Abstract:
A metal oxide threshold switch (MOTS, e.g., NbOx, x preferably fractionallyess than 2) series connected is used to sharpen the pulse rise to about one or less nanosecond, with a peak up to several kilovolts, and up to 100 amperes or more.

Description:
The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to sharpening the rise time of high voltage, high current pulses. 
     The interest in nanosecond high voltage pulse generations is rapidly increasing. At the present time, they are used in quantum physics, in nuclear physics, in particle accelerators, in x-ray generators, in high-speed photography, etc. The possibility of applying nanosecond generators to solve certain problems in experimental physics such as the production of powerful pulse lasers, strong-current accelerators of charged particles, and fast heating of plasma is of extreme interest. These pulses are also of interest for radar. The state of the art is covered by a book &#34;Formation of Nanosecond Pulses of High Voltage&#34; by G. A. Mesyats, A. S. Nasibor and V. V. Kremnev, translation available from National Technical Information Service, Springfield, VA. 22151, as document No. AD 733 130. 
     A conventional (silicon) solid state device in a pulse-sharpening circuit would, because of its 100 to 1000 picofarad capacitance, cause intolerable pre-switching currents. Pulse-sharpening with gaseous devices is unsatisfactory because of the long and uncertain switch-on delay (several nanoseconds) with these devices. Therefore pulses from conventional power sources for lasars and radars are not sharpened--they have a much longer than optimal risetime. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide shorter rise-times with more time precision (less jitter), while at the same time reducing complexity, cost and size of a pulser circuit. 
     According to the invention, a metal oxide threshold switch (MOTS) is connected in series between a pulse generator and a load. 
     One type of MOTS uses NbOx (x equal to or preferably fractionally less than 2) as the switching material, either as a thin film formed on NbO, or in bulk form. The threshold voltage may be from about 60 volts up to several kilovolts, with a current in the &#34;ON&#34; state of several amperes or more than 100 amperes. The &#34;OFF&#34; state capacitance is only a few picofarads, and the switching time less than a nanosecond. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a block and schematic drawing of an embodiment of the invention; and 
     FIGS. 2A, 2B and 2C are graphs of voltage and current waveforms. 
    
    
     DETAILED DESCRIPTION 
     Further information on the operation of metal oxide threshold switches and some applications is found in two papers by G. Gaule&#39;, P. LaPlante, S. Levy and S. Schneider; the first titled Pulse Sharpening with Metal-Oxide Bulk Switching Devices in IEEE International Pulse Power Conference, November 1976; and the second titled Metal Oxide Devices for Rapid High Current Switching in the 1976 IEDM Technical Digest, pages 279-282, published December 1976. These papers are made a part hereof. They include a disclosure of the invention claimed herein. 
     Metal oxide threshold switches, MOTS, have been used to suppress transient pulses such as induced by lightening, etc. See, for example, U.S. Pat. No. 3,973,224. When applied in that manner, the device is placed in shunt across a transmission line to absorb the transient pulse current, and thereby protect a load device at the end of the line. 
     When a metal oxide threshold switch is subjected to a fast rising pulse from a low impedance source, it goes from a high impedance state (characterized by typically 100 kilohms to one megaohm in parallel with 1 to 4 picofarads) to a low impedance within less than a nanosecond. The voltage then appears clamped to the threshold value, and subsequently drops to a &#34;holding voltage&#34; of typically 20 volts. After termination of the applied pulse, the high impedance state, and the previous threshold voltage value is essentially recovered within typically 6 microseconds. 
     One form of the metal-oxide (MOX) chip is a NbO (a semiconductor, acting as ohmic contact) crystal with a 10 micrometer NbOx layer thermally grown on top by oxidation. (See U.S. Pat. No. 3,962,715.) The stoichiometric value of x is 2, but values lower than that, for example, x equal to about 1.9 have been found preferable for switching material. The thin film form yields threshold voltages from 80 to 600 volts. For higher voltages, single crystal chips of NbOx are used. Threshold voltages are controlled by the thickness of di-oxide and also the deviation from stoichiometry. The current flow in the low impedance state is typically several amperes, and values of 250 amperes have been sustained. 
     Referring now to FIG. 1, the basic circuit comprises a pulse generator 10 supplying pulses to a load represented by a 100-ohm resistor 24. To sharpen the pulses, a metal oxide threshold switch device 20, for example a NbOx device, is connected in series between the source and the load. 
     In an experimental arrangement, a Cober pulse generator 10, type 605-P, was set to produce a slow rising pulse (100 nanoseconds). The reversing switch 12, transformer 14, and 1500-picofarad capacitor 16 may be included as a part of the pulse source. To observe the results an oscilloscope, type Tektronix 7904 and a 10-ohm resistor 22 are connected as shown. 
     The applied voltage from the pulse source is shown in FIG. 2A. The effectiveness of pulse sharpening is tested by replacing the MOTS device 20 with a short. In that case, the shape of the current pulse resembles that of the applied voltage (FIG. 2B). Insertion of the MOTS device 20 causes sharpening of the current pulse, as shown in FIG. 2C. The current rise time is thus reduced to a few nanoseconds. During repetitive pulsing there is some variation in the time of the onset of the sharp rise of the current (jitter). However this &#34;time jitter&#34; can be quite small compared to that obtained with prior high voltage, high current switching devices.