Abstract:
A spark gap switch for controlling the output of a high voltage pulse from a high voltage source, for example, a capacitor bank or a pulse forming network, to an external load such as a high gradient electron gun, laser, pulsed power accelerator or wide band radar. The combination of a UV laser and a high vacuum quartz cell, in which a photocathode and an anode are installed, is utilized as triggering devices to switch the spark gap from a non-conducting state to a conducting state with low delay and low jitter.

Description:
GOVERNMENT RIGHTS 
     This invention was made with government support under Grant No. DE-FG03-02ER83402 awarded by the U.S. Energy Department. The government may have certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention provides a high voltage spark gap switch controlled by a laser-photocathode subsystem. A double-triggering mechanism is used for reliably closing the spark gap switch while keeping the low jitter properties of the laser triggering. 
     2. Description of Prior Art 
     High voltage switch is one of the elementary devices employed in pulsed power techniques. Ideally, it controls the flow of current in a circuit in two states: either the current flows at a value determined by the other components in series with it, or the current does not flow at all. There are many considerations when choosing a switch; for example, large current and/or voltage handling capacity, compact size, price, durability and reliability. A switch with minimized delay and jitter is also needed in many applications. 
     A spark gap is one of the most widely used switches. It is relatively simple to build and easy to operate and its application range is flexible. It can conduct current from a few tens of amperes to multi-mega amperes and it can also withstand voltages up to several megavolts. The basic spark gap usually consists of two current-carrying electrodes separated by a gap filled with isolating medium, which is made to break down by overvolting the gap or by some other means such as applying a triggering pulse through a third electrode, i.e. a trigger electrode, injecting an electron beam or shining an optical beam into the gap. 
     Different methods of inducing electrical breakdown in the spark gap have their own advantages and drawbacks. For example, the electrical pulse triggered switch is comparably simple, but it needs a separate electrical pulse generator, and the delay and jitter of the breakdown induced by this method are relatively larger. For precise timing and synchronization, the laser-triggered switch has been extensively studied in the past several decades. Though great progress has been made, some problems remain for the laser-triggered switches. One of them is how to make use of laser optical energy efficiently because many mediums used in the spark gap are transparent and their absorption coefficients are rather low to the photons generated by a common laser source whose wavelength is in ultra-violet (UV) spectrum or longer. For example, SF 6  and N 2  are two of the most often used gaseous mediums in spark gap switches. But neither SF 6  nor N 2  under standard conditions has an absorption coefficient higher than 0.002/cm for the photons with a wavelength of 186 nm. It means that the ratio of the photons absorbed by the gases to the total photons in the laser beam is less than 6% after the laser beam with the wavelength passes through a 30-cm-long gas channel. Moreover, the number of absorbed photons tends to decrease with the increment of the photon wavelength in UV spectrum. Therefore, most of photons in the laser beam passing through a common gas gap whose length is generally less than 30 cm simply waste their optical energy. For this reason, a high-energy laser system is needed to trigger a traditional high voltage spark gap switch, thus incurring a very high cost for the laser system. Otherwise, the delay time and the jitter of the switch will be adversely impacted. 
     One of the purposes of this invention is to seek a viable method to convert the leftover optical energy of the laser beam to actual triggering energy in order that the laser energy can be utilized efficiently. In the mean time, depressing the jitter of a spark gap switch as well as closing it reliably is the concern of current invention, too. Further objects and advantages of the invention will become apparent from a consideration of the drawings and ensuing description. 
     U.S. Pat. No. 5,057,740 to George et al. describes the use of photoelectrons to trigger a backlight thyratron switch. The switch exposes photoemission materials having very low quantum efficiency directly to a low-pressure gas. The working range of their switch is limited to only the left hand side of the Paschen curve. The lifetimes of the photoemission materials are also limited. Furthermore, the medium in such switches can only be gases. 
     SUMMARY OF THE INVENTION 
     The present invention incorporates the advantages of the electrically triggered spark gap switches, which are relatively simple and easy to operate, with the merits of the laser triggered spark gap switches, which have less breakdown delay and jitter and which can be decoupled from the main switching circuit. It provides a high voltage switch that can be operated reliably through a double triggering mechanism. Such a switch will make use of the laser optical energy more efficiently. 
