The halo experiment presently being conducted at the Low Energy Demonstration Accelerator (LEDA) at Los Alamos National Laboratory (LANL) has specific instruments that acquire horizontally and vertically projected particle-density beam distributions out to greater than 10{sup 5}:1 dynamic range. They measure the core of the distributions using traditional wire scanners, and the tails of the distribution using water-cooled graphite scraping devices. The wire scanner and halo scrapers are mounted on the same moving frame whose location is controlled with stepper motors. A sequence within the Experimental Physics and Industrial Control System (EPICS) software communicates with a National Instrument LabVIEW virtual instrument to control the motion and location of the scanner/scraper assembly. Secondary electrons from the wire scanner 0.03-mm carbon wire and protons impinging on the scraper are both detected with a lossy-integrator electronic circuit. Algorithms implemented within EPICS and in Research Systems Interactive Data Langugage (IDL) subroutines analyse and plot the acquired distributions. This paper describes this beam profile instrument, describes their experience with its operation, compares acquired profile data with simulations, and discusses various beam profile phenomena specific to the halo experiment

Barr, D. S. (Dean S.)

Day, L. A. (Lisa A.)

Gilpatrick, J. D. (John Douglas)

Gruchalla, M. (Michael)

Kamperschroer, J. H. (James H.)

O'Hara, J. F. (James F.)

Stettler, M. W. (Matthew W.)

Valdiviez, R. (Robert)

