A beam emittance measurement of the 16 MeV S-band High Repetition Rate Linac (HRRL) was performed at Idaho State University's Idaho Accelerator Center (IAC). The HRRL linac structure was upgraded beyond the capabilities of a typical medical linac so it can achieve a repetition rate of 1 kHz. Measurements of the HRRL transverse beam emittance are underway that will be used to optimize the production of positrons using HRRL's intense electron beam on a tungsten converter. In this paper, we describe a beam imaging system using on an OTR screen and a digital CCD camera, a MATLAB tool to extract beamsize and emittance, detailed measurement procedures, and the measured transverse emittances for an arbitrary beam energy of 15 MeV

S. Setiniyaz, T.A. Forest, K. Chouffani, Y. Kim, A. Freyberger

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TRANSVERSE BEAM EMITTANCE MEASUREMENTS OFA 16 MeV LINAC AT THE IDAHO ACCELERATOR CENTERS. Setiniyaz∗, K. Chouffani, T. Forest, and Y. KimIdaho State University, Pocatello, ID, 83209, USAA. Freyberger, Jefferson Lab, Newport News, Virginia, 23606, USAAbstractA beam emittance measurement of the 16 MeV S-bandHigh Repetition Rate Linac (HRRL) was performed atIdaho State University’s Idaho Accelerator Center (IAC).The HRRL linac structure was upgraded beyond the capa-bilities of a typical medical linac so it can achieve a repeti-tion rate of 1 kHz. Measurements of the HRRL transversebeam emittance are underway that will be used to optimizethe production of positrons using HRRL’s intense electronbeam on a tungsten converter. In this paper, we describe abeam imaging system using on an OTR screen and a digi-tal CCD camera, a MATLAB tool to extract beamsize andemittance, detailed measurement procedures, and the mea-sured transverse emittances for an arbitrary beam energy of15 MeV.INTRODUCTIONThe HRRL is an S-band electron linac located in thebeam lab of the Physics Department at Idaho State Uni-versity (ISU). The HRRL accelerates electrons to energiesbetween 3 and 16 MeV with a maximum repetition rate of1 kHz. The HRRL beamline has recently been reconfiguredto generate positrons to be used as a secondary beam.An Optical Transition Radiation (OTR) based viewerwas installed to allow measurements at the high electroncurrents available using the HRRL. The visible light fromthe OTR based viewer is produced when a relativistic elec-tron beam crosses the boundary of two mediums with dif-ferent dielectric constants. Visible radiation is emitted atan angle of 90◦ with respect to the incident beam direc-tion [1] when the electron beam intersects the OTR targetat a 45◦ angle. These backward-emitted photons are ob-served using a digital camera and can be used to measurethe shape and intensity of the electron beam based on theOTR distribution.Emittance is a key parameter in accelerator physics thatis used to quantify the quality of an electron beam pro-duced by an accelerator. An emittance measurement canbe performed in a several ways [2, 3]. This work used theQuadrupole scanning method [4] to measure the emittance,Twiss parameters, and beam energy.THE EXPERIMENTQuadrupole Scanning MethodFig. 1 illustrates the apparatus used to measure the emit-tance using the quadrupole scanning method. A quadrupole∗Email: sadik82@gmail.comis positioned at the exit of the linac to focus or de-focus thebeam as observed on a downstream view screen. The 3.1 mdistance between the quadrupole and the screen was cho-sen in order to minimize chromatic effects and to satisfy thethin lens approximation. Assuming the thin lens approxi-Figure 1: Apparatus used to measure the beam emittance.mation,√k1L << 1, is satisfied, the transfer matrix of aquadrupole magnet may be expressed asQ =(1 0−k1L 1)=( 1 0− 1f1), (1)where k1 is the quadrupole strength, L is the length ofquadrupole, and f is the focal length. A matrix represent-ing the drift space between the quadrupole and screen isgiven byS =( 1 l0 1), (2)where l is the distance between the scanning quadrupoleand the screen. The transfer matrix of the scanning regionis given by the matrix product SQ. In the horizontal plane,the beam matrix at the screen (σs) is related to the beammatrix of the quadrupole (σq) using the similarity transfor-mationσs = MσqMT. (3)where the σs and σq are defined as [5]σs,x =( σ2s,x σs,xx′σs,xx′ σ2s,x′), σq,x =( σ2q,x σq,xx′σq,xx′ σ2q,x′).