     The present invention utilizes a photocathode to collect the leftover optical energy of the laser beam in a laser triggered spark gap switch and convert the energy to photoelectron emissions. After the emitted photoelectrons are collected by an anode, they are used as additional electrical triggering power in a similar way as the conventional electrical triggering spark gaps. In this way, the laser energy is not only utilized more efficiently, but it also provides a second triggering power to secure the closing of the switch. Compared with the switches using laser beam triggering only, the switch in the present invention is expected to have less delay and jitter because more triggering energy is fed back into the spark gap. 
     The switch in the present invention consists of an ultra-violet (UV) laser system and a housing, in which all components of a spark gap and a high vacuum transparent cell are installed. The spark gap comprises a plurality of main electrodes and trigger electrodes, no external electrical triggering pulse generator being required. A first laser beam enters the switch housing through a window on the sidewall for the initial triggering of the spark gap. A second laser beam, which can be the continuation of the leftover of the first laser beam, enters a small transparent cell containing a photocathode and an anode. The transparent cell is made of a durable material such as quartz. It is pumped to high vacuum before being sealed and is installed beside the two main electrodes of the spark gap and is located in the opposite side to the laser window relative to the gap region. The photocathode and the anode are electrically connected to a lower-potential main electrode of the gap and to the trigger electrode of the switch respectively. 
     In operation to close the switch, the first triggering pulse, the laser beam, is incident into the gap medium region where it ionizes a portion of the medium. After the beam passes through the region, it enters the vacuum cell and is incident on the surface of the photocathode. The beam will generate a great number of photoelectrons. Under the action of the electric field, the photoelectrons move toward the anode and are collected by it. The photoelectrons produce a voltage between the trigger electrode and the higher-potential main electrode. Depending on the capacitance between the two electrodes, the voltage can be so high that it acts as a second triggering pulse to cause a fast breakdown first in the gap between the trigger electrode and the higher potential main electrode, and finally a major breakdown in the gap between the two main electrodes. 
     Two embodiments of the switch are disclosed in the present invention. The first embodiment is a trigatron type gap switch combined with a triggering laser. In the switch, a trigger electrode is located in the axis of the higher-potential main electrode. The second embodiment is a field distortion type gap switch combined with a triggering laser. In this switch, two electrically connected trigger electrodes are placed at opposite positions relative to the axis of the two main electrodes and both of them are near the higher-potential main electrode. Both embodiments need isolating medium to prevent a stochastic voltage breakdown and both of them adopt high quantum efficiency photocathodes. A UV laser system, e.g. Q-switch Nd:YAG laser operating at its fourth harmonic of 1064 nm, is also required to provide the triggering pulse. To diminish the jitter of the entire switch, the laser system should have a very low jitter. Commercial laser systems have the ability to control jitters on the order of picoseconds or lower. 
     For both embodiments, a moderate energy laser system is sufficient to provide the energy and the amplitude of the photoelectron triggering voltage pulse needed to induce electrical breakdown in the gaps. 
     The switch of the present invention can be used in pulsed-power accelerators, weapon effect simulators, fusion research devices, lasers and synchronizable high voltage pulsers. These systems require spark gap reliability, fast energy transfer rates, and low jitter. For example, a high-gradient dc/rf electron gun in accelerator research field needs a low jitter pulser in order to synchronize the voltage pulse with its electron bunch extractions whose durations last only several picoseconds. A high voltage pulse of short duration produced using the switch in the present invention can be applied to the acceleration gap of the pulsed dc gun. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention and further features thereof, reference is made to the following descriptions which are to be read in conjunction with the accompanying drawings wherein. 
         FIG. 1  is a view of the first embodiment of the present invention; 
         FIG. 2  is the magnification of the main spark gap portion for the first embodiment; 
         FIG. 3  is a model for the first embodiment to calculate the properties of the photoelectron triggering pulse; and 
         FIG. 4  is a perspective view of the second embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , the first embodiment of the switch of the present invention is illustrated. It comprises an UV laser coupled with optical system  18  and a gas-tight housing  10 . 
