In December 1999 a first 30 GHz high gradient experiment [1] was performed using a single cell excited directly by the high-charge drive beam of the CLIC Test Facility (CTF II) [2]. Since this experiment showed quite promising results (peak surface fields of 300 MV/m were measured) it was decided to remeasure the cavity with improved vacuum, diagnostics and data acquisition. In addition an experiment was prepared to cool the cavity with liquid nitrogen and heat it with a hot air gun. The electrical breakdown behaviour was measured as a function of the cavity temperature. The breakdown threshold was found to be at a maximum surface field of 380 MV/m and remained unchanged in the accessible temperature range between 100 K to 500 K. Large data samples were taken to provide statistics of unforseen delays and frequency shifts that occur during breakdown event

Achard, Claude

Braun, Hans Heinrich

Döbert, Steffen

Groening, L

Wilson, Ian H

Wuensch, Walter

CERN Document Server

CERN – European Organization for Nuclear ResearchEuropean Laboratory for Particle PhysicsÿC L I C  I ÿÿÿÿÿCLIC Note 498PS/AE Note 2001-010ÿÿA 30 GHZ BEAM DRIVEN HIGH GRADIENT SINGLE CELLCAVITYTESTC. Achard, H. H. Braun, S. Döbert, L. Groening, I. Wilson and W. WuenschAbstractIn December 1999 a first 30 GHz high gradient experiment [1] was performed using asingle cell excited directly by the high-charge drive beam of the CLIC Test Facility(CTF II) [2]. Since this experiment showed quite promising results (peak surface fields of300 MV/m were measured) it was decided to remeasure the cavity with improvedvacuum, diagnostics and data acquisition. In addition an experiment was prepared to coolthe cavity with liquid nitrogen and heat it with a hot air gun. The electrical breakdownbehaviour was measured as a function of the cavity temperature. The breakdownthreshold was found to be at a maximum surface field of 380 MV/m and remainedunchanged in the accessible temperature range between 100 K to 500 K. Large datasamples were taken to provide statistics of unforseen delays and frequency shifts thatoccur during breakdown eventsGeneva, Switzerland10 August 2001CERN-OPEN-2001-06310/08/200111. IntroductionThe Compact Linear Collider (CLIC) [3] currently being studied at CERN aims for anaverage accelerating gradient of 150 MV/m using 30 GHz travelling wave acceleratingstructures with an RF pulse length of 130 ns. In such a structure the maximum surfacefield can exceed the accelerating gradient by a factor 2 to 3 depending on the design ofthe cavity and is thus in the 300 to 500 MV/m range. In CTF II high gradient experimentsusing a 30 GHz power extraction structure are limited to a pulse length of 16 ns.The use of standing wave single cell cavities powered directly by the drive beam enablesinvestigations of very high surface fields for longer pulse durations. Cavities are drivenby 12 to 24 electron bunches having a 3 GHz time structure and they then ring freelywhile decaying exponentially according to their loaded quality factor. The advantages ofthis approach are a simple experimental set-up, low cost and a fast turn-around time.However there are also some particularities like the presence of the drive beam, thespecial pulse shape and the standing wave field pattern.The first single cell cavity experiment revealed some very surprising breakdown features.The RF frequency was observed to shift up during the energy dissipation and a delay of10 to 60 ns was measured between the onset of breakdown and the time when the fieldmaximum occurred. The second experiment using the same cavity focused on obtainingstatistical distributions of these two features.The explosion of RF heated dust particles is one model, which was inspired by theoriginal experiment [4] and was able to explain the short delay times. Because somepossible mechanisms of initiation of electrical breakdown are associated with some sortof heating process (heating dust particles, ohmic heating of copper tips or heating byelectron bombardment) an experiment was set up to investigate the effect of the cavitytemperature on breakdown behaviour.Figure 1: Photo of the 30 GHz high-gradient single-cell