(4)MOPPR087 Proceedings of IPAC2012, New Orleans, Louisiana, USAISBN 978-3-95450-115-1990Copyrightc ○2012byIEEE–ccCreativeCommonsAttribution3.0(CCBY3.0)—ccCreativeCommonsAttribution3.0(CCBY3.0)06 Instrumentation, Controls, Feedback and Operational AspectsT03 Beam Diagnostics and InstrumentationFigure 2: The OTR Imaging system.By defining the new parameters [4]A ≡ l2σ2q,x, B ≡1l+σq,xx′σ2q,x, C ≡ l2ǫ2xσ2q,x, (5)the matrix element σ2s,x, the square of the rms beam size atthe screen, may be expressed as a parabolic function of theproduct of k1 and Lσ2s,x = A(k1L)2 − 2AB(k1L) + (C +AB2). (6)The emittance measurement was performed by changingthe quadrupole current, which changes k1L, and measur-ing the corresponding beam image on the view screen.The measured two-dimensional beam image was projectedalong the image’s abscissa and ordinate axes. A Gaussianfitting function is used on each projection to determine therms value, σs in Eq. (6). Measurements of σs for severalquadrupole currents (k1L) is then fit using the parabolicfunction in Eq. (6) to determine the constants A, B, and C.The emittance (ǫ) and the Twiss parameters (α and β) canbe found using Eq. (7).ǫ =√ACl2, β =√AC, α =√AC(B +1l). (7)The OTR Imaging SystemThe OTR target is 10 µm thick aluminum foil with a 1.25inch diameter. The OTR is emitted in a cone shape withthe maximum intensity at an angle of 1/γ with respect tothe reflecting angle of the electron beam [1]. Three lenses,2 inches in diameter, are used for the imaging system toavoid optical distortion at lower electron energies. The fo-cal lengths and position of the lenses are shown in Fig. 2.The camera used was a JAI CV-A10GE digital camera witha 767 by 576 pixel area. The camera images were taken bytriggering the camera synchronously with the electron gun.Quadrupole ScanningThe quadrupole current is changed to alter the strengthand direction of the quadrupole magnetic field such thata measurable change in the beam shape is seen by theOTR system. Initially, the beam was steered by thequadrupole indicating that the beam was not entering alongthe quadrupole’s central axis. Several magnetic elementsupstream of this quadrupole were adjusted to align the in-cident electron beam with the quadrupole’s central axis.First, the beam current observed by a Faraday cup locatedat the end of beam line was maximized using upstreamsteering coils within the linac nearest the gun. Second,the first solenoid nearest the linac gun was used to focusthe electron beam on the OTR screen. Steering coils wereadjusted to maximize the beam current to the Faraday cupand minimize the deflection of the beam by the solenoidfirst then by the quadrupole. A second solenoid and the laststeering magnet, both near the exit of the linac, were usedin the final step to optimize the beam spot size on the OTRtarget and maximize the Faraday cup current. A configura-tion was found that minimized the electron beam deflectionwhen the quadrupole current was altered during the emit-tance measurements.The emittance measurement was performed using anelectron beam energy of 15 MeV and a 200 ns long,40 mA, macro pulse peak current. The current in the firstquadrupole after the exit of the linac was changed from− 5 A to 5 A with an increment of 0.2 A. Seven measure-ments were taken at each current step in order to determinethe average beam width and the variance. Backgroundmeasurements were taken by turning the linac’s electrongun off while keep the RF on. Background image and beamimages before and after background subtraction are shownin Fig. 3. A small dark current is visible in Fig. 3b thatis known to be generated when electrons are pulled off thecavity wall and accelerated.The electron beam energy was measured using a dipolemagnet downstream of the quadrupole used for the emit-tance measurements. Prior to energizing the dipole, theelectron micro-pulse bunch charge passing through thedipole was measured using a Faraday cup located approxi-mately 50 cm downstream of the OTR screen. The dipolecurrent was adjusted until a maximum beam current wasobserved on another Faraday cup located just after the45 degree exit port of the dipole. A magnetic field mapof the dipole suggests that the electron beam energy was15 ± 1.6 MeV. Future emittance measurements are plannedto cover the entire energy range of the linac.Data Analysis and ResultsImages from the JAI camera were calibrated using theOTR target