A superconducting undulator is installed in the ANKA electron storage ring. Electron clouds could potentially contribute to the heat load of this device. A microwave transmission type electron cloud diagnostic has been installed for the undulator section of the ANKA machine. We present the system layout with particular emphasis on the electron machine aspects. Hardware transfer function results and e-cloud data for different machine settings are discussed. Special care has been taken for front end filter design both on the microwave injection and pick-up side

Birkel, I

Casalbuoni, S

Caspers, Friedhelm

Huttel, E

Mashkina, E M

Muller, A S

Saez de Jauregui, D

Smale, N J

Sonnad, K G

Weigel, R

CERN Document Server

OBSERVATION OF ELECTRON CLOUDS IN THE ANKA UNDULATORBY MEANS OF THE MICROWAVE TRANSMISSION METHOD ∗Kiran G. Sonnad (University of Karlsruhe, Karlsruhe), Fritz Caspers (CERN, Geneva),Ingrid Birkel, Sara Casalbuoni, Erhard Huttel, David Saez de Jauregui, Anke-Susanne Mu¨ller,Nigel John Smale, (Forschungszentrum Karlsruhe GmbH)Ralf Weigel (Max-Planck Institute for Metal Research, Stuttgart)AbstractA superconducting undulator is installed in the ANKAelectron storage ring. Electron clouds could potentiallycontribute to the heat load of this device. A microwavetransmission type electron cloud diagnostic has been in-stalled for the undulator section of the ANKA machine.Several tests were first performed to optimize the transmis-sion of the microwave across the hardware in the absenceof a beam. Initial tests with beam indicated the presence ofside bands in the output microwave spectrum at harmonicsof the the revolution frequency. While such side bands areexpected to be created by electron clouds, one cannot dis-count the presence of inter modulation caused by the beam,which also creates a similar effect. Work is underway to de-tect the extent of inter modulation and also minimize thisundesired effect.INTRODUCTIONCold bore wigglers or undulators provide higher fieldsfor a given gap and period length compared to magnets atroom temperature. In light sources, the usage of these de-vices offers the advantage of providing higher brilliance ata lower cost. Their use has also been proposed for linearcolliders, namely the ILC and CLIC. An important issuefaced by these devices is the heat load on the cold vacuumchamber caused by the beam. The proper understandingof the heat load mechanism is of paramount importance inoptimally designing these devices. There is considerableevidence that the formation of low energy electrons makesa significant contribution to the overall heat load. Currentdata obtained at various facilities cannot be explained onthe basis of heating from synchrotron radiation or resistivewall heating [5]. Besides this, low energy electrons havebeen observed and their effects been studied at the super-conducting undulator at the ANKA storage ring. Further,it has been recently shown that pressure rise in the ANKAcold bore superconducting undulator can be explained onthe basis of electron multipacting [6].We are currently working on studying these low energyelectrons using the technique of microwave transmission.This technique was first proposed for the SPS at CERN[1, 2] and was demonstrated at the PEP II low energy ring∗This work has partly been supported by the Initiative and NetworkingFund of the Helmholtz Association under contract number VH-NG-320for varying electron cloud densities [3]. The low energyelectrons cause the microwave to undergo a phase advancethat is different from that in the absence of the electrons,leading to a so called ”Phase shift” that depends upon theelectron density, which can be quantified under certain ide-alized conditions [4]. Due to a gap in the bunch train, aperiodic clearing of the electrons occurs leading to phasemodulation in the microwave propagating across the sec-tion. One can thus observe this ”Phase shift” in the formof modulation side bands in the spectrum of the microwaveafter it propagates through the section of interest.THE FACILITYANKA is an electron storage ring used as a synchrotronradiation source. The maximum achievable energy is 2.5GeV and the maximum current is 200 mA. The revolutionfrequency is 2.7 MHz and the machine is normally operatedwith three trains, each composed of 32 bunches separatedby 2 ns. The cold bore superconducting undulator builtby ACCEL Instr. GmbH, Bergisch Gladbach, Germany,is