     The UV laser system need have a short pulse width, e.g. on the order of hundreds of picoseconds or shorter. Under this circumstance, all of its pulse optical energy can be output in a very short time. Such an UV laser system can be found from common commercial products, too. Numeral  3  in  FIG. 1  indicates the laser beam emitted from the UV laser system  18 . 
     The sealed housing  10  consists of a sidewall  44 , end cover  38  and end cover  46 . It is full of high voltage isolating mediums such as gases, water or even low vacuum. Cylindrical main electrodes  24  and  34  are welded with high voltage ceramic insulators  22  and  36  individually, while the high voltage insulators  22  and  36  are secured on the end covers  46  and  38 , respectively. The main electrodes  24  and  34  are made of a highly conductive and durable electrode material, e.g. brass. Main electrode  34  has a higher potential compared to the main electrode  24 . Trigger electrode  66  is made by a rod and it is set on the axis of the main electrode  34 . A laser window  28  is opened on the sidewall  44 . It is covered by a quartz window plate  26  that is transparent to UV laser beams. 
     There is a high vacuum cell  20  inside the housing  10 . The vacuum cell  20  comprises a photocathode  48  supported by a metallic supporter  52  and an anode supported by the second metallic supporter  58 . Another metallic support  54  not only supports the cell  20  securely, but also connects the photocathode  48  to the main electrode  24  electrically. Opposite to the photocathode  48 , the anode  56  is installed. The anode  56  is also connected to the trigger electrode  66  electrically through the metallic support  58  and a conductive wire  62 , while an insulator  64  electrically isolates the circuit of the trigger electrode  66  from the main electrode  34 . The wall of the cell  20  is made of quartz or any other durable transparent materials that can withstand certain inward pressure. The cell  20  is hermetically sealed, but before the sealing, it is pumped to a vacuum better than 10 −9  torr because only in a high vacuum environment, the photocathode  48  can have sufficiently long lifetime. It is also an advantage to avoid undesirable electrical breakdowns between the photocathode  48  and the anode  56  under high vacuum condition. The preferential materials for the photocathode  48  are those of having high quantum efficiency in UV light such as Magnesium. 
       FIG. 2  magnifies the gap defined by the main electrodes  24  and  34  of the first embodiment that is shown in  FIG. 1 . In  FIG. 2 , main spark gap  74  refers to the space between the main electrode  24  and the main electrode  34 . Trigger gap  72  refers to the space between the main electrode  34  and the trigger electrode  66 . 
     In an operation to close the switch, the laser beam  3  is first directed into the main spark gap  74 , where one portion of the beam optical energy is absorbed by the gap medium and makes some of the medium ionization. However, most energy of the laser beam  3  will reach the surface of the photocathode  48  in the cell  20  since the gap medium that is for holding a high stand-off gap voltage is nearly fully transparent to the UV laser beam, as analyzed in the previous paragraph. The beam will extract a great number of photoelectrons from the surface of the photocathode  48 . Then, the photoelectrons will move toward the anode  56  under the attraction force of the electric field. Those photoelectrons will be collected by the anode  56  finally and, along the metallic connection, reach the trigger electrode  66 , where they will enhance the localized electric field in the trigger gap  72 . Depending on the quantity of the photoelectrons collected and the capacitance between the circuit of the trigger electrode  66  and the main electrode  34 , the enhancement of the field can be so strong that the trigger gap  72  will break down immediately, which, like those of conventional electrical triggering, will further induce a large amount of active ions to the main spark gap  74  and cause the major breakdown in the main spark gap  74  if such a breakdown does not happen yet during the laser beam triggering. This is our so-called second time triggering mechanism. It leads more activating energy into the main gap  74  compared with the laser beam triggering only and thus ensures the occurrence of the major breakdown that closes the spark gap switch reliably. 