frame. An LED was used to illuminate theOTR aluminum frame that has a known inner diameter of31.75 mm. Image processing software was used to in-scribe a circle on the image to measure the circular OTRinner frame in units of pixels. The scaling factor can beobtained by dividing this length with the number of pix-Proceedings of IPAC2012, New Orleans, Louisiana, USA MOPPR08706 Instrumentation, Controls, Feedback and Operational AspectsT03 Beam Diagnostics and InstrumentationISBN 978-3-95450-115-1991 Copyrightc ○2012byIEEE–ccCreativeCommonsAttribution3.0(CCBY3.0)—ccCreativeCommonsAttribution3.0(CCBY3.0)0 200 400 6000100200300400500x (pixel)y (pixel)  0204060(a)0 200 400 6000100200300400500x (pixel)y (pixel)  0204060(b)0 200 400 6000100200300400500x (pixel)y (pixel)  0204060(c)Figure 3: Background subtracted to minimize impact ofdark current; (a) a beam with the dark current and back-ground noise, (b) a background image, (c) a beam imagewhen dark background was subtracted.els observed. The result is a horizontal scaling factor of0.04327 ± 0.00016 mm/pixel and vertical scaling factor of0.04204 ± 0.00018 mm/pixel.Digital images from the JAI camera were extracted in amatrix format in order to take projections on both axes andperform a Gaussian fit. The observed image profiles werenot well described by a single Gaussian distribution. Theprofiles may be described using a Lorentzian distribution,however, the rms of the Lorentzian function is not defined.The super Gaussian distribution seems to be the best op-tion [6], because rms values may be directly extracted.Fig. 4 shows the square of the rms (σ2s ) vs k1L for x(horizontal) and y (vertical) beam projections along withthe parabolic fits using Eq. 6. The emittances and Twissparameters from these fits are summarized in Table 1. Anautomatic MATLAB based emittance measurement tool isunder development [7].CONCLUSIONSA diagnostic tool was developed and used to measurethe beam emittance of the High Rep Rate Linac at theIdaho Accelerator Center. The tool relied on measuringthe images generated by the optical transition radiation ofthe electron beam on a polished thin aluminum target. The0 0.2 0.42.42.62.83k1L (m−1)σ2 x (mm2)  DataFit−0.4 −0.2 02.42.62.83k1L (m−1)σ2 y (mm2)  DataFitFigure 4: Square of rms values and parabolic fittings.Table 1: Emittance Measurement ResultsParameter Unit Valueprojected emittance ǫx µm 0.37± 0.02projected emittance ǫy µm 0.30± 0.04βx-function m 1.40± 0.06βy-function m 1.17± 0.13αx-function rad 0.97± 0.06αy-function rad 0.24± 0.07micro-pulse charge pC 11micro-pulse length ps 35energy of the beam E MeV 15 ± 1.6relative energy spread ∆E/E % 10.4electron beam profile was not described well using a sin-gle Gaussian distribution but rather by a super Gaussian orLorentzian distribution. The larger uncertainties observedfor σ2y are still under investigation. The projected emit-tance of the High Repetition Rate Linac, similar to medicallinacs, at ISU was measured to be less than 0.4 µm as mea-sured by the OTR based tool described above when accel-erating electrons to an energy of 15 MeV. The normalizedemittance may be obtained by multiplying the projectedemittance by the average relativistic factor γ and β of theelectron beam. We plan to perform similar measurementsover the energy range of the linac in the near future.ACKNOWLEDGMENTWe would like to acknowledge the outstanding effortsof Idaho Accelerator Center. This work was supported byDOE award # DE-SC0002600.REFERENCES[1] B. Gitter, Tech. Rep., Los Angeles, USA (1992).[2] K.T. McDonald and D.P. Russell, Fron. of Par. Beams Obser.Diag. and Cor. 08544, (1988).[3] Y. Kim et al., in Proc. FEL2008, Gyeongju, Korea.[4] D.F.G. Benedetti, et al., Tech. Rep., DAFNE Tech. Not.,Frascati, Italy (2005).[5] S.Y. Lee, Accelerator Physics, (Singapore: World Scientific,2004), 61.[6] F.J. Decker, NASA STI/Recon Tech. Rep. N 1996, (1994)10487.[7] C.F. Eckman et al., in Proc. IPAC2012, New Orleans,USA.MOPPR087 Proceedings of IPAC2012, New Orleans, Louisiana, USAISBN 978-3-95450-115-1992Copyrightc ○2012byIEEE–ccCreativeCommonsAttribution3.0(CCBY3.0)—ccCreativeCommonsAttribution3.0(CCBY3.0)06 Instrumentation, Controls, Feedback and Operational AspectsT03 Beam Diagnostics and Instrumentation

Transverse Beam Emittance Measurements of a 16 MeV Linac at the Idaho Accelerator Center

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