installed in one of the four straight sections of the ring.The rest of the ring is warm, while the temperature of thisdevice is maintained at about 4.5 K.Possible heating mechanisms of the cold bore device are:(1) synchrotron radiation from upstream magnets, (2) highfrequency image currents on the cold surface also calledresistive wall heating, (3) ions and electrons acceleratedto the walls by the transverse field of the ultra-relativisticbeam. The condensed gas layer adjacent to the surface,which is formed at low temperatures is a likely source ofelectrons impacting back into the chamber walls. The vac-uum chamber surface is very similar to one of the LHCbeam screens (300 m stainless steel with 30 m of electro-plated copper). For such a surface the dominant desorbedgases are H2 , CH4 , CO, CO2 and H2 and O2 [7].MICROWAVE TRANSFER IN THEABSENCE OF A BEAMThe cross section of the device is close to a rectangle,with chopped corners. The vertical gap of the chambercan be varied in steps of 16, 12, and 8 mm respectively.The width of the cross-section is 66.2 mm and the undu-lator is 1.5 m long, with 100 periods. Using the formulafor rectangular waveguides as an approximation, the cut-Proceedings of PAC09, Vancouver, BC, Canada TH5RFP044InstrumentationT03 - Beam Diagnostics and Instrumentation 3549off frequency for the fundamental mode was estimated tobe about 2.3 GHz. Both sides of the insertion device areequipquiped with 4 Beam Position Monitors (BPMs) each.The ones on the same side of the chamber cross-sectionwere connected and the two pairs were used as electrodesfor launching the wave on one end and receiving the waveat the other respectively. The transmission was examined inthe absence of a beam for different frequencies using a net-work analyzer. We found enhanced signal at around 2.54GHz. This corresponds to a trapped mode where the trans-fer of the signal from the BPMs into the vacuum chamberis well matched.Figure 1: Enhanced transmission at trapped modeOBSERVATIONS IN THE PRESENCE OFAN ELECTRON BEAMMeasurements of the transmitted spectrum were madewith a beam at 2.5 GeV with a set up shown in Fig. 4. Theinput frequency of the microwave was 2.52 GHz from anHP 86608 synthesized signal generator. We had 180 deghybrids connected to the input and output segment of thecircuit. The output was amplified using a an Avantek solidstate amplifier before being fed into an HP 8562E spectrumanalyzer. The magnetic field of the undulator was switchedoff.Figure 2 shows the output spectrum. We see that thecarrier frequency is well above the noise floor displayingsufficient transmission. The side bands are visible at har-monics of the revolution frequency upto the sixth order onthe right and the with order on the left of the carrier sig-nal. This disparity is most likely due to loss in transmissionat the 6th lower frequency sideband. The signal from theelectron beam itself is clearly visible at harmonics of therevolution frequency. The 180 deg hybrids serve the pur-pose of minimizing the signal from the beam affecting thespectrum analyzer, amplifier and the signal generator. Thiscould be be further improved by using front end filteringwhich eliminates unwanted frequencies. Also, the use ofan isolator/circulator would further minimize the transmis-sion of the beam signal generator. As explained below, it isessential to minimize the signal produced by the beam.Figure 2: Left and right sidebands with carrier signal in thepresence of beamWhile it is well known by now that the low energy elec-trons or electron clouds produce side bands, the same side-bands could also be attributed to the nonlinear mixing ofthe carrier signal and the beam signal, leading to undesiredinter modulation effects. Despite all measures to minimizethe signal from the beam, one can never eliminate its pres-ence. Several tests have been planned for the future wherethe conditions producing these low energy electrons canbe varied. We will be able to analyze the signal at dif-ferent beam currents and beam energies. This would varythe intensity of the synchrotron radiation produced in theundulator and thereby also vary the extent of low energyelectrons produced. Another planned test is to repeat thisacross a warm, straight section where we believe the pres-ence of low energy electrons is negligible. If the propertiesof these side bands vary based on the extent of low energyelectrons believed to