NANOSECOND LASER-TRIGGERED MICROWAVE SWITCH∗ Matthew M. McQuage, Andreas A. Neuberξ, James C. Dickens Texas Tech University Center for Pulsed Power and Power Electronics Lubbock, TX 79409-3102
Abstract The design and experimental testing of a laser-triggered microwave switch with a nanosecond activation time is described. The objectives of the project include, confirming that a nanosecond to sub-nanosecond risetime is achievable in the X-band waveguide at 9 GHz with the laser-triggered switch and to determine the minimum laser energy necessary to obtain the fastest possible risetime. A 1 kW pulsed X-band source with a 500 ns output pulse provides the microwave power for the system. A variable power Nd:YAG laser with a maximum 450 mJ at 532 nm, 10 ns FWHM output pulse is used in conjunction with an applied high voltage pulse to trigger the microwave switch. The microwave signal is switched with the rapid formation of plasma caused by the breakdown of a gas contained by a quartz tube inserted through a section of waveguide. The centerpiece of the waveguide system is a magic tee, which controls the direction of power flow through the system. Compared to tests in air and N2, the best results have been obtained in Argon. Risetimes below 2 ns have been obtained using Argon at a reduced pressure of 150 Torr and a high voltage pulse of 28 kV from a spark gap. The impact of gas pressure, applied voltage pulse and applied laser pulse on the risetime of the microwave switch are discussed.
I. INTRODUCTION Electromechanical, diode, ferrite and other types of commercially available microwave switches exist [1]. Each type of switch has good risetime or power handling capability but not both. The characteristics of laser and electrically triggered switches have been well understood for quite some time [2,3]. This project seeks to use the knowledge of these switches to develop a microwave switch with nanosecond risetime and high power handling capability. The primary objective of the nanosecond laser and electrically triggered microwave switch project is to create a fast switch for an X-band waveguide with a nanosecond or faster activation time. Additionally, it is desired to determine the minimum laser energy necessary to achieve nanosecond risetime, as laser cost is largely
dependent upon output power. The design of the switch centers on the microwave waveguide system and the gas breakdown system. The two are related in that the speed of the gas breakdown determines the speed of the plasma formation and therefore the risetime of the microwave switch. Some brief discussion of the theoretical aspects of waveguides and dielectric breakdown is included in this report as well.
II. THEORY OF OPERATION The switching of microwave power is accomplished in an X-band waveguide. The switching action is created by the breakdown of a gas inside a quartz tube inserted through the waveguide. The tube is inserted parallel to the electric field of the dominant TE10 mode inside the rectangular waveguide. The gas breakdown is created with an applied high voltage pulse, a laser pulse or both. When the electron density in the waveguide gap reaches approximately 1x1012cm-3 or higher due to the gas breakdown, the electron plasma frequency becomes greater than the microwave frequency and the plasma reflects the microwave signal. The breakdown voltage of the 10 mm gap across the waveguide can be adjusted by varying the gas and pressure inside the quartz tube while maintaining vacuum or atmospheric pressure in all other regions of the waveguide. Increasing the local electric field across the gap can also reduce the breakdown voltage. This can be accomplished by creating a sharp edge at either side of the gap for charge to accumulate. In addition to the electric field due to the applied high voltage pulse, the focused laser beam produces an extremely high electric field at a small spot inside the waveguide. The short, 10 ns FWHM, laser pulse rapidly propagates the discharge across the gap, which is initiated by the high voltage pulse.
III. EXPERIMENTAL SETUP The nanosecond laser-triggered microwave switch experimental test setup consists of a waveguide system for the microwave signal and a gas breakdown system utilized in the switch waveguide section. Various
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This work was supported by Accurate Automation Corporation Subcontract Number 466 under US Army Space & Missile Defense Command, Contract No. DAGS60-01-C-0088. ξ Email:
[email protected] 0-7803-7915-2/03/$17.00 ©2003IEEE.
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waveguide sections and the 1 kW, 8.9-9.2 GHz pulsed Xband microwave source comprise the microwave portion of the setup, see Figure 1. The microwave travels from the source through an E-H Tuner into the magic tee. An E-H tuner, and the adjustable size of the source’s magnetron cavity, provides the necessary frequency matching between the source and the waveguide system section. The magic tee is the centerpiece of the waveguide system, as it controls the power flow through the system. The magic tee divides the source power between the switch section and a matched termination, ideally preventing any power to flow directly from the source to the test port. After the reflection is created by the gas breakdown, the magic tee directs the reflected power into the test port. The section of waveguide where the breakdown occurs is a custom piece with modifications to allow gas input, vacuum output, laser power input and electric field enhancement. A special viewport is attached to the end of the switch waveguide section to allow visual confirmation of the breakdown across the waveguide. The cutoff frequency of the cylindrical pipe added to the waveguide bend is such that no microwave power is lost and the breakdown across the gap can be viewed or photographed. Additionally, a photo-multiplier tube, PMT, can be attached to the viewport to amplify the broadband radiation produced by the breakdown in the waveguide.
