The LEP Collider was operated in 1997 with 60 superconducting four-cavity accelerating modules (about 2600 MV available) installed at the four interaction points. During operation for physics it was o bserved that the dissipated heat in the superconducting cavities is not only a function of the acceleration gradient but it also depends on beam characteristics: number of bunches, bunch length and cu rrent per bunch. These beam effects were not foreseen in the original heat budget of the LEP refrigerators. Three days of LEP Machine Development were dedicated in August 97 to clarifying the correlat ion of the losses with the beam characteristics. The beam dependent heat load of the cryogenic system for the superconducting cavities is described. The dependence on various beam parameters is presen ted and scaling laws are given. A possible explanation will be presented and the consequence for LEP operation will be discussed

Cavallari, Giorgio

Gayet, P

Geschonke, Günther

Jiménez, J M

Kaiser, D

CERN Document Server

SL/Div RepsEUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN — SL DIVISIONCERN-SL-98-010  RFBeam Related Thermal Losses on the Cryogenic and Vacuum Systemsof LEPCavallari, G.;  Gayet, Ph.; Geschonke, G.; Kaiser, D.;Jimenez, J.M.AbstractThe LEP Collider was operated in 1997 with 60 superconducting four-cavity acceleratingmodules (about 2600 MV available) installed at the four interaction points. During operationfor physics it was observed that the dissipated heat in the superconducting cavities is not onlya function of the acceleration gradient but it also depends on beam characteristics: number ofbunches, bunch length and current per bunch. These beam effects were not foreseen in theoriginal heat budget of the LEP refrigerators. Three days of LEP Machine Development werededicated in August 97 to clarifying the correlation of the losses with the beam characteristics.The beam dependent heat load of the cryogenic system for the superconducting cavities isdescribed.  The dependence on various beam parameters is presented and scaling laws aregiven.  A possible explanation will be presented and the consequence for LEP operation willbe discussed.Presented at the Eighth Workshop on RF Superconductivity, Abano Terme (Padova), Italy, 6-10 October 1997Geneva, SwitzerlandApril 1998BEAM RELATED THERMAL LOSSES ON THE CRYOGENIC AND VACUUMSYSTEMS OF LEPG. Cavallari, Ph. Gayet, G. Geschonke, D. Kaiser, J.M. JimenezCERN, 1211 GENEVA 23 (Switzerland)AbstractThe LEP Collider was operated in 1997 with 60 superconducting four-cavityaccelerating modules (about 2600 MV available) installed at the four interaction points.During operation for physics it was observed that the dissipated heat in the superconductingcavities is not only a function of the acceleration gradient but it also depends on beamcharacteristics: number of bunches, bunch length and current per bunch. These beam effectswere not foreseen in the original heat budget of the LEP refrigerators. Three days of LEPMachine Development were dedicated in August 97 to clarifying the correlation of the losseswith the beam characteristics. The beam dependent heat load of the cryogenic system for thesuperconducting cavities is described.  The dependence on various beam parameters ispresented and scaling laws are given.  A possible explanation will be presented and theconsequence for LEP operation will be discussed.1. INTRODUCTIONThe CERN Large Electron Positron (LEP) Collider is a 26.6km circumference e+ e-storage ring. The accelerating system consists of 84 five-cell room temperature Cu cavitiesand of 240 four-cell superconducting (SC) cavities at 352 MHz. The SC cavities are operatedat the design field of 6 MV/m.Some evidence of an additional beam induced power loss was seen on the cryogenicsystem and when was investigated the LEP impedance related to the bellows. A systematicmeasurement of warm bellows temperature along the beam pipe [1] and between the cryo-modules showed that the power deposited in the bellows placed near the RF modules is higherthan expected from their loss factor. This heating is higher near RF modules and decreaseswith increasing distance from the RF modules. Since the cold inter-cavity bellows are ofsimilar shape, special attention was given to the effects of this additional power loss on thecryogenic system [2]. The measurements done show similar results (see Fig. 1).Preliminary assumption: attribute the losses to the Higher Order Modes (HOM)escaping from the SC cavities through the beam pipe. The use of ferrite absorbers mountedbetween the modules to trap the HOM circulating inside the beam pipe did not reduce thecryogenic losses despite an absorbed power higher than 1 kW (80.5 GeV, eight bunches, totalbeam current 5 mA and bunch length 10 mm) [3][4].3051015202530354022:00 23:12 0:24 1:36 2:48 4:00Time elapsed (hh:mm) Temperature (C)Thermal losse variation (W)0.00.51.01.52.02.53.03.54.0Beam current (mA)RampingStabilisationperiodcoast beam dumpCurrentBellows temp.Thermal lossesFigure 1: Variation of Cryo Losses and Warm Bellows Temperature as function of Beam Current.Investigations of the beam related power load for the cryogenic system started at theend of the 1996 LEP running period [5].  