     Calculations on one concrete example of the first embodiment were performed. In this example, a Q-switch Nd:YAG laser operating at its fourth harmonic of 1064-nm with pulsed energy at 4 mJ and a pulse width at 100 ps is adopted to trigger a 200 kV switch. SF 6  gas with a pressure at one atmosphere is filled in the switch as isolating medium. The calculations are performed mainly on the following aspects: 
     (1) Total Charge Extracted by the Laser Beam 
     From the analysis in prior paragraph, it is theorized that 80% of laser beam energy, i.e. 3.2 mJ, arrives at the surface of the photocathode  48  finally. There will be 4.29×10 15  photons reaching the cathode per laser pulse because the energy of a single photon is 6.64×10 −19  J for 266-nm light. Mg cathode&#39;s quantum efficiency is around 5×10 −4  at this wavelength. Thus, the number of photoelectrons generated by the residual energy of the laser beam would be 2.14×10 12 . The total charge, Q pe , of the photoelectrons is about −3.43×10 −7  Coulomb. 
     (2) The Lowest Voltage Generated by the Photoelectrons Above Over the Trigger Gap  72   
     The lowest voltage, V pe , produced by the photoelectrons between the main electrode  34  and the circuit of the trigger electrode  66  depends not only on the total charge Q pe , but also on the capacitance between the two electrodes&#39; circuits. The capacitance, C trig , composes three parts: (a) C 1 , a capacitance between the trigger electrode  66  and the main electrode  34 ; (b) C 2 , a capacitance between the anode  56  and the main electrode  34 ; (c) C 3 , the stray capacitance of the connecting wire. 
     C 1  was computed based on the model indicated in  FIG. 3 . The value of C 1  is about 9.44×10 −13  F according to this model. The largest value of the C 2  was estimated to be 6.26×10 −13  F based on a simplified parallel plate capacitor model, in which the radius of the anode  56  is 1.5 cm and the smallest gap between the anode  56  and the main electrode  34  is 1 cm. Actually, C 3  is flexible and can be minimized by several methods such as increasing the radius of the hole that leads wire inside the main electrode  34  and choosing a connecting wire with a proper small radius. After such managements, the value of C 3  for a 4-cm-long wire was estimated to be less than 5×10 −13  F if the radius of the wire is no larger than 1 mm in this example. Therefore, the overall capacitance, C trig , for the circuit of the trigger electrode  66  should be smaller than 2.07×10 −12  F and thus 
               V   pe     =              Q   pe            C   trig       =     165.7   ⁢           ⁢     kV   .               
In  FIG. 3 , the group of curves represented by numeral  70  are electric field potential lines generated by the SUPERFISH code, a prior art program known in the accelerator research field and developed by Los Alamos National Laboratory, which show the distributions of the field around the three electrodes when the V pe  reaches 165.7 kV. The much higher density of the potential lines in the trigger gap than that in the main spark gap indicates that the most possible first breakdown position.
 
     The value of the V pe  above is apparently very high even when it is compared to that of the switching voltage between the main electrodes  24  and  34 , which is 200 kV in this example, although the V pe  can&#39;t be higher than 200 kV because the action of the electric field in the cell. It is one of the indicators which shows the feasibility to use photoelectrons to trigger the breakdown in the trigger gap  72 , even though in reality V pe  may not climb so high because breakdown may occur at any time once V pe  is equal to or higher than the self-breakdown voltage of the trigger gap  72 . To find out the properties of the photoelectron triggering pulse clearly, it is assumed that there is no breakdown in the trigger gap when the following calculations are performed. 
     (3) The Highest Photoelectron Triggering Energy Stored in the Trigger Electrode 
     The highest photoelectron triggering energy, E stored , is calculated as 28.4 mJ by using the stored energy formula of a capacitor, i.e. E stored =C trig ×V pe   2 /2. Compared to the 4 mJ optical energy reaching the surface of photocathode, the stored photoelectron electrical energy in the trigger electrode  66  is much higher. It is evident that the triggering energy stored sources from the field acceleration of the photoelectrons in the gap between the photocathode  48  and the anode  56 . After the acceleration, the triggering agility of the photoelectron current is greatly enhanced. The amplified triggering energy is the second indicator that the photoelectron pulse is viable to trigger a breakdown in the trigger gap  72 . 