UNT Digital Library

LA-UR-01- 2459 e./ N A T  I O N  A L TITLE: A UTHOR(S): SUBMITTED TO: L A B O R  BEAM-PROFILE INSTRUMENTATION FOR BEAM-HALO MEASUREMENT: OVERALL DESCRIPTION, OPERATION, AND BEAM DATA J. Douglas Gilpatrick Dean S. Barr Lisa A. Day Debora M. Kerstiens Matthew W. Stettler Robert Valdiviez Michael E, Gruchalla James F. O'Hara James H. Kamperschroer DIPAC May 13 - May15 Crenoble, France LANSCE-1 LANSCE-1 LANSCE-8/CON LANSCE-8 SNS-04 ' LANSCE-1 Honeywell Corp. Honeywell Corp. General Atomics mos A T O R Y  Los Alamos National Laboratory, an afflrmative action/equal opportunity employer, is operated by the University of Califomla for the U.S. Department of Energy under contract W-7405-ENG-36. By acceptance of thls article, the publisher recognizes that the U.S. Government retelns a nonexclusive, royaltyfree license to publish or reproduce the published form of thls contribution, or to allow others to do so, for U.S. Government purposes. The Los Alamos National Laboratory requests that the publisher identify this article as work perfoned under the auspices of the U.S. Department of Energy. Form No. 838 R5 ST2829 1Wl EAM-PROFILE INSTR~UMENTATION FOR BEAM-HALO MEASUIREM ENT: IVIERAIL DESCRIPTION, OPERATION, AND BEAM DATA’ I). Barr, L. Dily, J. D. Gilpatrick, D, Kerstiens, M. Stettler, R. Valdiviez, Los Alamos National Laboratory M. Gruchalla, J, O’l-Iara, Honeywell Corporation J. Kamnpa.schrcse.r, General Atomics Corporation A bslract The halo experiment prcsently being conducted at the Low Energy Demonstration Accelerator (LEDA) at Los Alanms Nationill Laboratory (LANL) has specific instruments that acquire horizontally and vertically projected particle-density bcam distributions out to greakr than 1@:1 dynamic range. We measure the core of the distributions using lradilional wire scanners, and the tails of the distribution using water-cooled graphite scraping devices. ‘Thc wire scanner and halo scrapers are mounted on the same moving tiarne whose location is controlled with stepper motors. A sequence within the Experimental Physics and Industrial Control System (EPICS) sortware communicates with a National Lnstrurnent Lal~wEw virtual instrument to control the motion and location of the scannerhxraper assembly. Secondary clectrons from the wire scanner 0.03-mm carbon wire and protons impinping on the scraper are both detected with a lossy- integrator elcctronic circuit. Algorithms implemented within EPICS i d  i n  Rescarch Systems’ Interactive Data Language ( U X )  subroutines analyse and plot the acquired distributions. This paper describes this beam profile instrument, describes our experience with its operation, compares acquired profile data with simulations, and discusses various beam profile phenomenon specilic to the halo experiment. 1 HALO INS‘IL‘MUMENTATIOW The L,PDA facility contains a 13” source and radio ficquency quadrupole (RFQ) that generate and iiijccts a 1 OO-mA, 6’7-MeV beam into a SLquadr upole magnet lattice (see fig. 1). Within this 1 l-rn FODO latticc, there are nine wire scanner/halo scraper/wire scanner (WS/HS) stations, fivc: pairs of steering magnets and beam position monitors, five loss tnonif ors, three pulseti-beam current monitors, and two image-current monitors for monitoring beam energy. Thc WS/HS instrunlent purpose is to measure the beam’s transvase projected disi ributiorrs with sufficient detail to relate the acquired data of the generated beam halo to the distributions generated in various arrP1rr:itnr rhnrtrrrl n:irtic+ si mii1:itinn rnrlrq Tkrr f‘irst WSMS station, located after the fourth quadrupole magnet, verifies the beam’s transverse characteristics after the beam exits the KFQ. A cluster of four H S N S  located after magnets #20, #22, #24, and #26 provides phase space information after the beam has debunched but prior to a full formed beam halo. After magnets #45, #47, #49, and #51 resides the final four WS/HS stations. These four WS/HS acquire projected beam distributions under both matched and mismatched conditions. These conditions are generated by adjusting the fields in the first four lattice quadrupole magnets so that the RFQ output beam may be matched or mismatched in a known fashion to the rest of the lattice. Because the halo takes many lattice periods to fully dcvelop, this final cluster of WSMS are positioned within the lattice to be most sensitive to halo generation. Figure 1. The 1 1-m, 52-magnet FODO lattice includes nine WS/HS stations that measure the beam’s transverse projected distributions. As the RFQ output beam is mismatched to the lattice, what is actually observed by the WS/HS is a variety of distortions to a properly matched gaussian-like distribution. These distortions appear as distribution tails or backgrounds added to beam’s core distribution. It is the size, shape, and extent of these tails that predicts specific type of halo. However, not every lattice WS/HS observes tht: halo generated in phase space because the resultant distribution tails may be hidden from the projection’s view due to the halo's orientalion in phase spaci.:. This is why multiple WWHS are used in clusters to observe these various distributions tails no miltter what the phase space oiientation is of either the beam core or its