The NIST-Los Alamos Racetrack Microtron (RTM) is designed to deliver a low-emittance electron beam of up to 0.5 mA cw over an energy range of 17 MeV to 185 MeV. Fed by a 5 MeV injector, the RTM contains two 180/degree/ end magnets that recirculate the beam up to 15 times through a 12 MeV RF linac. The linac, which operates in a standing-wave mode at 2380 MHz, has been tested to nearly full RF power. At present, the injector has undergone beam tests, and the beam transport system is complete through the 12 MeV linac. A temporary beam line has been installed at the exit of one end magnet to measure the beam energy, energy spread, and emittance after one pass through the accelerator. Preliminary results indicate that the accelerated beam energy spread and emittance are within design goals. 4 refs., 7 figs

Ayres, R. L.

Cutler, R. I.

Debenham, P. H.

Lindstrom, E. R.

Mohr, D. L.

Penner, S.

Rose, J. E.

Wilson, M. A.

Young, L. M.

UNT Digital Library

/. TAJ ft(L Ce»vf$ NIST-Los Alamos Racetrack Microtron StatusM.A. Wilson, R.L. Ayres, R.I. Cutler, P.H. Debenham, E.R. Lindstrom,D.L. Mohr, S. Penner*, J.E. RoseNational Institute of Standards and Technology (formerly NBS)Gaithersburg, MD 20899L.M. YoungLos Alamos National LaboratoryLos Alamos, NM 87545CONF-881049--75DE89 016083IntroductionThe NIST-Los Alamos Racetrack Microtron (RTM) isdesigned to deliver a low-eroittaoce electron bean of upto 0.5 mA cw over an energy range of 17 MeV to 185 MeV.Fed by a 5 MeV injector, the RTM contains two 180° endmagnets that recirculate the bean up to 15 times througha 12 MeV RF l inac. The l inac, which operates in astanding-wave mode at 2380 MHz, has been tested to near-ly fu l l RF power. At present, the injector has under-gone beam tests,1 and the beam transport system is com-plete through the 12 MeV l inac. A temporary beam linehas been instal led at the exit of one end magnet tomeasure the beam energy, energy spread, and emittanceafter one pass through the accelerator. Preliminaryresults indicate that the accelerated beam energy spreadand emittance are within design goals.Accelerator DescriptionThe RTM and injector, shown in Figure 1, are con-nected by a 180° achromatic bea î transport system (shownin Figure 2), wnich injects tne 5 MeV beam onto the RTMaccelerator axis. The two end nagnets recirculate thebeam through the 12 MeV linac up to 15 times by way ofseparated return l ines. A single, 500 kW, cw klystrondelivers RF power to four separate accelerating sections(two on the injector and two on the RTM axis) by way ofa waveguide RF distr ibut ion system. The phase andamplitude of each linac section are independently con-t r o l l ed . Tests of the RF system2 have confirmed thatphase and amplitude s tab i l i t y are well within designrequirements.To measure the effect on the electron beam of tnefirst-pass acceleration through the RTM l inac, a tempor-ary beam l ine has been installed in place of the returnlines at the exit of end magnet El, parallel to the 'inacaxis at a displacement of 66 cm. This configuration isshown in Figures 3 and 4. Included on this one-passreturn beam l ine are three viewscreens, spaced threemeters apart, to determine the beam posit ion, shape, andapproximate size. A wirescanner assembly3 is includednear each viewscreen for more precise measurements ofthe beam size. A Hall probe, calibrated by an adjacentNMR, is used to measure the end magnet f i e l d .Beam Transport, Acceleration, and MeasurementConditions calculated to provide achromatic beamtransport have been verif ied experimentally for each 90°section of the 180° transport beam l ine between the 5MeV injector and the RTM. The f inal dipole magnet inthe transport system deflects the 5 MeV beam through anangle of 15° onto the RTM accelerator axis. Three low-f ie ld steering magnets (S16-S1S, Figure 4) , one locatedat each end of the linac and one between the two linacsections, are used to keep the beam on the acceleratoraxis over the 12.5-meter distance between end magnets.Quadrupole doublets (Q6 ?and Q8 g, Figure 4) are locatedat each end of the RTM Tinac. 'S - - ' . -Figure 1. Plan view of the completed RTM.•Present address: 10500 Pine Haven Terrace, Rockville, MD 20852.~*- DISTRIBUTION OF THIS DOCUMENT IS UNLIMITEDMASTERFigure 2. View of the RTM from the too of end magnet £2, prior to completion of the one-pass return line. The1SO= beam transport line, with the magnetic shields removed, is to the left and bottom in the picture.Figure 3. View of the RTM frorr tne top of end magnet £1, showing the one-pass return beam line to the left of theRTM accelerator axis.Figure 4. Plan view of the RTM with the one-pass return l ine.The 5 MeV beam was aligned on the RTM acceleratoraxis and deflected by end magnet El onto the one-passreturn beam l ine . The RF power in the f i r s t linac sec-tion was raised to about 100 kW, and the accelerator RFphase was adjusted to maximize the bean energy gain, asdetermined by the magnetic f ie ld in El . This sequencewas repeated with the second linac section. The maximumbeam energy attained with power in both accelerators was16.2 MeV, limited during these preliminary beam tests bythe voltage gradient sustainable in the second l inac.This l im i t w i l l be overcome as linac conditioning con-tinues.The power in the accelerating sections was reducedby about 10% for the sustained operation required tocarry out the beam tests. Figure 5 shows the acceler-ated beam spot produced on viewscreens along the one-pass return beam l ine axis. The.beam is about 1 mm high(y) by 2 mn wide (x) . With no steering applied beyondthe middle of the linac to the end of the one-passreturn l i ne , the beam passed within 1 mm of the centerof each viewscreen, indicating good alignment of thebean with both the accelerating f ields and magneticguide f i e lds .1 mm (a)1 mm(b)(0Figure 5. Image of accelerated beam on viewscreenslocated three meters apart on one-pass returnl ine axis, a) 1 m from El ex i t , b) 3 in from(a), c) 6 m from (a).