Gold, S

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* Work supported by Department of Energy Contract DE-AC03-76SF00515KLYSTRON GUN ARCING AND MODULATOR PROTECTIONS.L. GoldStanford Linear Accelerator Center (SLAC), Menlo Park, CA USAAbstractThe demand for 500 kV and 265 amperes peak to power an X-Bandklystron brings up protection issues for klystron faults and the energydumped into the arc from the modulator.  This situation is made worsewhen more than one klystron will be driven from a single modulator,such as the existing schemes for running two and eight klystrons.  Highpower pulsed klystrons have traditionally be powered by line typemodulators which match the driving impedance with the load impedanceand therefore current limit at twice the operating current.  Multipleklystrons have the added problems of a lower modulator sourceimpedance and added stray capacitance, which converts into appreciableenergy at high voltages like 500kV.  SLAC has measured the energydumped into klystron arcs in a single and dual klystron configuration atthe 400 to 450kV level and found interesting characteristics in the arcformation.  The author will present measured data from klystron arcspowered from line-type modulators in several configurations.  Thequestions arise as to how the newly designed solid-state modulators,running multiple tubes, will react to a klystron arc and how much energywill be dumped into the arc.1. INTRODUCTIONThe amount of protection required for a gun arc or a defocused beam in a microwave tube is acontinual source of controversy and debate.  Historically, tube companies have set protectionrequirements by their own experience in test.  Body current interception was thought to be limitedto 10 joules maximum and shutdown within 10 microseconds.  In high power klystrons theengineers found out that as the voltage increased, the rate of rise of fault current needed to belimited.  1000A/msec with a less than10 microsecond maximum shutdown became a typicalnumber used for CW or long pulse t bes with crowbars or series switches.  Tube engineershesitated to put a hard number on the energy that could be dumped into an arc, but 10 joulesbecame a defacto rule of thumb.  Thomson in France has listed the highest number I am aware offor allowable arc energy at 40 joules.High power pulsed klystrons have been classically pulsed using Line-type modulatorswhose impedance is matched into the klystron by a pulse transformer.  In this type modulator themaximum peak current delivered into a gun arc is two times the normal operating current.  Aklystron was considered to be inherently protected, because of single pulse shutdown andtherefore no real attempts were made to measure arc energy levels.  SLAC has successfully usedthis type modulator at high peak power for over 30 years.  SLAC continues to require higherpeak pulse power and for the NLC (Next Linear Collider) and its Test Accelerator it was decidedto operate two klystrons in parallel on the same modulator, which means that the arc current canrise to four times the normal operating current.  In fact, the modulator design of choice for theNLC is a solid-state induction modulator, which runs eight klystrons.  I decided to try to measurearc energy to see where we are today and feel more comfortable about protecting a klystron.Digital storage oscilloscopes have made this task easier.2. ARCING IN A VACUUMTo begin to try and understand a vacuum gun arc in a microwave tube one must first consider thefact that the electrode shape, spacing and therefore electric field is chosen to be well below thethreshold where an arc would occur.  Additionally, the focus electrode (at cathode potential) isintentionally operating at a much lower temperature than the cathode to avoid it becoming anemitter.  The most likely cause of the creation of an arc is therefore field emission from the focuselectrode.  Classically for field emission the potential is raised towards the breakdown threshold ofthe vacuum gap.  At some potential a small electron emission begins (microamperes) after whichsmall changes in voltage will generally produce large changes in emitted current.  As the gap isover-voltaged the emitted electrons traverse the gap in less time.  Spark gaps, either two pole ortriggered, fire in tens of nanoseconds.  However, in a microwave tube the cathode voltage remainswell below the breakdown threshold and the current builds up slowly (several hundrednanoseconds).  This is clearly seen in the experimental data shown below.  At the 2000 ModulatorSymposium, I reported that Richard Adler of North Star Research “believes, for short pulses thefirst stage of breakdown is plasma electron emission (private communication).  The currentincreases monotonically until the expanding plasma reaches the anode.  The velocity of thisexpanding plasma tends to be 2-3 cm/microsecond or slower.  This is in somewhat agreement withthe rise of arc current measured at SLAC[1]. It should be noted that these klystron operate with avacuum of 10-8 or 10-9 torr, which enhances voltage hold-off.  