Since this unexpected effect can limit the futureperformance of LEP, extensive measurements were done as soon as stable beam conditionswere available in 1997 in order to establish the dependence on the beam parameters and tounderstand the origin of this effect.2. MEASURED PARAMETERSMeasurements directly related to the cryogenic loss in the modules will be discussedand data collected on warm vacuum bellows mounted between the cavity modules will bepresented to confirm the similarity of the mechanisms of power losses (see Fig. 2).2.1. CryogenicsTwo methods are available for the measurement of the cryogenic power:a) The opening of the outlet valves of the He return line from the four-cavity modulesdepends on the total power dissipated in the modules, as described in [5].  During the 1997commissioning the calibration of these valve positions was refined on all modules, in orderto allow the thermal load variation to be measured.  The total cryogenic consumption of acavity consists of the static losses, the dynamic RF losses, the electric heater power andthe beam dependent losses.  Reliable measurements of the beam-induced power can bestbe obtained under conditions where the electric heater and the RF powers are keptconstant.b) The cryogenic plants have a reservoir, called “pot”, in which the level of liquid He is keptconstant by means of electric heating.  The power of this heater is a measure of theavailable spare cryogenic power.  A measurement of this electric power can thus be used4to determine changes of the cryogenic power load with varying beam parameters.  Thisgives the integral consumption of all modules of a whole LEP point.010203040506070809010009:30 10:42 11:54 13:06 14:18TimeEnergy[GeV], Cryo Loss[W], Bellow Temp.[C]02468101214Bunch Length[mm], Current[mA]Beam EnergyBeam currentBellows temp.Cryo loss variationFigure 2: Effect of the beam parameters on the cryo losses and the temperature increase of the warmbellows.Both measurements depend on the stability of cryogenic conditions; the relaxation timeafter load changes is of the order of 30 min - 1 hour for the first method and 2 hours for thesecond.  Results obtained with these methods can therefore only be compared at least 2 hoursafter establishing stable conditions.  The accuracy of these measurements is estimated to benot better than r 20 %2.2. Cold Inter-cavity Bellows temperaturesThe temperature of some of the normal conducting cold bellows inside the cryostatswas logged during all measurements, as described in [5].2.3. Bunch lengthThis year a systematic on-line measurement of the bunch length was available duringall our measurements [6].2.4. Warm Vacuum BellowsThe warm bellows are mounted between the cavity modules to interconnect thevacuum pipes. The shape of these bellows is similar to that of the inter-cavity bellows.Thermocouple probes were installed on these bellows which were thermally insulated toestimate the power dissipated on these structures. The same bellows but far away from any5modules was also equipped to measure the contribution of the beam itself [5]. In total, morethan 40 bellows temperatures were measured in LEP in April 1997.3. MEASUREMENTSSystematic measurements were done in order to answer the following questions:How does the cryo load depend onx circulating beam current, bunch length, Does the cryo load depend onx cavity accelerating gradient, beam energy,x module position, bunch train bumps,x synchrotron radiation.Measurements were done with one beam of positrons with eight equally spacedbunches, no bunch train bumps, total current up to 4.5 mA.  Some modules were always run atmaximum gradient, some were left idling without RF.In the first session, the beam was ramped up to 91.5 GeV with intermediate stops at60, 80 and 85 GeV.  At each plateau we tried to adjust the bunch length V to between9.7 and 10 mm by changing the total RF voltage and the RF frequency, and allowing thecryogenic conditions to stabilise.In the second session, we tried to measure the effect of bunch length, which was variedby switching off wigglers at 45 GeV, and again by varying RF voltage and frequency.  At 45GeV the bunch length V was varied between 5.5 and 13.3 mm, at 87 GeV between 9.2 and 13mm.In a third session, we measured the current dependence at 80 GeV and at 60 GeV.These measurements were complemented by data taken during normal physics coasts.4. RESULTS4.1.  Influence of synchrotron radiation from bunch train bumps or from the arcsAt 91.5 GeV the collimators at IP4 (Interaction Point) were fully opened and laterbunch train bumps were introduced into the previously flat machine.  Fig 3 shows as anexample the valve positions measured on one module running at constant field of 40 MV.  