Figure 2. Switch waveguide section. The switch section of waveguide utilizes two 34 mm vacuum tees for gas input and vacuum output to the gap across the waveguide. This enables lowering the breakdown voltage of the gap (d ~ 10 mm) by filling the gap with gases such as Ar or N2 and lowering the pressure. The gas is contained to the waveguide gap with a quartz tube sealed at either side with custom 34mm flanges with grooves for o-rings. The upper tee is fitted with a Pyrex viewport to allow the focused laser beam to pass through to the gap. The upper electrode fits into the custom flange and extends to the upper edge of the waveguide inside the quartz tube. The hollow cylinder design of this electrode allows the laser light to pass through to the waveguide gap and still provides a sharp edge at the top of the gap. The lower tee attaches to a coaxial high-voltage feedthrough. The outer conductor of the coax is attached to the tee, while the inner conductor and insulator pass through to the lower edge of the waveguide. An electrode is attached to the inner conductor to provide the optimal field enhancement across the waveguide gap.
Figure 1. Block diagram of experimental setup. Black arrows indicate power flow in switch off-state, white arrows in on-state. The gas breakdown portion of the experimental setup contains nearly all of the switch parameters that are varied for optimization, see Figure 2. This includes the high voltage and optical configuration for the switch section of the waveguide. The high voltage is applied to the gap with either a 10 kV pulse generator or a spark gap triggered coaxial line pulse generator charged to 30 kV. Opposite to the coaxial high voltage feed-through, the output beam of a 450 mJ, 532 nm pulsed Nd:YAG laser is focused into the breakdown volume utilizing a 500 mm focal length lens. The focused 70 MW optical power of the 10 ns laser pulse produces an electric field of about 1 GW/cm for a 1.5 µm spot size, initiating breakdown.
Figure 3. Various electrode geometries corresponding to results listed in Table 1.
IV. RESULTS AND DISCUSSION To obtain the fastest possible risetime with the minimum amount of laser power, several parameters were 310
shape was found to be a sharp circular edge at either sides of the gap across the waveguide, cf. Figure 3. HV Anode Cathode Gas Pressure Pulse
Laser Risetime
Plane Plane Plane Plane Plane Cone Bowl Bowl Bowl Bowl Bowl
OFF ON ON OFF OFF OFF OFF OFF ON ON OFF
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Ar Ar Ar Ar Ar Ar Ar Ar Ar Ar Ar
1 atm 1 atm 1 atm 100 Torr 100 Torr 50 Torr 150 Torr 150 Torr 150 Torr 150 Torr 150 Torr
9.1kV OFF` 9.1kV 9.1kV 9.1kV 9.1kV 9.1kV 28 kV OFF 28 kV 50 kV
20 ns 10 ns 3.5 ns 2.8 ns 19 ns 11 ns 2.0 ns 1.6 ns 8.8 ns 5.2 ns 1.6 ns
160 4 120 3
80 40 0
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varied during the testing procedure. The original setup used a hermetically sealed pin with attached target electrode as the DC high voltage feed-through in the switch section. This setup was replaced with a coaxial feed-through for better insulation, along with reduced inductance, and a high voltage pulse for faster breakdown. A spark gap was later utilized to apply a faster and larger voltage pulse than the solid-state high voltage pulse generator was capable of handling. In addition to varying the feed-through, assorted gap geometries were used to enhance the electric field as much as possible. Pointplane, point-point, plane-multipoint and multipointmultipoint electrode geometries were all employed in various tests to find the best possible geometry for the microwave switching application. The multipoint-tomultipoint geometry led to the fastest multi-channel breakdown and therefore the fastest risetime microwave switching action. The type of gas and gas pressure were also varied to achieve a faster switch risetime. Compared to tests in air and N2, the best results have been obtained in Argon. The pressure of the gas inside the gap was lowered to further speed up the breakdown development in the gap. The early electrode geometry consisted of a cylindrical plane as the target electrode (anode) and a hollow cylinder as the upper electrode (cathode), see Figs. 2 & 3. The use of the hollow cylinder allowed the laser to pass through to its focal point inside the waveguide, yet still provided electric field enhancement across the gap. In Ar at 1 atm, the 10 kV high voltage pulse generator produced risetimes of 20 ns while the laser produced risetimes of 10 ns. The two used simultaneously resulted in risetimes of nearly 3 ns. A change in electrode geometry to a sharp edged bowl target electrode and a reduction of the pressure of the Ar to 150 Torr produced better results. The high voltage pulse generator at 10 kV just by itself produced risetimes of 2 ns and the spark gap coaxial pulser at 28 kV charging voltage produced risetimes of less than 1.6 ns. Higher voltage pulses, up to 50 kV charging voltage, were applied with the spark gap, with little risetime improvement. Using the spark gap and laser in conjunction produced a slower risetime than the high voltage pulse alone. This is primarily due to the risetime of the laser pulse, which is near 5 ns. The risetime of the switched microwave signal using the HV pulse and laser pulse was over 5 ns, significantly longer than the risetime obtained using the HV pulse alone. As seen in Figs. 6 & 8, an overshoot and oscillation is apparent in the reflected microwave signal. This is due to the interaction of the incident and reflected pulses in the magic tee, as it takes some time for the two signals to travel through the tee. The speed of the arc formation, and therefore switch risetime, is highly dependent upon the risetime of the voltage and/or laser, see Figure 5.