associated tail. 2 WIRE SCANNEN/HALO SCRAPER Each station consists of a horizontal and vertical actuator assembly (see fig. 2) that can move a 0.03-nim crubon monofilament and two graphite/copper scraper sub- assemblies. The airbon wire and scriipers are connected to the same nrovablr: frame. Attached to this nioviible fmme is a linear actuator that provides the wire and s c r a p  eclgrs' relative position to within 0.2 pm, an additional linear potentiometer provides an absolute approximate position for LEIIA's run permit systems. A stepper motor and ball lead scew drives the actuated wire ml scrapeis, 'and microswitches and motor brakes limit the wire and scraper movement. Figurc 2 1 hc WS/II5 aswnihlv co~rtaiiis a nlovablc frame on which ii 0.03-mm carbon wire resides between two water-cooled gmpliite scrapers. The caibon wile, which sciises the beani'!i core, is cooled by therind radiation. If the bcam macropulse is too long. the wire temperature continues above the 1800 K iesultiiig ill the onset of thermionic emission. Thermionic emission gives the distribution an inaccurate appeamce of a larger distribution core current density. To diminate these effects for the hdo experinient, the milximum pulse lcngth iuld repetition rate is liiniti:d to approximiitely 30 11s and I Hz, respectively. The halo scrapers am composed of a 1.5-mm thick graphite phte limrrd to a wiiter-cooletl 1.5-tnin copper plate. Since 6.7-MeV protons iivi:rage r'ange in carbon is approximate 0.3 mni, the beam is completely stoped in graphite plate. The cooling via contluction lowers the avenige temperature of scraper sub-:asembly and allows the scraper to be cooled more rapidly than the wire (see fig. :3). The lower avcrage tenipeiature and faster cooling ;~llows the scraper to be driven in as fir as 2 rms widths from the beam distribiition peak without the penk tc~nper:itore increasing above I800 K. The inovment and positioning of each wire aid scraper pair is controlled by a nioveinent control system that contains ii steppc:r motor, steppr motor controller, a linear encoder, and an electronic tlrivcr amplitier. The controller's digital PIIS loop controls the speed iud accuracy at which the assembly is moved arid placed. 1 0.0 1.0 2.0 3.0 4.( Time (s) Figure 3: The temperature of the wire and scraper are liliown to be below 1800 K under normal pulsed-&ani conditions, of a 1-mm round beam with a 30-ps 100-mA beam pulse. The target position, as defined by the WSlHS operator, is relayed from the EPICS control screen via a software process variable to a National Instruments LabVIEW Virtual Instrument (VI). Based on the EPICS command, the VI then instructs the controllers to move the wire nnd scrapx to a target position. The VI also calibrates the relative position of the linear encoders based on the measured position of the limit switches. and provides some error feedback information. The total erron between the target wire position and the actual position attained am <+/-2% of a 1-mm width beam (see Fig. 4). J Positioii Error (mm) -- _- I_____-___ F'igure 4. The above histogram above shows a typical distribution of wire/scraper movement errors of WS/W #45 vertical axis. The distribution core i s  sensed by carbon wire as the bean intercepts the wire. A small portion of the beam's energy is imparted to the wire causing secondary electron emission to occur. The secondary electrons leaving the wire are replaced by negative charge flowing from the electronics. This c a n t  flow for both axes is conneded through a bias battery to an electronic lossy integrator circuit and followed by an amplification stage. The integrator capacitance and amplifier gain are set to allow a very wide range of values of accumulated charge. The lossy integrator circuit reset time constant is 1 ins. Data are a c q d  by digitising the accumulated charge through the lossy integrator at two different times within the beam pulse. The acquired charge difference acquired by subtracting the two values of charge within the macropulse provides a low noise method of relative beam charge acquisition. For the wire scanner, the bias is set to optimise secondary emission and limit the effects of positive ion collection by the wire. The scraper is biased slightly positive, sufficient to inhibit secondary emission and collect only 6.7-MeV proton charge. The wire and scraper accumulated charge signals are digitised using 12 bit digitisers. The analog noise floor has been measuIBd to be 0.03 pC, a noise level slightly lower than the scraper digital LSB noise level of 0.15 pC using the highest gain settings within the detection electronics. The frontend electronic circuitry, mounted on a daughter printed circuit board, is connected to a mothehard that ha5 all of the necessary interface electronics to communicate to EPICS via a VXI controller module. A software. sequence was written within EPICS to control and operate WS/HS instrumentation. The sequence instructs the VI to move the wire and scraper to a specific location, a q u k  a distribution data point from either the wire or scraper, normalise the acquired charge with a nearby toroidal current meawernent. graph the normalised data, and write the distribution to a file. The sequence also instructs IDL to calculate