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Two-Electrode Spark Gap Preamble

INTRODUCTION PRINCIPLES OF OPERATION The range of two-electrode spark gaps offered by e2v A simple two-electrode spark gap is shown in Fig. 1. At low technologies comprises hermetically sealed gas-filled the gap is an insulator. As the increases, the available with DC breakdown voltages from 400 V to 50 kV. few free (present in the gap as a result of cosmic Two-electrode spark gap applications include voltage surge radiation and other ionising effects) are accelerated to higher protection, protection, single-shot pulse generators, velocities, until they are able to ionise atoms of the gas filling. high energy switches and turbine engine ignition circuits. This leads to an avalanche effect as the additional electrons produce further ionisation and as the current builds up, the PRINCIPAL FEATURES voltage falls (see Fig. 2). There is a range of current over which emission from the cathode takes the form of a glow discharge * No Standby Power Consumption and the voltage is almost constant. * Consistent * High Current Capability GAS FILLING MAIN ELECTRODE GAP 4169B * Fast Switching * Rugged and Reliable over Temperature Range * Lightweight TWO-ELECTRODE SPARK GAP SELECTION When considering the choice of spark gap, the following factors should be taken into account: * Application * Peak Current and Waveform * Coulombs per Shot INSULATOR * Maximum Repetition Rate * Main Gap Voltage Fig. 1. Basic structure of a typical 2-electrode spark gap * Environmental Conditions A further increase in current produces cathode heating by ion The e2v technologies two-electrode spark gap preamble should bombardment, leading to the formation of emission sites and be read in conjunction with the relevant product data sheet to another voltage drop across the gap. The arc discharge formed gain a general understanding of the spark gap performance in this stage is able to carry very high currents at a few tens of characteristics. volts across the gap and the peak current value is determined e2v technologies provides a technical customer support service by the external circuit. for any queries and assistance. e2v technologies limited, Waterhouse Lane, Chelmsford, Essex CM1 2QU England : +44 (0)1245 493493 Facsimile: +44 (0)1245 492492 e-mail: [email protected] Internet: www.e2vtechnologies.com Holding Company: e2v holdings limited e2v technologies inc. 4 Westchester Plaza, PO Box 1482, Elmsford, NY10523-1482 USA Telephone: (914) 592-6050 Facsimile: (914) 592-5148 e-mail: [email protected] # e2v technologies limited 2003 A1A-Two-Electrode Spark Gap Preamble Issue 4, May 2003

527/5949 BREAKDOWN 4108A VOLTAGE Impulse Ratio The ratio of the impulse breakdown voltage to the DC breakdown voltage.

Life The passage of current through a gap has a cumulative effect, causing a gradual reduction of the DC breakdown voltage. The life of the spark gap can be expressed as the cumulative charge GLOW DISCHARGE in coulombs that can be passed through the device without changing its breakdown voltage by more than 10%. Life shortens with increasing charge per discharge and increasing ARC DISCHARGE repetition rate. For further details see Operating Notes. GAP VOLTAGE CURRENT Maximum Charge Transfer The proven charge transfer, single discharge handling capability Fig. 2. Voltage-current characteristic of a 2-electrode of the spark gap with a defined pulse wave shape unless spark gap otherwise stated. The cumulative charge transfer figure used to express life is based on figures below this value specified at a The arc persists until the current falls to a value insufficient to relative pulse repetition discharge rate. maintain arc conditions; this current is lower than that required to establish the arc initially. Current flow then ceases and the Recommended Repetition Rate ionisation of the gas decays until the gap has returned to its The maximum repetition discharge rate per second for optimum original state. spark gap performance. The amount of natural ionisation present in the gap described above varies according to the intensity of ionising radiation and Capacitance should normally be very small. This might cause wide variations in the breakdown voltage; for example the value in darkness The inter-electrode capacitance is measured across the might be double that when illuminated. The time taken to terminals and typical values for e2v technologies spark gaps establish the arc would also be undesirably long and variable. are: GXK and GXN series typically 0.5 pF; GXH, GXS and The performance can be improved and stabilised by providing a GX2001 series typically 5.0 pF; GXF series 50 pF maximum. consistent source of ionisation within the gas of the gap. One method is to maintain a small at the normal Insulation Resistance operating DC voltage by providing sharp corners on the The resistance of a two-electrode spark gap measured at 100 V electrodes or including some high dielectric constant material. dc between the terminals is typically 410,000 MO. Another method frequently used is to include a small quantity of radioactive isotope, normally tritium gas, which emits high Radioactivity energy electrons continuously. Some devices contain tritium to ensure consistency of The value of the breakdown voltage is determined by the gap operation. The activity of the device will not exceed the limit geometry, gas composition and pressure, ionisation conditions stated. and the waveform of the gap voltage. A given type of spark gap can be made to break down at any given voltage selected Gas Content from a wide range by appropriate adjustment during The gas content of a spark gap might be inflammable when manufacture. Immediately after a current pulse, the voltage mixed with air. Devices should not be operated if damage to the hold-off capability of the tube is limited by the residual envelope is evident. ionisation. This decays rapidly by recombination, the recovery time depending on the gas composition, electrode material and pulse energy. FACTORS AFFECTING LIFE Life TWO-ELECTRODE SPARK GAP It might be possible to extend the life of a spark gap by TERMINOLOGY operating the device well below the stated operating DC Breakdown Voltage characteristics. The voltage at which discharge occurs. This is normally the Peak Current lowest voltage that will cause breakdown. The spark gap will break down within +10% of the specified voltage unless The life of a spark gap is governed primarily by the deposition of otherwise stated. A device can be manufactured to any electrode material on the insulating surfaces. The rate of erosion voltage within the range stated. of the electrodes may be related to peak current as follows: 1.6 Rate of erosion ! (Ipk) Impulse Breakdown Voltage Hence, limiting the peak current can increase spark gap life. The voltage at which discharge occurs when a rapidly rising voltage is applied across a spark gap. Impulse breakdown Pulse Duration voltage normally increases with increasing rate of rise. A rate of For a given peak current, longer life will be obtained with rise of 15 kV/ms is usually used at e2v technologies. shorter pulses.