cavity.20 50 100 150 200 250050100150200250300350400450Time (ns)MaximalSurfaceGradient(MV/m)0 50 100 150 200 2503030.0530.130.1530.230.25Time (ns)Frequency(GHz)0 50 100 150 200 2503030.0530.130.1530.230.25Time (ns)Frequency(GHz)0 50 100 150 200 250050100150200250300350400450Time (ns)MaximalSurfaceGradient(MV/m)0 50 100 150 200 2503030.0530.130.1530.230.25Time (ns)Frequency(GHz)0 50 100 150 200 250-400-300-200-1000100200300400Time (ns)MaximalSurfaceGradient(MV/m)3. High gradient single cell experimentThe vacuum was improved to 10-9 mbar for the second experiment since it was probablyvery poor during the first one (~ 10-6 mbar). To reduce outgasing and limit the oxidelayer, the cavity was vacuum fired at 600 oC for two hours and in-situ baked at 140 oC fortwo days. The experimental set up consists of a TM010-mode pill-box with a smallcoupling aperture to a WR28 waveguide (see Fig.1).Figure 2: Typical RF pulses (left column) and corresponding plots of resonant frequencyof the cavity during the pulse (right column).3The power coupled out of the cavity is measured with a -57 dB vacuum-to-air directionalcoupler before being dumped in a vacuum high power load. A more detailed descriptionof the cavity and the set up can be found in [1]. The cavity was driven with an 8 ns longbunch train with a total charge > 100 nC. After excitation, the field in the cavity decaysaccording to its loaded quality factor (QL = 3800). The coupling factor was measured tobe β = 0.33 leading to an unloaded Q of Q0 = 5054. The 30 GHz cavity output signal ismixed down to an intermediate frequency (IF) of 500 MHz and captured on a fast scope.In this way amplitude and frequency are measured. In order to improve the timeresolution a second detection electronics was developed using an IF of 1 GHz. Utilizing ahigher IF frequency gave more confidence in the shape of the measured waveforms butdidn’t reveal new information.Typical measured signals are shown in Fig.2 with and without breakdowns. Plots offrequency, determined by analysing zero-crossings, during the RF pulse are shown to theright. The top left plot shows a 500 MHz raw signal and the corresponding envelope for atypical pulse without breakdown. One sees that the resonant frequency stays constantduring the pulse. The two following rows in Fig. 2 show an example of RF breakdownevents occurring at different delays after the field maximum was reached (60 ns in themiddle row and 10 ns in the bottom row). The resonant frequency of the cavity shifts upconsiderably during energy dissipation in a breakdown event (150 MHz in the middlerow and 200 MHz in the bottom row). The definition of a breakdown event in this paperis the very fast dissipation of the available stored energy in the cavity. It appears as ashortening of the RF pulse.Figure 3: Conditioning history of the 30 GHz single cell cavity.0 1 2 3 4 5x 105150200250300350400Number of shots @ 5 HzMaximumsurfaceGradient(MV/m)40 5 10 15 20050100150200No.ofshotsDelay (ns)50 100 150 200 250050100150200Frequency shift (MHz)No.ofshots202228 cmneeeεpiλωω=∆The cavity was conditioned for a few days at a repetition rate of 5 Hz to accumulateabout 5·105 pulses as shown in Fig.3. Although the conditioning of the cavity wasstopped because of time limitations, the plot in Fig.3 reveals some sort of saturation inthe conditioning progress. The maximum surface field in the cavity for stable operationwas found to be 380 MV/m.4. Frequency shift and delayed breakdownBreakdown events in such a single cell cavity can be characterized by the frequency shift∆f and the delay τ of the energy dissipation after the field maximum. The breakdownevents happened up to 60 ns after the field maximum, but the statistical analysis showedthat most of the events occurred about 10 ns after the maximum. The maximumfrequency shifts measured were about 250 MHz while the average shift was 110 MHz. Itwas found that breakdowns can occur as single events but are most often observed inlimited periods of continuous breakdown on every pulse. Usually the first breakdown in aseries produces a bigger frequency shift than the subsequent ones. Very delayedbreakdowns (up to 60 ns) are