Duel, K. L.

Tan, C. Y.

Zwaska, R.

UNT Digital Library

AN IMPROVED RETARDING FIELD ANALYZER FOR ELECTRONCLOUD STUDIES ∗C.Y. Tan† , K.L. Duel‡ , R. Zwaska§ , Fermilab, Batavia, IL 60510, USAAbstractWe have designed a retarding field analyzer (RFA) anda rad-hard amplifier which improves the sensitivity overthe present RFA installed in the Main Injector of Elec-tronc Cloud measurements. From computer simulationsand bench measurements, our RFA will have a 20% im-provement in sensitivity compared to the Argonne NationalLaboratory (ANL) design. And when we couple our RFAto the matched rad-hard amplifier, S/N will also be im-proved.INTRODUCTIONThe electron cloud is a collective instability that maylimit the performance of high-intensity particle accelera-tors [1]. The electron cloud consists of an accumulation ofelectrons in the vacuum vessel of an accelerator. The elec-trons are non-relativistc, but are accelerated transversely bythe beam’s electromagnetic field and are multiplied throughsecondary-electron emission on the vacuum vessel. Theelectron cloud can interact adversely with the beam throughthe direct driving of instabilities and through tune shiftsonto other, pre-existing instabilities.Retarding Field Analyzers (RFAs) have been routinelyused for electron cloud detection [2, 3]. An RFA directlymeasures the flux of electrons at the interior surface of thevacuum vessel above a certain energy. Electrons are al-lowed to ballistically exit the vessel through slots cut onthe surface. Beyond the surface, there is some grid, ring,or other potential surface that provides a retarding electricfield that prevents the transit of electrons below a certainenergy. After the retarding field, the electrons are collectedon an electrode configured like a Faraday cup. The currentfrom the cup is a direct measure of the number of electronsentering the device with energies above the cutoff estab-lished by the retarding field.The flux of electrons at the surface of the vessel is di-rectly related to the density of electrons within the vacuum.For protons bunches in the range of the Main Injector’sspacing, electrons will typically bombard the surface onceper bunch passage. Thus, the flux at the pipe surface canbe related to the density as:F ≈λC· fRF ·bh(1)∗Work supported by Fermi Research Alliance, LLC under ContractNo. DE-AC02-07CH11359 with the United States Department ofEnergy.† cytan@fnal.gov‡ kduel@fnal.gov§ zwaska@fnal.govHere,λ is the linear electron density,C is the tranverse cir-cumference of the vacuum vessel,fRF is the RF frequency,b is the number of bunches within the machine, andh is theharmonic number. The relation is approximate becuase itis possible for an electron to not impact the surface on onebunch passing, and it is possible for there to be multipleimpacts. In practice, a detailed simulation of the electroncloud dynamics must be performed to make a prediction ofthe flux; such simulations have confirmed the above rela-tion for the Main Injector to within 10% [4]. For a 1% neu-tralization of the Main Injector beam at typical operationalintensities (λbeam = 1011 protons per5.7 m, C = 48 cm,fRF = 53 MHz, b = 500, h = 588), we would expect aflux of 0.26 µA/cm2.An RFA borrowed from Argonne National Laboratory[5], herein known as the ANL RFA, was installed in theMain Injector in 2006 and has been operating since then[6]. This device was used for successful measurementsof high-intensity electron cloud formation. However, ithas suffered from significant noise, limiting the measurablesignal and making energy spectrum measurements imprac-tical. Currents as high as 4µA were measured. In typicaloperation the noise floor limits us to measurements of morethan0.1 µA, though careful noise cancellation allowed spe-cific runs to be measured to0.001 µA. One of the discov-eries was that the Main Injector pipe conditions over timesuch that the signal was below the measurable threshold.The need for RFAs required new construction. This pa-per describes an evolution of the ANL design for the pur-poses of the Main Injector. The desired attributes are highsignal efficiency, low noise, and sharp energy discrimina-tion. A very-high signal efficiency will allow us to measurethe weak electron cloud in a conditioned beam pipe, andperhaps allow measurement of the residual electron fluxwhich would provide a baseline signal and possibly cali-bration. A primary source of the noise on the previouslyinstalled ANL RFA was the long cable run from the detec-tor to the amplifier on the surface, and the poor quality ofthat cable. This noise will be addressed by amplificationand filtering in the tunnel and a higher-quality cable.DESIGNWe have usedSIMION[7] to simulate the various trialRFA designs. We have used it to calculate the electric fieldsand the capture efficiency for each trial design before com-ing up with our final design with dimensions shown in Fig-ure 1. The improvements to the ANL design[5] are as fol-lows:• Increased surface area. The collector area has been in-creased by1.8× compared to the ANL RFA. Clearly,more electrons will be captured and thus sensitivityimproved because of this increase.