the first through fourth moments, fit a gaussian distribution to the wire scanner data, end calculate the point at which the beam distribution disappears into the distribution background noise. To plot the complete beam distribution for each axis, the wire scanner and two scraper datn sets must be joined. To accomplish this joining, several analysis tasks a~ performed on the wire and scraper data including. I )  Scraper data are. spatially differentiated, 2) Wire and scraper data am acquilrd with sufficient spatial overlap. and 3) Differentiated scraper data are normalised to the wire beam core data. The scraper data need only be normalised in the relative charge axis since the distances between each wire and scraper edge are known within 0.25-mm. In addition, the first four moments and the point at which the beam distribution disappears into the noise m also calculated for the combined distribution data. 3 TYPICAL ACQUIRED DISTRIBUTIONS Figure 5 shows a data from WSiHS #26 with some slight mismatch gene& by increasing the field above nominal by 5% in the first matching quadmpole magnet. These typical WS/HS profiles show distributions with a dynamic range of > 10':l. The calculated rms widths 1.10 and 1.13 mm for the horizontal and vertical distributions, respectively. The point at which the &, LE40 2 l.E-01 d l.E-02 1.E-03 E 1.E-04 ~4 1.E-05 I P 1.E-06 distributions rise out of the noise floor m 5.9 and -8.2 mm for the horizontal axis and 5.8 and -6.7 mm for the vertical axis. The WS/HS instrumentation is capable of providing distribution information to > 5X to 7X times typical rms widths of the beam. I -15 -10 -5 ' 0 5 10 15 Position (mm) I Figure 5.  WS/HS distributions, such as #26 shown here, have a typical dynamic range of > 10s: 1. 4 WIRE/BEAM AND SCRAPER/BEAM PHYSICS The choice of bias potential was determined by measuring the wWscraper potential versus h - r e l a t e d  current. The resulting data displayed in fig. 6 show that the wire is optimally biased at -6 to -12 V and the scraper is optimally biased at +20 to +30 V. Sonpar Elan. L d l  Horlaonlnl I a6000 r: Figure 6. The above graph shows the scraper relative current as a function of the bias potential on the scraper. The net wire current near the 0 V potential is approximately 15% higher than at either -1-6 V or -lOV bias. One proposed cause of the elevated net wire current is the interception of electrons and ions from offenergy protons ionising msidual background gas - both of these ionised species creating further secondary emission. As the wire is biased negatively, the intercepted electrons me mjected causing a reduction in secondary emission, and as the wire is biased positively, the intercepted ions me rejected causing a reduction in secondary emission. If the intercepted electron-ion pairs are the mechanism for the elevated net cumnt. the wirc should be biased to not include this additional 15% net cumnt component. Also note that as the wire bias is positively increased from 0 V to > +IO0 V. the wire secondary electron emission is inhibited and the net wir6 current reduces to very near zero. As expected, it is clear that a large positive bias reduces the wire detection signal. Furthermore, it appears that the wire collects positive ions with e -25 V bias potentials well after the beam pulse. This additional limits how the mount of negative bias that is applied to the wire for proper secondary emission operation. The scraper detection goal is to inhibit secondary emission and detect only 6.7-MeV protons. With 0 V applied to the scmper, the scraper net current is also elevated. With approximately t 2 5  V bias applied to the scraper, the secondary emission is almost entirely inhibited. Furthermore, with this +25 V bias, no ions appear to be collected after beam pulse. 4 SUMMARY A combination of a wire scanner and a halo scraper has been integrator into a single very wide dynamic range beam profile instrument. Nine of these pmfde instruments have been developed, installed, and operated at Los Alamos to measure the predicted beam halo generatwl by a magnetic lattice. These instruments are performing as expected and am present acquiring beam halo data. REFERENCES [l] P. L. Colestock, et al., “The Beam Halo Experiment at the LEDA Facility: a First Test of the Core-Halo Model,” LINAC’OO, Monterey, CA, August ??-??, 2000. [2] J. D. Gilpatnck, et al., ‘I Beam Diagnostics Instrumentation for a 6.7-MeV Proton Beam Halo Experiment”, LINAC’OO, Monterey, CA, August ??- ??, 2000. [3] T. Wangler, et al., “Beam Halo in Proton Linac Beams,” LINAC’OO, Monterey, CA. August ??-??, 2000. [4] R. Valdiviez, et al.. “Intense Proton Core And Halo Beam Profile Measurement: Beam Line Component Mechanical Design,” LINAC’OO, Monterey, CA, [5] L. Day, et al., “Control System for the LEDA 6.7- MeV Proton Beam Halo Experiment”, this conference. [6] D. Barr, et al., to be published PAC paper, Motor August ??-??, 2000. control Paper. 

BEAM-PROFILE INSTRUMENTATION FOR BEAM-HALO MEASUREMENT : OVERALL DESCRIPTION, OPERATION, AND BEAM DATA.

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BEAM-PROFILE INSTRUMENTATION FOR BEAM-HALO MEASUREMENT : OVERALL DESCRIPTION, OPERATION, AND BEAM DATA.

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