Two-Electrode Spark Gap Preamble, page 2 # e2v technologies # over voltage breakdown gap spark of life plot Typical operational 4. Fig. DC BREAKDOWN VOLTAGE may but acceptable be may rates life. repetition rate. shorten repetition higher decreasing with or increases gap Burst spark a of life The Rate dependent) Repetition (circuit pulse current typical A 3. Fig. CURRENT = V and capacitance = C not where areas, shaded CV, the voltage. of charging charge (see sum gap the stored spark is the transfer the charge by the conducted 3), pulse Fig. current other ringing the and mC also. For currents 0.8 considered at peak be charge C should 6000 relative characteristics to decreasing the circuit rising GXH discharge, However, with per the discharge. C for increases 1.0 per at life C gap charge 600 cumulative is spark series the example, a For of transfer. life The Transfer milliseconds. Charge transfer of tens charge many be high may time At recovery decreases. the the levels, the rate and (in dominate repetition to short begin maximum very effects per transferred thermal is charge increases, the discharge operation as However, duty microseconds). of low order with time Recovery and erosion Recovery local excessive to discharge lead preferred might life. a turn the reduced produce in by to which generated sufficient path fields be may around electromagnetic discharge discharge The the of electrodes. distribution even the on depends life Long Symmetry Circuit preferred. is circuit but damped reversal, critically current a high life with maximum achieved for be can life Reasonable Reversal Current LIFE 2 technologies e2v TIME 7801 7800 odofvlae h eitnesoni eiswt h gap the The with series normal. in be shown its to can resistance recovered gap has The returned gap spark voltage. the until the hold-off has re-close before not must circuit voltage half- and the destroyed, following the open in may must if again but contactor cycle, down AC even break the start in to cycles will zero sufficiently it to surge, ionised falls rapidly voltage current be a the operate by as fired recover is not short to gap with the would surges When voltage times. this against rise protection a included, provide Although to primary. is enough breaker the with circuit its parallel 2-electrode and in a transformer gap by power over-voltages spark against a transient of against example protected transformer typical load, a power shows 5. Fig. a of Protection transients. voltage 5. Fig. device, electro-mechanical an than protection. transient greater giving voltage thereby faster gas-filled respond a will of and than durable to use energy more more is the handle component, will is solid-state gap a ways spark a of effective Generally, One gaps. most to spark necessary. is and magnitude, impossible protection simplest and the of the form events, predict some these so to them, of difficult prevent voltage frequency is and of Internal It switching waveform etc. failures, etc. loads, component arcing, switching, inductive used. from surges, arise general being power transients by strikes, is caused be lightning equipment might wherever transients problem voltage and External ever-increasing instrumentation an electronic are transients Voltage Protection Surge * * * * * * NOTES parallel APPLICATION opposite the onto the material and erosion electrode surface. by electrode the the caused of be of reduction also The deposits can film. coating voltage metallic by slowly breakdown a caused DC reduces be with can gap surface This insulator spark 4). the Fig. the (see of life operational voltage over breakdown DC The Modes Failure Life of End ewe ieaddshrertnscnb aidoe wide a over varied be type. compromise given can any the ratings for reached; discharge range longer and no discharges life is of to between number limit been the related voltage as have are measured the gap, are ratings before spark may conditions, energy the of and operating they life current the of although The set adaptable. gap, particular very one spark for of developed types Most 4111A ouao opnn Protection Component Modulator Switches Energy High Lithotripsy Medical Circuits Ignition Engine Turbine Circuits Protection Surge Voltage Generators Pulse Single-shot w-lcrd pr a rabe ae3 page Preamble, Gap Spark Two-Electrode LOAD might be necessary for installations where very large currents The radioactive content used in most spark gaps is tritium gas, can be drawn under fault conditions. an isotope of hydrogen. Its emission consists of b particles A more specialised application is shown in Fig. 6. In a (electrons) at relatively low energy. The total activity per tube is transmitter, if the magnetron misfires once, the following pulse in the classification category band of 55.5 MBq, class 1 (code from the modulator may be at a much higher than of practice Def. Stan. 59-60) and 537 MBq. normal and this could cause rapid destruction of the pulse The spark gap should not be stored near photographic film and transformer and other modulator components. The spark gap must be in a correctly marked package. There are some connected across the secondary of the pulse transformer limitations on the action to be taken when large numbers of provides the necessary protection. Most pulse magnetrons radioactive tubes (e.g. 55.5 MBq) are disposed of, broken or operate at some thousands of volts, but the short-circuit involved in a fire (consult relevant local authorities), but the current of the modulator is relatively small; similar types of gap handling of single tubes requires no special precautions. may be used to protect other types of tube and equipment operating under similar conditions. Mounting and Connections.