rare and seem to occur more frequently in the transitionbetween no breakdown and continuous breakdowns. An example of the distribution ofthe parameters discussed above is shown in Fig. 4.Figure 4: Distributions of measured frequency shift and delay observed for breakdownevents in the single cell cavity.The frequency shift towards higher frequencies could be interpreted as a sign of massiveelectron loading or plasma formation. For a flow of free electrons across the gap of thecell the frequency shift would be [5]:The frequency shift is proportional to the electron density ne and to the wavelength λsquared. This electron current corresponds to an additional parallel inductance. A secondapproach would be to assume not only free electrons but the formation of a neutralplasma. The frequency of the cavity filled by a plasma would change due to the plasma50 100 200 300 400 500 60010203040506070Temperature (K)Rs(mOhm)HIGGS-Data 20.12.2000HIGGS-Data 18.12.2000Rs(T)-DC-Datafrequency (ωp2 = e2ne/ε0me)  yielding ω2 = ω02 + ωp2 [6]. For ∆ω << ω this leads to thesame results as the pure electron loading description.A frequency shift of 200 MHz corresponds to an electron density of ne = 1.5·1017 m-3 or3·1010 electrons. If the stored energy of about 30 mJ in such a cavity is dissipated byelectron acceleration across the gap, about 2·1011 electrons are needed.The observed delays for the discharge indicate that the process needs typically 10 ns todevelop the conditions that lead to sudden energy dissipation. It seems to be most naturalthat this phenomenon is due to some sort of heating process. From pulsed DCexperiments [7] it is known that mechanisms like explosive electron emission on thecathode or melting by electron beam impact on the anode can have similar short timescales. As shown in [4] RF heating of dust particles combined with run-away thermalelectron emission would also occur on a time scale of 10 to 20 ns. As a consequence onemight expect that the cavity temperature could affect the breakdown level.5. Temperature scanThe cavity was attached to two copper blocks, which are directly cooled by a flow ofliquid nitrogen. The breakdown behaviour was then measured while the cavity warmedbackup to room temperature, which took a few hours. In a similar way the cavity washeated and the breakdown level was measured as it cooled down to room temperature. Asimple hot air gun served for heating the cavity above room temperature. The resonantFigure 5: Measured surface resistance and published data for DC-experiments.60 100 200 300 400 500 6002000400060008000100001200014000Temperature (K)QunloadedHIGGS-Data 20.12.2000HIGGS-Data 18.12.2000SQRT-Fitfrequency shift due to the temperature changes was consistent with a simple thermalexpansion using data found in the literature. This shift of the resonant frequency thereforeturned out to be the most accurate measurement of the cavity temperature because it wasmeasurable on every shot together with the RF power data. A copper/constantantemperature sensor attached to the cavity confirmed the temperature readings but wasmuch slower and showed some dependence on the insulation of the cavity. Tocompensate for the shift of the resonant frequency of the cavity, the bunch-to-bunchdistance within the train had to be adjusted to assure efficient power production at30 GHz. Since the external Q of the cavity is expected to remain constant as the cavityexpands and contracts, the unloaded Q and the surface resistance at the differenttemperatures can be calculated. Figure 5 shows a comparison of the measured surfaceresistance and published data for DC experiments. The measured data is consistent withthe DC-data found in the literature, confirming that the assumption of a constant externalQ was valid and that we actually reached the temperatures indicated on the inner cavitysurface. The unloaded Q varied by a factor of 3 over the temperature range, this is shownin Fig.6 together with a fit of the expected ν-1/2 dependence.The change in the quality factor implies also a large variation for several parameters thatare suspected to be important for electrical breakdowns such as the power dissipated