Noeffect can be seen anywhere of either of these two experiments.  Therefore synchrotronradiation shining into the modules appears not to have an effect on the cryogenic heat load.4.2. Influence of circulating beam currentMany measurements were done with different beam currents.  An example is shown inFig. 4.  These data were taken during a physics run with a bunch length of between 11.5 and12.1 mm.  The additional cryogenic heat load due to the beam was measured for most of themodules at IP 4.  The two solid lines give the average over all measured modules, and the heatload measured using the previously described “pot” heating.  Both measurements are in fairly6good agreement.  The discrepancy at high beam currents at the beginning of the fill is due toinstabilities in the cryogenic plant after the ramp.The beam-induced power dissipated by module varies from module to module,between 1.3 W/mA2 and 3 W/mA2.  The average over the measured modules gives1.7 W/mA2, the average “pot” power gives about 2 W/mA2. In order to compare differentbunch configurations, an impedance Z can be defined in analogy to the loss factor, in such away that the beam induced cryogenic power Pbcr can be calculated for any configuration ofkb bunches/beam with a bunch current ib, and nb circulating beams. :All measurements in this paper were done with eight bunches in either one or twobeams, therefore for convenience most of the time the power normalized to itot2 = 1 mA2 isgiven, with itot being the total circulating beam current.  A power of 2 W/mA2 corresponds toZ=16 M: per module.4042444648505254565813:0013:3014:0014:3015:00TimeValve opening051015202530354013:38 coll. Out14:00 bumps onFigure 3: Valve positions during stable running.  The valve positions, i.e. the thermal load, did notchange when all collimators were moved out, nor when bunch train bumps were introduced.2bbbbcr iknZP  7y = 1.7103xR2 = 0.9531051015202530354045500 5 10 15 20 25Total Beam current^2P [W]"Pot" powerAverageFigure 4:  Cryogenic heat load due to the presence of beam.  The points were measured using the valvepositions of most of the modules during physics at 91.5 GeV.  The solid lines give the average over allmeasured modules and the power consumption derived from the spare capacity of the cryo plant.  Thebunch length was measured to be between 11.5 and 12.1 mm.  The "Pot" power line above 15 mA2 andthe module measurements above 24 mA2 are perturbed due to stabilisation of the cryo plant after theramp.y = 1.04 xR2 = 0.970510152025300 5 10 15 20 25 30Total Beam Current^2 (mA^2)Temperature increase (ºC)0123456Power dissipated in the bellows (W)Figure 5: Quadratic dependence of the temperature increase of the warm bellows placed between twocavity modules with the beam current. Estimation of the equivalent power dissipated.8The quadratic dependence of the temperature increase of the warm bellows placedbetween two cavity modules with the beam current is shown in Fig. 5. An estimation of thepower dissipated can be obtained by an in situ thermal calibration of the bellows.4.3. Influence of bunch lengthMany measurements were done with different bunch lengths at different energies.  Fig.6 shows as an example the power (normalized to itot2=1 mA2), measured using the valvepositions. These measurements include all energies between 22 and 91.5 GeV.  The data donot show any energy dependence.  Measurements were done at similar bunch lengths atdifferent energies.y = 93.425e-0.4268xR2 = 0.9597y = 48.701e-0.4147xR2 = 0.87740246810125 6 7 8 9 10Bunch length [mm]Pcryo/Itot^2 [W/mA^2]Figure 6:  Beam induced cryogenic heat load at IP6 as function of bunch length.  Eight equally spacedbunches of positrons were used.In Fig. 7 the cryo load due to the beam is shown for modules not driven with RF, as afunction of bunch length.  The data do not show a dependence on beam energy.Measurements of the global consumption using the “pot” of the cryo-plant were taken at thesame time.  They give at 45 GeV 7 W/mA2 for V=5.5 mm and 1.9 W/ mA2 for V=9.5 mm permodule, in agreement with the data shown in Fig. 7.Fig. 8 shows the dependence of the temperature increase of the warm bellows placedbetween two modules when the bunch length is decreased. This figure which contains the datacollected at different energies confirms both the exponential bunch length dependence of thelosses and the lack of correlation with beam energy.9Further measurements will be carried out since these components which can beequipped easily reproduce the power deposition mechanisms observed in the inter-cavity coldbellows.0246810125 6 7 8 9 10 11 12 13Bunch length sigma [mm]P cryo / I^2 [W/mA^2]Figure 7:  Beam induced cryogenic load measured on modules without RF.  Eight equidistant bunchesof positrons were used.  The data were taken at energies of 45, 60 80, 87, and 91.5 GeV.051015205 6 7 8 9 10 11 12 13Bunch length (mm)Temp. increase of the bellows (degre C)Figure 8: Variation of the temperature increase of warm bellows at different energy when the bunchlength is decreased (45, 60 80, 87, and 91.5 GeV).104.4. Both beamsA series of measurements was done with both beams with four bunches each, at60 GeV.  