be produced, we can conclude that thesidebands are indeed produced by the low energy electrons.In this process, we would obtain an independent confirma-tion of the presence of these electrons, already done withthe help of a retarding field analyzer [6].The experiment will need to be repeated with the undu-lator magnetic field on. This can influence the side bandsin two ways. The enhanced synchrotron radiation from theundulator could influence the creation of low energy elec-trons. Besides this, the external magnetic field could leadto cyclotron resonance leading to amplification of the sidebands. This effect occurs when the cyclotron frequencydue to the external magnetic field is equal to the carrier fre-quency, which was demonstrated through simulations andlater in an experiment carried out at the PEP II high en-TH5RFP044 Proceedings of PAC09, Vancouver, BC, Canada3550InstrumentationT03 - Beam Diagnostics and Instrumentationergy ring [8]. Several factors will need to be consideredbefore expecting to see this resonance effect in this system.First of all, the fundamental mode of propagation of the mi-crowaves in this geometry corresponds to a case where thewave electric field has mostly a vertical component. This isbecause the cross section of the beam pipe is much widerand relatively short. In such a case, the external magneticfield will need to have a horizontal component, perpendic-ular to the wave electric field in the area where low energyelectrons are present. This corresponds to the so called ex-traordinary wave or X-wave, necessary to create the cy-clotron resonance (which is the same as upper hybrid res-onance in low plasma frequency limit). An ideal undulatorfield would offer only ordinary, or O-wave propagation atlower modes which are unaffected by undulator magneticfields (which are parallel to the wave electric field). On theother hand, several practical features such as fringe field ef-fects and alterations near the surface of the chamber couldproduce conditions leading to the cyclotron resonance.Figure 3: The bunch fill pattern captured by an oscilloscope(span = 300 ns)signal generatorhybrid180 deg 180 deghybridamplifieranalyserspectrumbeam pipeFigure 4: Schematic of the layout of the experimentA DEMONSTRATION OF THEUNDESIRED INTER MODULATIONEFFECTTo demonstrate the inter modulation effect from thebeam, which also leads to side bands, we conducted thefollowing test. An RF generator was pulse modulated witha 3 pulse sequence that was 500 MHz sinusoidal with a 2.7MHz repetition frequency that resembled the bunch fillingpattern in the storage ring (Fig.(5). This signal was super-imposed upon the the 2.53 GHz carrier signal used in theexperiment. The output of this mixed signal was fed intothe same receiver that was used in the microwave transmis-sion experiment. There were clear signs of inter modula-tion producing sidebands as shown in Fig(5). These resultssuggest that the sidebands produced by the low energy elec-trons could be contaminated by inter modulation effects.More work needs to be done to eliminate the inter modu-lation of the two signals in the real experiment so that theside bands are created entirely from the low energy elec-trons present in the undulator section.Figure 5: Signal generated to resemble fill pattern of beam,and demonstration of sidebands produced purely from intermodulationREFERENCES[1] Kroyer, T ; Caspers, F ; Hfle, Wolfgang ; Jimnez, J M ;Malo, J F ; Tckmantel, Joachim, 31st Advanced ICFA BeamDynamics Workshop on Electron-Cloud Effects, Napa, CA,USA, 19 - 23 Apr 2004, pp.89-94[2] T. Kroyer, F. Caspers, E. Mahner Proceedings of PAC, 2005,Knoxville TN[3] S. De Santis et al. Phys. Rev. Lett. 100, 094801 (2008)[4] K. G. Sonnad, M. Furman et al., Proceedings of PAC, 2007[5] S. Casalbuoni et. al. Proceedings of EPAC, 2008, Genoa, Italy[6] S. Casalbuoni et al., Phys Rev ST Accel. and Beams, 10,093202 (2007)[7] S. Casalbuoni, S. Schleede, D. Saez de Jauregui, M. Hagel-stein, in these proceedings[8] M. T. F. Pivi et al Proceedings of EPAC, 2008, Genoa, ItalyProceedings of PAC09, Vancouver, BC, Canada TH5RFP044InstrumentationT03 - Beam Diagnostics and Instrumentation 3551

Observation of Electron Clouds in the ANKA Undulator by Means of the Microwave Transmission Method

Observation of Electron Clouds in the ANKA Undulator by Means of the Microwave Transmission Method

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