2 0
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Figure 3. Pressure vs. reflected power and risetime for bowl-pipe geometry in Ar.
Laser
Spark Gap
Solid-State Pulser
2 ns/div Figure 4. High voltage and laser pulses used to initiate gas breakdown.
Table 1. Selected microwave switch risetimes. Risetimes below 2 ns were obtained in Ar at 150 Torr with a HV pulse applied from the spark gap. The optimal electrode
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Power [W]
400 300 200 100 0
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200 300 400 Time [25ns/div]
Figure 5. Shot 163: Ar @ 150 Torr, 28 kV HV pulse, 1.7 ns risetime, 200 W reflected power (thick line).
Power [W]
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V. CONCLUSION
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Time (2ns/div)
Figure 6: Shot 143: Ar @ 150 Torr, 28 kV HV pulse, 1.63ns risetime, 340W reflected power (thick line). 500 400 Power [W]
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Figure 8. Shot 175: Ar @ 150 Torr, 50 kV HV pulse, 1.63 ns risetime, 140 W reflected power. The PMT waveform, representing the radiation emitted by the breakdown, closely follows the fall in forward power.
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Laser Pulse
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New electrode geometries and gas pressure configurations may also improve the switch risetime. A high-speed camera and holographic notch filter at the laser wavelength of 532 nm may be utilized in the future to aid the process of determining the optimal electrode configuration.
300
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PMT
300
0
500
500
Tests have illustrated that laser or electrically initiated gas breakdown within an X-band waveguide can be used to rapidly switch a microwave pulse. Microwave switch risetimes below 2 ns can be obtained using solely a 28 kV pulse applied to the waveguide gap. This result utilized the electrode geometry of two sharp circular edge electrodes and the gas configuration of Argon at a pressure of 150 Torr. The electrode geometry, gas type and gas pressure were all varied over numerous tests to maximize the switch risetime. Tests performed with the spark gap and the Nd:YAG laser produced slower risetimes than the risetimes produced by the HV pulse alone. Laser and voltage risetimes limitations, along with limitation due to the dimensions of the waveguide gap, may prevent further improvements in risetime. Further testing is required to determine if sub-nanosecond risetimes are possible.
Time [2ns/div]
Figure 7. Shot 165: Ar @ 150 Torr, 28 kV pulse & laser @ full power, 10 ns risetime, 175W reflected power. The darker lines correspond to microwave power. The HV pulse initiates the breakdown, which is enhanced by the laser pulse. The use of a faster risetime laser or voltage pulser may increase the risetime of the switch. In addition to the risetime of the voltage and/or laser pulses, the arc formation time and gap inductance also limit the risetime of the switch. Sub-nanosecond switch risetimes may still be obtained with the use of faster risetime laser or voltage pulses.
VI. REFERENCES [1] Dow-Key Microwave Corporation, Design-guide, http://www.dowkey.com/dk/pdf/DesignGuideSect2.pdf [2] A. H. Guenter and J. R. Bettis, “The laser triggering of high-voltage switches” in J. Phys. D: Appl. Phys., Vol 11, 1978. Printed in Great Britain. [3] G. Schaefer, M. Kristiansen and A. Guenther. Gas Discharge Closing Switches. New York: Plenum Press, 1990.
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