The modules were driven at constant field of 35 MV.  Again the power deposited bythe beam at a bunch length of 9 mm was 1.9 W/mA2, measured using the valve positions.  Nodifference can be seen compared to the experiments with only one beam of positrons.Also the measurements taken during physics, as shown earlier, show no significantdifference from the conditions with only one beam.5. TEMPERATURES OF COLD (NORMAL CONDUCTING) BELLOWSIn order to allow alignment and thermal movements, the four cavities in a cryostatmodule are separated by three bellows of 193 mm aperture, 62 mm warm length andundulations with 7 mm depth and 3.1 mm width.  Two bellows of the same geometry aremounted on the ends of the first and the last cavity before the conical transitions to roomtemperature.  The only normal conducting components of the beam vacuum inside the cryostatare these five bellows and the transition cones.  Cryogenic temperature gauges are installed onthe cold inter-cavity bellows of some modules, as described in [5]. It is assumed that all thepower deposited in the bellows and detected by a temperature rise is finally dissipated in theliquid He bath and thus contributes to the overall cryogenic consumption. The computed lossfactors of a complete four-cavity module and of the five bellows of a module [7] are seen inFigs. 9 and 105.1. Influence of Synchrotron Radiation etc.During the test described in 4.1 no temperature variation was detected.01234567894 5 6 7 8 9 10 11 12 13 14 15 16bunch length (mm)k (V/pC)ktot of complete modulek without intercavity bellowsk for f>2.2 GHzk for f<1.1 GHz 00.20.40.60.814 5 6 7 8 9 10 11 12 13 14 15 16bunch length [mm]k [V/pC]Figure 9: Loss factor a complete four-cavitymodule [7]Figure 10: Loss factor of the five bellowsalone [7]115.2. Temperature dependence on beam currentThe temperature dependence on beam current during a normal physics coast is plottedin Fig.11. The dependence is linear in itot2.5.3 Dependence on bunch lengthHaving determined that the temperature is linear in itot2, all the temperature data vs.bunch length have been normalized to 1 mA2. Temperature data at various beam energies andbunch lengths are plotted in Fig. 12.  These show a single relationship on bunch length,irrespective of the energy.00.511.522.533.540.0 5.0 10.0 15.0 20.0Ibeam^2Temp [K]00.20.40.60.811.24 6 8 10 12 14Bunch Length [mm]Kelvin/Ibeam^2T@45 T@60 T@80 T@85 T@87 T@91.5Figure 11: Temperature rise vs. beamcurrent2 at 91.5 GeVFigure 12: Bellows temperature vs. bunchlength at various beam energies6. CONCLUSIONSNot all measurements taken could be presented here.  The conclusions are:xThe beam deposits additional power Pbcr into the cryogenic system, with a large spreadbetween individual modules.  The reason for the differences is not understood yet,however, the loss factor of the bellows strongly depends on the exact bellows geometry.The power measurements done with two independent methods agree within the errors incases where both measurements are available.12xPbcr scales with the square of the beam current.xPbcr is strongly bunch length dependent.  Above 9-10 mm bunch length the slope levels offand operation above 9–10 mm is recommended, above which the impedance Z is 16 M: atthe most.xNo dependence of Pbcr  on beam energy, collimator positions or bunch train bumps wasseen.xNo dependence on operation with one beam or both beams was seen.xThe additional cryogenic losses can be explained by power deposited in the normalconducting parts of the cavity system.  Temperature increase measured on cavity bellowsshows exactly the same behavior as Pbcr .  The loss factor calculated for the bellows couldexplain the power deposition.xUpgrading of the cryogenic installation therefore is essential, especially in view of operationat higher gradients (>6 MV/m). In fact, the additional power losses, estimated to be closeto 50 W per module at 6 MV/m and 5 mA, are no longer negligible compared to the 280 Wof the dynamic RF loss.AcknowledgementWe are grateful to G.von Holtey and L. Vos for their participation in discussions andfor their help in measurements and to the CERN CryoLab for the supply and installation of thecryogenic thermometry system.References[1] J.M. Jimenez and al., CERN 6th LEP Workshop, Chamonix (1996)[2] P. Gayet and al., CERN 6th LEP Workshop Chamonix (1996)[3] J.M. Jimenez and al., CERN 7th LEP Workshop, Chamonix (1997)[4] P. Gayet and al., CERN 7th LEP Workshop Chamonix (1997)[5] G.Cavallari, P.Gayet, G.Geschonke, M.Jimenez, S.Myers, Beam Related ThermalLosses in the LEP SCRF System,   CERN LEP2 note 97-40[6] L.Vos, CERN SL MD note 214, July 1996[7] E. Haebel, E. Plawski, CERN LEP2-Note 94-19

Beam related thermal losses on the cryogenic and vacuum systems of LEP

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