In another theory field emittedelectrons knock off ions on the opposite surface.  The ions travel back creating more electrons.The iterative process continues until either breakdown occurs or the voltage goes away.3.     EXPERIMENTAL DATA3.1 Line-Type ModulatorDevelopment klystrons at SLAC are tested or operated in a number of different modulators withdifferent transformer ratios and impedances.  They are Line-type modulators with 25kV chargingpower supplies, resonant charge and their transformer ratios vary from 15 to 23.  Table 1 is asummary of part of the data.  Some of this data has been presented previously.  The devices testedwere S-band and X-band.  XL4 klystron arc data was obtained in two different modulators andsingle and dual tube operation.  Examining the data shows that the voltage fall time and thcurrent rise time is much slower than conventional wisdom has believed and takes at least severalhundred nanoseconds.  During this arc transition, energy is being dissipated and the calculated arcenergy is larger than the maximum limitsTable 1Measured Arc Data - Line-Type Modulatorspreviously thought allowable. The gap spacing, anode to cathode (focus ring) is large (approx. 2-4cm) as compared with standard spark gap spacing.  It takes the arc longer to form and close thegap.  It is hard to predict the energy in an arc because it is very dependent upon the arcformation, which is I believe an uncontrolled variable.  In the early stages of XL4 development atSLAC, it would take a long time to recover from a gun arc.  Discussions about this problem ledthe klystron engineers to believe that there was poor vacuum at the gun area since the vacuumpump was located a long distance from the gun and there was poor conductance through the X-band circuit.  A floating ion pump was mounted at the gun of the XL4. The pump power wassupplied from power supply using the klystron heater supply for its input.  No other changes weremade to the klystron gun and the problem of recovery after a gun arc was gone. Figure 1 is the voltage waveform taken on an XL4 klystron in the Test Lab on Test Stand08 and the arc current calculated from the cathode and collector current waveforms.  The bodycurrent shown at the beginning of the pulse isrepresentative of the capacitive charging currentduring the rise time of the voltage.  Thismodulator will allow an arc current of approx.800 amperes.  In this case the peak arc currentis not very large but the fall time of the voltageis long (1600 nanoseconds) which accounts forthe arc energy of almost 60 joules.  Figure 2 isan XL4 klystron arc taken in an NLCTAmodulator powering two XL4 klystrons.  Themodulator limiting impedance will allow the arccurrent to rise to four times klystron operatingcurrent, approx. 1600 amperes.  During this arcthe voltage fall time is faster and the current risetime slower.  The current reaches a much highervalue but the voltage initially falls inFigure 1. XL4 klystron arc on Test Stand 08450 nanoseconds to 50kV and then falls to 0volts in and additional 400 nanoseconds andthe energy in the arc is about 60 joules.  Thearcs of Figures 1 and 2 are different in currentand time but dissipate about the same energy.This data reinforces my belief that the arcformation is an unknown variable with somedependence on the external circuit. The next version of the 75 MW PPM(periodic permanent magnet) focussedklystron, XP3 (formally known as the DFMklystron) is under construction.  The XP3Diode, a beam stick with a portion of themagnet stack has recently been initially testedin Test stand 13 at SLAC.  This tube wouldnominally operate at 490 kV andFigure 2.  XL4 klystron arc in Two-tubeConfiguration265 amperes.  Figure 3 is a picture of an arc waveform captured while processing and operatingthis tube.   In this case the arc causes the voltage to drop in 800 nanoseconds and the voltageessentially drops directly to 0.  The current rises to almost 500 amperes in about 600nanoseconds.  This arc has a calculated energy of 42 joules.  Two arcs were captured on thisdiode and both were about the same energy although different in waveshape.  It should be notedthat the XP3 electron gun has a microperveance of 0.75 and therefore has larger cathode to anodespacing than the XL4 klystron whose microperveance is 1.2.  It is hard to relate the gap spacing inthe electron gun to the rise of arc current or fall of arc voltage at this time.  It should take ionslonger to traverse the longer gap but there is a larger accelerating potential from the highervoltage.During modulator testing at NLCTA, a 5045diode had a arc while operating above 400 kV.We have operated 5045’s at this level in the past.However, this time, the 5045 diode would nolonger hold voltage.  Another diode was still inoperation.  The diode was returned to the tubeshop for a careful autopsy.  The arcing tookplace between the focus electrode and the bellhousing near the high voltage seal joint.  