1、Wire Setup Calibration of Beam Position MonitorsD. Wang, B. Binns, M. Kogan, A. ZolfaghariMassachusetts Institute of TechnologyBates Linear Accelerator CenterMiddleton, MA 01949, USAABSTRACTMany button type and strip line type beam positionmonitors are employed in the South Hall Ring (SHR) at MIT-Ba
2、tes. It is desired to calibrate and routinely check the readouts ofthose monitors by simulating the beam. A wire setup was built toserve this purpose. It consists of a button type beam positionmonitor body with a wire running through it. To eliminate end-effects both ends are extended with 10 inch l
3、ong uniform beampipes. RF matching is carefully performed with ECCOSORB AN75 microwave absorber forms. The wire is positioned 3 mm offsetwith respect to the X-pair buttons and symmetric with respect tothe Y-pair buttons. Thus the same setup can be used to check bothsensitivity and zero-offset.INTROD
4、UCTIONA number of stripline and button beam position monitorsare used in the SHR. The readouts of those beam position monitorschange due to zero-drifting of electronic circuits (thermal and/oraging), radiation effects on electronic components, inadequatelymatched 3-dB hybrids, cable bends, etc., To
5、get reliable data, it isnecessary to perform periodic field calibrations and routinely checkthe zero-offsets and sensitivities of those beam position monitorelectronics using rf to simulate the beam.The normal rf test of the electronics does not calibrate theentire system. We have built a calibratio
6、n setup with an actualbeam position monitor and a wire, excited by rf, to activelysimulate the beam.The SHR is a 0.3-1 GeV, multi-bunch (containing 1812buckets) electron beam storage ring1. The rf frequency of thering is equal to the rf frequency of the linear accelerator, that is2856 MHz. Consequen
7、tly, all beam position monitors operate onthis frequency23.The setup is basically straight forward: a wire runningthrough a button type beam position monitor. The beam positionmonitor is stimulated through the wire by a 2856 MHz rf source.The displacement of the wire is a simulation of the beam in a
8、noffset position.DESCRIPTION OF THE FIXTUREThe construction of the wire setup is shown in Fig. 1. Thefixture consists of a button type beam position monitor body with25 cm long uniform extension pipes at both ends. The innerdiameter of the monitor and pipes is 60 mm. The function of thesepipes is to
9、 eliminate the end-effects (local modes effects). A wirewith diameter of 0.5 mm runs through the beam position monitorand the uniform pipes with a displacement of 3 mm from the axialcenter in the horizontal plane. The wire is tightly stretched tominimize sag and vibration. A dielectric plate with a
10、clearance holeis at both ends to assure good alignment.Fig. 1 Construction of the wire setupAn SMA bulkhead feedthrough jack receptacle ismounted on each end-plate, and the wire is soldered to the innerconductor of the SMA bulkhead. One end is the rf input, the otherend is terminated with an rf term
11、inator. Because the characteristicimpedance of the wire-pipe system is 60*ln(30/0.5)=287 G53,parallel and serial resistance matching is needed in order to match2536 1996 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promot
12、ional purposes or for creating new collective works for resale or redistribution to serversor lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.to the outside 50 G53 system. Also, we placed some microwaveabsorber forms, ECCOSORB AN 75, inside to
13、further improvematching.RF RELATED CONSIDERATIONSThe beam position monitor rf signal processing blockdiagram is shown in Fig. 2. The beam induced signals on thetransducer pairs (horizontal and vertical) are transmitted to the 90G45hybrids. The amplitude difference of these pairs of beam inducedsigna
14、ls, which is a function of beam position, is converted to aphase difference by the 90G45 hybrids. The electronics box processesthis phase difference information. The output of the phase detector,which is transmitted to the Central Control Room, is themeasurement of the beam position.Fig. 2 BPM elect
15、ronics block diagramA very important characteristic of the system is thedynamic range, which is determined by the mixers used in thecircuits (Mini-Circuits ZEM-4300MH). From the benchmeasurements, shown in Fig. 3, we note the following: when the rfsignal power level to the hybrid is within the range
16、 of -40 dBm to0 dBm, the output of the electronics circuit is only dependent onthe phase difference of the input signals to the mixers, andindependent of signal amplitude. In other words, the beam positioninformation will not be deteriorated by the beam intensity if thebeam induced rf signal power l
17、evel transmitted to these hybrids iswithin the above mentioned range. Fig. 3 Dynamic range of the BPM electronics circuitAfter the fixture was built we measured the transition, S ,21of this wire setup from the input (rf stimulating port) to the output(BPM pickup), which was around -50 dB. Thus, the
18、stimulating rfsignal power level should be within the range of 10 dBm to 50dBm. We set the stimulating rf power level at about 20 dBm,which is easily available.BENCH TEST AND FIELD OPERATIONTo investigate the overall response of the beam positionmonitor to the displacements of the wire, we did bench
19、measurements using a HP8510B Network Analyzer. Table 1shows the results. Table 1: Bench measurements of position sensitivity_displacement signal diff. on phase diff. convertedof wire (mm) pair-transducers(dB) after hybrid (G45)3 3.7 23.46 7.2 43.09 10.7 57.112 15.4 70.515 19.2 77.8_From Table 1 we c
20、an see that over the range of half radiusof the beam pipe, 1 mm displacement of the wire corresponds with1.23 dB on average.One of the advantages of the wire setup calibration overthe normal electrical-zeroing by using rf only is that you can2537determine if the signal polarity is correct. For examp
21、le, in our casewe want a positive signal when the beam (wire) goes right or up,and negative when the beam (wire) goes left or down.Because the wire is positioned 3 mm off center withrespect to the horizontal (X) transducers and approximatelycentered with respect to the vertical (Y) transducers, by s
22、wappingX and Y connections we obtain both the sensitivity and the zero-offset of the beam position monitor electronics box under test (seeFig. 1 and 2). Table 2 lists some sample data on the field operation.Fig. 4 is a scope display of the wire setup field calibration asviewed in the Central Control
23、 Room.Table 2. Sample data of the wire setup calibration_BPM # *LPM17 *LPM18_Test date 09/27/94 09/27/94R to 1 (L to 2) 298 335R to 2 (L to 1) -325 -274R to 3 (L to 4) 35 94R to 4 (L to 3) -63 -12Zero-offset(mV) -14 41Sens. (mV/mm) 104 102_T to 1 (B to 2) 304 319T to 2 (B to 1) -296 -282T to 3 (B to
24、 4) 54 50T to 4 (B to 3) -52 -30Zero-offset(mV) 1 10Sens. (mV/mm) 100 100_* LPM17 and LPM18 are the two most important positionmonitors for SHR operationACKNOWLEDGEMENTThe authors would like to thank E. Ihloff, J. Grenham andall others in the Mechanical Group for their support in making thewire setup.Fig. 4 Wire calibration signal viewed in CCRREFERENCES1 J. B. Flanz, et al., Proceedings 1989 Particle AcceleratorConference pp34-36 (1989)2 J. B. Flanz, et al., Proceedings 1993 Particle AcceleratorConference pp2331-2333 (1993)3 A. Zolfaghari, C. Sibley, et al., Bates internal reports2538