     (4) The Longest Delay Time of the Photoelectron Pulse Relative to the Laser Beam Pulse 
     The delay time comprises two parts: (1) the last electron&#39;s transit time from the photocathode  48  to the anode  56 ; and (2) the electric field propagation time from the anode  56  to the trigger electrode  66 . For the first part, an expression of the transit time is derived as below in considering the relativistic effect of the photoelectrons: 
                   t   =           m   0     ⁢   c     eE     ⁢     arccos   (     1   -     eEl       c   2     ⁢     m   0           )               (   1   )               
where t is the transit time from photocathode  48  to anode  56 , m 0  is the rest mass of electron, e is the electron&#39;s charge, c is the light speed in vacuum, E is the electric field, and l is the distance between said photocathode  48  and said anode  56 . The longest transit time is determined by the last photoelectron at the circumstance that it just leaves the photocathode while the potential of the anode  56  is near its lowest, i.e. 34.7 kV, relative to the photocathode  48 . The gap distance is 1.5 cm in this example. In such circumstances, it is found that the longest transit time for the photoelectrons is 276 ps. Since electric field propagates at the speed of light in metal, just like that in vacuum, the field propagation time from the anode  56  to the trigger electrode  66  should be less than 333 ps, supposing that the total length of all of the metallic wire connections is less than 10 cm. So the overall longest delay time for the photoelectron pulse to the laser pulse is less than 609 ps, which is still very fast and can be acceptable in many applications.
 
(5) Minimum Rising Speed of the Trigger Gap Voltage
 
     The voltage, V pe , starts to rise once the laser beam reaches the surface of the photocathode  48  and it ends the rising when the field of the last photoelectron reach the trigger electrode. Therefore, the overall rising time of the trigger gap voltage is consisted of the laser pulse duration time and the delay time of the photoelectron pulse relative to the laser beam pulse. From the data in above paragraphs, we know its value is less than 709 ps. So the minimum trigger gap voltage rising speed is 234 kV/ns. The speed is much faster than those of triggering voltage used in conventional trigatron switches and field distortion switches. The latter ones are comparably difficult to be made higher than 100 kV/ns. The fast rising speed of the trigger gap voltage is another indicator of the feasibility of the photoelectron triggering pulse, too. 
     The calculations above also indicate that the triggering voltage pulse originating from the photocurrent is capable of triggering the high voltage switch by itself, even if the laser beam is not used as the first trigger pulse to pass through the main spark gap  74 . 
     In addition, the number of photoelectrons extracted and the capacitance between the circuit of the trigger electrode  66  and the main electrode  34  are the two critical factors to induce electrical breakdown. To enhance the photoelectron pulse triggering reliability, a photocathode with high quantum efficiency and a small capacitance are desired. 
     Referring to  FIG. 4 , the second embodiment of present invention, a field distortion type switch, is illustrated. Only the relative positions of all key components of the switch are plotted in  FIG. 4  since the second embodiment is similar to the first embodiment in many aspects such as the housing format, the material and position of the high vacuum cell, the support and electrical connections of the photocathode  86  and the anode  88 , and the direction of the laser beam  94 . The differences between the two embodiments are the number and the position arrangement of the trigger electrodes. In the second embodiment, two trigger electrodes  84 A and  84 B are utilized, instead of the only one trigger electrode in the first embodiment, and the two trigger electrodes  84 A and  84 B are set into the main spark gap defined by the main electrodes  82  and  92 , unlike the trigger electrode in the first embodiment, whose position is inside the higher potential main electrode. The two electrodes  84 A and  84 B are set at upper and lower positions of the laser beam individually and are near the higher potential main electrode  92  for triggering the breakdown easily. The advantage of this arrangement is that the trigger gap is a part of the main spark gap, which makes the main spark gap easier to be broken down. Furthermore, a very large portion of the trigger electrode rods in the second embodiment is far from the main electrode  92  except the tips of the rods. This fact is helpful to reduce the capacitances there between and therefore is benefit to raise the trigger gap voltage. 
     While the invention has been described with reference to its preferred embodiments, those skilled in the art will understand that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its essential teachings.