As canbe seen in Figure 4, the stainless teel coronaring shows an area where thin layers of materialhave been removed.  The second photoshowsthat material plated on the ceramic high voltageseal.  This is the same area we have seenFigure 3. XP3 Diode arcceramic seal punctures in the past.  The gap between the corona ring and the ceramic is muchsmaller than the anode to cathode gap.  We did not capture the arc that created the damage that wesee.  Obviously there is enough energy to ablate the surface material.  In fact, it is mostprobable that the damage did not all occur in one arc, but over a number of arcs.  Perhaps somedamage occurred  during prior operation in another modulator.  There was also activity on theanode.Figure 4. 5045 Diode Autopsy- Focus Ring (Upper left), H.V. Seal (Upper right), Anode (bottom)3.2    Low Impedance Switch3.2.1  10kV Marx Switch ModulatorDr. Anatoly Krasnykh designed a modified Marx modulator for a TWT application at 10kV[2].Although the voltage and power are much lower than the klystron modulators I was curious whatthe arc current would look like in this ‘Direct Switch’ modulator.  Dr. Krasnykh made thefollowing measurements with an air arc.  The current sensing threshold was set slightly above theoperating current and the measured delay in turn-off of the final switches was 700 nanoseconds.400 nanoseconds was due to the gate delay of the IGBT itself.  Figure5 are the voltage and arccurrent for arcs occurring at the beginning, the middle and the end of pulse. Figure 5. 10kV Marx Modulator Arc- 1) pulse beginning, 2) pulse middle, 3) pulse endThe normal operating load on the modulator was 1 ampere but the peak arc current rose to above30 amperes.  The energy is still low but what if this was extended to 500kV?  Would the very highcurrent create a problem or would the rate of rise be such that the voltage will be zero before thecurrent reaches its very high level, thereby limiting the energy?  These are unknowns that willonly be answered by testing a full scale model.  It is conceivable that testing of any of the lowimpedance ‘direct switch’ modulators would give enough insight to predict the performance ofthe others.  Diversified Technologies has another SBIR grant to work on a direct switch modulatorfor 500 kV either with the modified Marx approach or a straight series switch.3.2.2  Induction ModulatorThe data on preliminary arcing into a vacuum gap below 80kV was not fully available at the timethis paper was submitted.4. CONCLUSIONKlystron protection in very high peak power modulators remains an issue that is still not clearlyunderstood.   Clearly, a klystron with excellent vacuum (8 to 9 scale), can dissipate more energyin a current limited gun arc than conventionally believed.  However, recovering from an arc doesnot mean that some damage was not done.  Surfaces do get electroplated with copper or stainlesssteel.  All of the tubes tested had excellent vacuum in the gun region.  We know that the lowimpedance modulators whether a Marx switch, an induction design or a hybrid with a pulsetransformer have the capability of delivering larger peak arc current than a Line-type modulator.What we don’t know is how fast the arc will build up and if the current will indeed rise faster or ifthe voltage will fall at a slower rate.  These modulators can have the capability of terminating thepulse upon sensing an arc and depending on circuit speed can shut down the current.  The onlyway we will know is to operate a klystron on these modulators.  I am hoping that DiversifiedTechnologies, Inc. will deliver a Hybrid Modulator built on an SBIR for testing at SLAC in thefall.  Dick Cassel and company is planning to have a full 500kV version of the inductionmodulator (affectionately known as the four dog) in test this summer.  Therefore, I amencouraged that we should have a better picture of this phenomenon by the end of this year.Further investigation is necessary to understand klystron arcs and how much energy causesdamage.  A demountable gun on a diode body would be required to enable examination of thegun electrode surfaces after a set number of captured arcs.  The gun would then be reassembled,at least low temperature baked and re-tested.ACKNOWLEDGEMENTSI would like to thank Anatoly Krasnykh for his help in trying to understand the nature of the arc.In addition, I would like to thank Richard Cassel for the data on the Induction Modulator andJohn Eichner for taking data at NLCTA and in the Test Lab.REFERENCES[1] S.L. Gold, “Thyratron-PFN, IGBT Hybrid and Direct Switched Modulator R&D s itEffects Klystron Protection”, 2000 24th International Modulator Symposium, June 2000[2] A. Krasnykh, etal.,”A Solid-State Marx-Type Modulator for Driving a TWT”, 2000 24thInternational Modulator Symposium, June 2000BIBLIOGRAPHYG. Schaefer, M. Kristiansen and A. Guenther, “Gas Discharge Closing Switches”, (Plenum Press,New York, 1990)

Klystron Gun Arcing and Modulator Protection

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