onthe surface or the integrated power during a pulse. The main result from this experimenthowever was that the maximum surface field without breakdowns remained constantwithin the accuracy of the measurement. The breakdown threshold as a function oftemperature is plotted in Fig.7. No significant temperature dependence was found onbreakdown delays or frequency shift.Figure 6: Unloaded Q as a function of temperature.70 100 200 300 400 500 600200250300350400Temperature (K)MaximalSurfaceGradient(MV/m)Figure 7: Maximum surface gradient as a function of the cavity temperature.6. Conclusion and DiscussionThe breakdown threshold of a 30 GHz single cell cavity directly driven by the electronbeam showed no significant dependence on the cavity temperature. In the entiremeasured temperature range between 77 K and 550 K the surface gradient remainedconstant to within 10 % (which corresponds to the accuracy of this experiment). Themaximum surface field at room temperature after about 5·105 pulses was found to be380 MV/m. Breakdown events were found to be characterised by a delay of the energydissipation and a frequency shift during the discharge. The maximum surface fieldimproved from 300 MV/m to 380 MV/m with respect to the first high gradient test of thiscavity. This improvement could be due to the much better vacuum but is probably justdue to longer conditioning time.The cavity was inspected with an optical endoscope after the high gradient test. Thesurface in the high electrical field region appears to be rougher compared to the opticalsurface quality obtained after diamond machining. Since the resolution of the endoscopeis not sufficient an autopsy and inspection with SEM is planned to assess any possibledamage. The RF parameters of the cavity were remeasured and found to be unchangedafter the test.The independence of breakdown behaviour with cavity temperature can be eitherinterpreted as a result showing that the relevant mechanism for electrical breakdowndoesn’t involve a heating process at all or the heating process is too fast in thisexperiment to modify the breakdown threshold.80 2 4 6 8 10 12 14 16 180100200300400500600700800900RF-pulse lengthMaximumsurfacefield(MV/m)AEG-Structure LHAEG-Structure RHSection #7Planar-StructureSQRT-Fit to AEG-DataQuarter-Fit to Section #7Cubic Root Fit to PlanarHIGGSIn the first case a mechanism like multipacting could play a role. Preliminary estimationshave revealed that two-point multipactoring across the longitudinal gap should beinvestigated in more detail as a possible candidate to explain the measured threshold.From the observed frequency shifts during a breakdown event it seems obvious thatelectrons play an important role in the energy dissipation mechanism. Electrons will crossthe longitudinal gap in one half of a RF period (16 ps). If we assume that the impact ofthese electrons heats up the surface to the melting point in about 10 ns, the heating ratewould be 100 K/ns. Thus the difference of ± 200 K would be within our time resolution,and would support the second interpretation. Heating and subsequent explosion ofemission-tips in pulsed DC experiments is also known to be an extremely fast process,which leads to breakdown in less than 10 ns [7].A particularity of this experiment is the exponential decay of the RF field with a timeconstant proportional to QL. The time constant of the decay was varied by more than afactor of two therefore it seems that only the first nanoseconds with the highest surfacegradients are important for triggering the breakdown events.We believe nevertheless that the results from these beam driven single cell experimentsare also relevant to our high gradient studies of multicell travelling wave acceleratingstructures. It turns out that the surface gradient 14 ns after the maximum in the single cell(~ 300 MV/m) is very well comparable with the values obtained with the CLIC prototypestructures [7]. This statement is illustrated in Fig. 8 where a HIGGS field decay curveFigure 8: Comparison of RF pulse length dependence of surface gradients measured onCLIC prototype accelerating structure and a HIGGS surface-field decay curve.9was added to a plot