4113A The rise time of the arc current is determined mainly by the P.F.N. inductance of the external circuit and connections; where the fastest possible switching is required, the gap can be built into the end of a coaxial line, matched to the circuit impedance. In general, the connections should be kept short and well-spaced to minimise their inductance. For the same reason, the mounting position will be normally as close as possible to the TO MAGNETRON load or protected equipment. In protection and single-pulse applications, the heat dissipated by the spark gap is not enough to require any assisted cooling, and the maximum temperature rating of the gap will not usually restrict the choice of mounting PULSE TRANSFORMER position. SPARK Most spark gap applications involve high peak currents. GAP Although the mean currents are relatively small, so that small section conductors can be used safely, the connections must be secure. Any poor or loose contacts will be eroded rapidly Fig. 6. Protection of a magnetron against misfiring and are likely to fail completely. Figs. 7 to 10 show examples of suitable mounting methods for Pulse Generators several larger types of gap. It is possible to use a spark gap as the tube in a pulse modulator up to very high peak power levels. The life of the gap 4125A PLASTIC CLIP will be very much less than that of a developed for the same conditions, but might still be sufficient for applications requiring only a short or very intermittent operation. The advantages of spark gaps for this purpose include zero standby power, zero warm-up time, a long non-operating life ANODE CAP and low cost. Where the output of a modulator must be P471 R.S. accurately timed or synchronised, a 3-electrode spark gap is used. INSULATOR

Environmental The thermal, shock and vibration tolerances of most spark gaps are adequate for normal domestic and industrial applications, but certain types are especially rugged. Where an industrial application involves a corrosive or abrasive Fig. 7. Typical mounting arrangements for GXK Series atmosphere, spark gaps will require the same protection as most electronic equipment.

Storage Spark gaps should be stored preferably in the original packing in a non-corrosive atmosphere. If removed from their packing, they should be protected from dust and industrial atmospheres until required. The shelf-life is adequate for all normal storage requirements, although it is possible that the characteristics may change as the radioactive content decays. Since tritium has a half-life of twelve years, the change over normal storage periods is negligible.

Two-Electrode Spark Gap Preamble, page 4 # e2v technologies 4124A SPARK GAP ENQUIRY OPERATING PARAMETERS FORM Overleaf is e2v technologies’ Spark Gap Enquiry Operating Parameters Form. Please detach or photocopy, then complete SOLDER and return the form as indicated to the address shown, and e2v TAG technologies will recommend the most suitable spark gap. All information will be treated in the strictest confidence.

INFORMATION CHECK LIST Answering the following questions will enable e2v technologies to select the most suitable 2-electrode spark gap.

What is/are the: * application, * required DC breakdown voltage, INSULATOR * value of the charge storage , * maximum charge transfer, Fig. 8. Typical mounting arrangements for GXN Series * cumulative charge transfer, BRACKET 4126A * typical life shots expected, * peak current, waveshape and cycle time, * maximum switching rate per second, * ANODE CAP required dimension limits, P471 R.S. * environmental parameters, e.g. operating temperature, shock, etc?

INSULATOR

Fig. 9. Typical mounting arrangements for GXS Series

4122A

FLEXIBLE CONNECTION

TERMINAL

INSULATOR

Fig. 10. Typical mounting arrangements for GXF Series

Whilst e2v technologies has taken care to ensure the accuracy of the information contained herein it accepts no responsibility for the consequences of any use thereof and also reserves the right to change the specification of goods without notice. e2v technologies accepts no liability beyond that set out in its standard conditions of sale in respect of infringement of third party patents arising from the use of tubes or other devices in accordance with information contained herein.

# e2v technologies Printed in England Two-Electrode Spark Gap Preamble, page 5 e2v technologies Waterhouse Lane, Chelmsford, Essex CM1 2QU, UK Telephone: +44 (0)1245 493493 Facsimile: +44 (0)1245 453725 Internet: www.e2vtechnologies.com Spark Gap Enquiry Operating Parameters Please complete as accurately as possible to allow the most suitable spark gap to be recommended. All information is treated in the strictest confidence.

Company Name and Address

Contact Name E-mail

Position

Telephone No. Fax No.

Application

Discharge Circuit and Current Waveform (if known). Please sketch the discharge circuit below or attach a circuit diagram.

Spark Gap Operating Conditions Operating Voltage Range

Peak Current

Repetition Rate

Life Required

Other Requirements

11093A-4