(a)(b)0.78 mm(OFigure 6. Oscilloscope traces of the signal produced asthe y-scanning wires pass through theaccelerated beam on the one-pass returnl i ne .a) Beam y-prof i le about one meter from theexit of end magnet El .b) Beam y-prof i le 3 m from (a). Thehorizontal scale is magnified 2.5xrelat ive to (a) and (c) .c) Beam y-prof i le 6 m from (a).The quadrupole doublet, Q3 and Qg, located at theexit end of the RTM Linac (Figure 4), was adjusted toproduce a waist in the y-direction near the middle ofthe one-pass return beam line to fac i l i ta te emittancemeasurements with the three wirescanners. Figure 6shows the beam prof i le in the y-direction as measured bythese wi rescanners. The focusing produced a 0.6 mmvertical beam waist near the middle wirescanner.Following a technique described in an earl ier paper,1the normalized transverse y-plane emittance1* was deter-mined from these beam size measurements to be 2.35 \M.The x-plane emittance was not determSnable, from similarmeasurements, due to energy spread effects. Beam enve-lope measurements along the accelerator axis w i l l beincluded during further planned beam tests, in ordermeasure the x-plarte emittance and the beam energy spreadindependently.(a)(b)(OAs a f i r s t estimate of the accelerated beam energyspread, the beam was focused to as small a size as pos-sible in the x-direction near the f i r s t wirescanner onthe one-pass return beam l ine by the quadrupole doublet,Q3and Qg. Figure 7 shows the beam profiles at the threewirescanner positions along the one-pass beam returnl ine under these conditions. The minimum beam envelopewidth was measured to be about 1.6 mm. From the minimumy-waist measured from the data in Figure 6, and assumingequal x- and y-emittance for the accelerated beam, theemittance part is estimated to contribute a l i t t l e morethan 1/2 to the beam envelope size. Therefore thenoment.ua dispersion contribution to the beam size isestimated to be 0.7-0.9 mm, corresponding to a f u l lenergy spread of 16-20 keV.Summary and ConclusionsPreliminary measurements of the electron beam afterone acceleration through the RTM linac have been made.The maximum energy achieved thus far is 16.2 MeV. Themeasured normalized emittance after one pass is 2.35 urnand the estimated energy spread is 16-20 keV. Thedesign goals for normalized emittance and energy spreadat 185 MeV are 5 urn and 36 keV, respectively. Theenergy spread is not expected to increase signi f icant lywith multiple passes through the mic-otron because ofphase focusing.These tests were conducted with a 0.3 mA pulsedbeam, with no indication of beam loss. I t is evidentfrom these preliminary results that the electron beamcan be transported and accelerated through the RTM whilebeam quality is maintained well within design l im i t s .Additional one-pass beam tests are planned to i n -clude f u l l voltage conditioning of the RTM linac sec-t ions, comprehensive beam envelope measurements to de-termine the x-emittance and for a more accurate energyspread determination, and the transport and accelerationof cw beams up to 0.55 mA average current.References1. M.A. Wilson, et a l , "Performance of the 5 MeVInjector for the NBS-Los Alamos RacetrackMicrotron," Proc. 1987 IEEE Particle AcceleratorConference, 322, December 1987.2. R.I. Cutler and L.M. Voung, "Performance of the HighPower RF System for the NIST-Los Alamos RacetrackMicrotron," this conference.3. R.I. Cutler, et a l , "Performance of Wire ScannerBeam Profi le Monitors to Determine the Emittance andPosition of High Power CW Electron Beams of theNBS-Los Alamos Racetrack Microtron," Proc. 1987 IEEEParticle Accelerator Conference, 625, December1987.4. For def in i t ion of emittance, see: S. Penner, "RFLinac Based Free Electron Lasers," Proc. 1987 IEEEParticle Accelerator Conference, 185, December1987.Figure 7. Oscilloscope traces of the signal produced asthe x-scanning wires pass through theaccelerated beam on the one-pass returnl ine .a) Beam x-prof i le about one meter from exitof end magnet El .b) Beam x-prof i le 3 m from (a).c) Beam x-prof i le 6 m from (a).The beam was focused to a waist near (a) bythe quadrupole doublet, Qg and Qg.DISCLAIMERThis report was prepared as an account of work sponsored by an agency of theUnited States Government. Neither the United States Government nor any agencythereof, nor any of their employees, makes any warranty, express or implied, orassumes any legal liability or responsibility for the accuracy, completeness, or use-fulness of any information, apparatus, product, or process disclosed, or representsthat its use would not infringe privately owned rights. Reference herein to any spe-cific commercial product, process, or service by trade name, trademark, manufac-turer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof.The views and opinions of authors expressed herein do not necessarily state orreflect those of the United States Government or any agency thereof.

NIST--Los Alamos racetrack microtron status

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NIST--Los Alamos racetrack microtron status

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