• Better focusing. The electric fields of our RFA havebeen designed so that electrons which have– energy greater than the voltage on the grid at theentrance of the RFA and– momentum vectors within an azimuth and ele-vation of ±10◦ w.r.t. the symmetry axis of theRFAwill be focused onto the collector.• Fewer grids. From the results of the simulationsshown in Figure 2, we have found that each 25 linesper inch grid reduces the number incident electrons by∼ 20%. The ANL design has two grids while our de-sign only has one. (Note: The grid reduces the slopeof the energy filter and allows electrons below the gridvoltage to slip through because the centre of each holeis at a lower voltage than at the grid wire). We havealso tested our RFA and the ANL RFA on a test standand have found that the grid only shields the collectorby about 8 dB in voltage while the slots in the beampipe shield the collector by 17 dB (or 25 dB includingthe grid). Therefore, a single grid should be sufficientfor shielding the collector from the beam.Figure 1: This is the cross section of the RFA model, in-cluding the pipe which encloses it, used inSIMION. Themajor parts of the RFA are the grid shaded in green andcollector shaded in cyan. The equipotential lines are in red,and the trajectories of the electrons are in blue.Figure 2: The capture efficiency of the our RFA (red) andthe ANL RFA (blue) when the grid is ideal and set to−100V are compared here. The green trace shows the re-sult when the grid is non-ideal. Notice that electrons withenergy< 100 eV can slip through a non-ideal grid.RealizationA prototype of our RFA is shown in Figure 3. An elec-trical feedthrough with SHV coaxial connections is used tosupply a voltage to the grid and return a signal from thecollector cup. The grid is an electroformed copper meshwith 25 lines per inch which covers approximately 12% ofthe surface area. The clamps which hold the grid and thecollector cup are machined from OFHC copper and will becoated with graphite. The support plate, collector cup, andgrid are isolated from one another by the use of aluminatubing and washers. Figure 4 shows a cross section draw-ing view of our RFA.ElectronicsFrom our measurements of the ANL RFA in the MainInjector (MI), we have found that the noise is∼ 10 mVgoing into1 MΩ load. In fact, the noise spectrum is dom-inated by signals starting from10 kHz. We have designeda high gain low pass filter (LPF) around a low noise, highslew rate, rad hard opamp (HS-5104ARH)[8]. This filter isan 8th order Butterworth filter with its 3 dB point at 3 kHzand a voltage gain of 40 dB. Figure 5 shows the measuredresponse of this filter. We intend to install this filter in thetunnel so that the small electron cloud signal can be ampli-fied and filtered before it is sent upstairs to the electronicsroom. This high gain LPF should improve the S/N ratio ofthe signal seen upstairs.As a precaution, we have designed the circuit so that theelectron cloud signal can bypass the LPF in case the elec-tronics fail or the signal is much larger than10 mV. Thebypass is in effect when the LPF is not powered.CONCLUSIONA new installation is planned in 2009 for Main Injec-tor electron cloud experimentation. This installation willFigure 3: Side and top view of our RFA. The side viewshows the wires connecting the grid and the collector tothe feedthroughs which are clearly not 50Ω. The top viewshows the aperture and the grid.include a one-meter pipe coated with TiN, and an identi-cal uncoated tube, to validate TiN’s mitigation effect in theMain Injector. To measure the effect requires substantiallymore instrumentation. Four RFAs described in this paperwill be installated as well as microwave antennæ [9] forelectron cloud detection.REFERENCES[1] R. Caminoet al., “Can Low-Energy Electrons Affect High-Energy Physics Accelerators”, Phys. Rev. Lett.93, 014801(2004)[2] K. C. Harkay and R. A. Rosenberg, Phys. Rev. ST Accel.Beams6, 034402 (2003).[3] R.J. Maceket al., “Electron cloud generation and trapping ina quadrupole magnet at the Los Alamos proton storage ring”,Phys. Rev. ST Accel. Beams,11, 010101 (2008).[4] C. M. Celataet al., “Simulations of Electron Cloud Effects onthe Beam Dynamics for the FNAL Main Injector Upgrade”,Proc. of Particle Accelerator Conf., Albuquerque, New Mex-ico, 25-29 Jun 2007, pp 3985.[5] R.A. Rosenberg and K.C. Harkay, “A Rudimentary Elec-Figure 4: Cross section drawing view of our RFA.Figure 5: The frequency response of the LPF. The mea-sured gain is 39 dB and the 3 dB point of the filter is2.6 kHz which is close to the SPICE model calculation.tron Energy Analyzer for Accelerator Diagnostics”, Nucl. In-strum. MethodsA453, 507 (2000).[6] X. Zhanget al., “Electron cloud studies at Tevatron and MainInjector”, Proc. of Particle Acceleartor Conf., Albuquerque,New Mexico, 25-29 Jun 2007, pp 3501.[7] SIMION 8, Scientific Instruments Inc., http://simion.com.[8] HS5104-ARH,“HS5104-ARH, Low Noise Quad OperationalAmplifier”, Intersil, http://intersil.com.[9] N. Eddyet al, “Measurement of Electron Cloud Developmentin the Fermilab Main Injector Using Microwave Transmis-sion”, this conference, 2009.

An Improved Retarding Field Analyzer for Electron Cloud Studies

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An Improved Retarding Field Analyzer for Electron Cloud Studies

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