showing the RF pulse length dependence of the achieved gradientsfor different accelerating structures [8].7. References[1] W. Wuensch, W. Wilson, H. H. Braun, M. Luong, ‘A very-high-gradient test of a 30GHz single-cell cavity’, Proc. 7th EPAC, Vienna, Austria 2000, 271[2] H. H. Braun et al., ‘Experimental Results and technical results and development atCTF II’, Proc. 7th EPAC, Vienna, Austria 2000, 48[3] The CLIC Study Team (Ed: G. Guignard), ‘A 3TeV e+e- Linear Collider Based onCLIC Technology’, CERN 2000-008, 2000[4] G. Geschonke, W. Schnell, ‘Delayed RF breakdown in a microwave cavity’, CLICNote 431, April 2000[5] W. Schnell, private communication[6] J. D. Jackson, ’Classical Electrodynamics’, Wiley, New York 1998[7] G. A. Mesyats, D. I. Proskurovsky, ’Pulsed Electrical Discharge in Vacuum’,Springer ,Heidelberg 1989[8] H. H. Braun, S. Döbert, L. Groening, I. Wilson, W. Wuensch, F. Zhou, ‘High-powertesting of 30 GHz accelerating structures at the CLIC Test Facility (CTF II)’, CLIC Note475, April 20018 Appendix IPlease note that the surface fields quoted in references [1] and [4] are unfortunatelywrong by a factor pi 0.5 due to a calibration error, which was discovered during the secondexperiment.9. Appendix IIPower and field calibrationIn order to determine field levels in the cavity a complete calibration of the RF network isrequired. The Qext of the coupling determines the part of the RF energy deposited in thecavity by the drive beam that is coupled out of the cavity. A vacuum-to-air directionalcoupler picks up a small fraction of this signal while the main part is absorbed in avacuum load. The 30 GHz signal is then transported by a waveguide into the klystrongallery. In the gallery, the signal amplitude can be adjusted with a rotary attenuator tomatch the input power requirements of a microwave mixer, which converts the signaldown to 500 MHz. The output signal of the mixer is amplified and transported with low-loss cables to the control room where the waveform is digitised on a fast oscilloscope.During the calibration the following quantities are measured:· The loaded Q and the coupling factor β of the cavity using a network analyser.10· The attenuation between the cavity output flange and the non-vacuum side of thedirectional coupler also with the network analyser (dcatt [dB]).· The attenuation of the waveguide from the directional coupler to the input of the mixerbox in the gallery (wgatt [dB]).· The signal amplitude on the scope in the control room as a function of the 30 GHzinput power of the mixer box (calibbox [W/V 2]). A synthesizer is used to provide areference input power source. A frequency scan is also made.The value of the rotary attenuator in the gallery will be called (gallatt [dB])From cavity simulations using for example HFSS or URMEL we need:· The ratio between maximal surface gradient and maximal gradient on axisEs /Ezmax = 1.1 ( URMEL)· The ratio between gradient and stored energyEzmax / W 0.5 = A =17.635⋅108 [V m-1 J-0.5] (URMEL)· The expected surface field as a function of drive beam chargeEsurf =3.894 [MV/m]⋅ q/[nC] (see Fig. 9)The RF power coupled out of the cavity is then given by using the envelope of the 500MHz signal:PRF = calibbox ⋅10 ((dcatt+wgatt+gallatt)/10)⋅ envelope 2 [W]The stored energy is given by:W = Qext /ω0 ⋅ PRF (J) with Qext = (1/β+1) ⋅ QLFinally the maximum surface field is given by:Esurf = Es /Ezmax ⋅ A ⋅ W0.5 [V/m]As a consistency check during operation the field levels expected from the drive beamcharge were compared to those determined from the RF calibration. An example of sucha comparison is shown in Fig.9.110 20 40 60 80 100050100150200250300350400Drive beam charge in (nC)SurfaceField(MV/m)24 Bunches MeasurementLoss factor simulationsSingle bunch measurementFigure 9: Comparison of measured and simulated loss factor for the 30 GHz single cellcavity. The agreement between the simulations and the measurement is very good(Es simulated / Es measured =1.02). The inefficiency due to bunch length and multi buncheffects appears to be negligible.

A 30 GHz beam driven high gradient single cell cavity test

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