through
7 leak resistor 36, the voltage of the control electrode C
8 with respect to the cathode K is zero. This places the
9 operating condition of the control gap at point 89 in FIG. 4.
When on-switching is desired, for example at
11 point tl, switch 42 is closed to impress a pulse between the
12 cathode K and the control electrode C to bring the voltage
13 therebetween into the conductive region of the crossed-
14 field breakdown curve, for e~ample to point 86 in FIG. 4
which represents about 1 kilovolt between the cathode and
16 the control electrode. Under these conditions, a discharge
17 in the control gap initiates~; Wi.th that gap conducting
18 in the low pressure glow discharge mode, the discharge
19 plasma seeps through the transmissive cathode structure
to cause conduction of the main ,gap 18. The mechanism
21 by which the conduction starts is not clearly understood,
22 because under the initial conditions the gap is in an
23 unconducting region at point 84. However, the presence
24 of glow discharge plasma, which does not seem to need to
be sufficient to reach across the entire gap, starts the
26 main gap discharge. With the beginning o~ that discharge,
27 within microseconds after the on-pulse, and thus both indicated
28 at time tl in FIG. 5, the main gap conduction current in-
29 creases to 140~ and the voltage across the main gap decreases
to,the discharge voltage drop, typically SOOV.
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'! 1 Conduction continues either to exhaustion of t2le
2 interelectrode gas, or until magnetic field is suppressed.
3 As is seen in the bottom curve in FIG. S, the voltag~ across
4 the ignition gap quickly falls to a lower valuel due to con-
duction through impedance 36, and the dis~pation of the
6 charge on capacitor 38. Conduction continues to take place
7 from the anode to both the cathode and the control electrode,
8 Off-switchin~ is then accomplished by pulsing the ma~netic
; g field below Bc. This pulsing appears at time t2 in FIG. 5
and moves the operating point to point 88 which is in the
11 nonconductive region 82 of the srosse~-field breakdown curve
12 of FIG. 4. This causes termination of the main gap discharge
13 with a buildup of voltage between the anode and cathode and
14 between the cathode and the control electrode and a termination
of the current flow therebetween. Finally current flow
- 16 through resistor 36 ceases and the control electrode comes
17 to the potential of the cathode and the magnetic field is
18 then reapplied. In this way the on-of cycle is completed.
~ 19 The squirrel cage type o~ structure shown in ~IG. 2
-; 20 LS an example of a preferred structure. Basically, cathode
21 electrodP 14 must have sufficient material to develop the
22 electric field and participate in the -discharge. However,
23 it must be open enough to permit the glow discharge plasma
24 to seep through from the outer ignition discharge gap to the
main gap. A perforated tube can accomplish the same result.
26 The preferred maximum open area has not yet been experi-
27 mentally established, however the cathode electrode 14 of
28 about 30 percent open area has been found to be feasible.
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1 In the device actually redùced to practice,
2 resistor 36 wa`s about 1 kilohm and such a resistor seemed
3 to be necessary to produce the correct discharge. FIG. 5
4 illustrates the maximum voltage applied as being 9 kilovolts,
but the limitation to this value was introduced by the
6 isolation resistors between the electrodes rather than
7 a fundamental limitation of the physical process. In fact,
8 ignition at 50 kV has been performed in the above described
9 manner.
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