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COMMISSIONING OF THE DUKE STORAGE RING


COMMISSIONING OF THE DUKE STORAGE RING*
V.N.Litvinenko, Y. Wu, B.Burnham, J.M.J. Madey, F.Carter, C.Dickey, M.Emamian, J.Gustavsson, N.Hower, P.Morcombe, S.H.Park, P.O'Shea, R.Sachtshale, D.Straub, G.Swift, P. Wang, and J.Widgren Duke University, Free Electron Laser Laboratory , Durham, NC 27708-0319 USA Abstract The commissioning of the 1 GeV Duke Storage Ring began in November, 1994 with the demonstration of injection, storage and ramping to 1 GeV at the first attempt. The ring is now operational. The Duke project is unique in that the storage ring and linac were designed, constructed and commissioned by a small new University laboratory, operating on a low budget. The team is comprised of six accelerator physicists and graduate students, eight engineers, and fifteen technicians.

Figure 1. Layout of the Duke FEL storage ring and 280 MeV linac-injector. I. INTRODUCTION Storage Ring: The layout of the Duke storage ring and linac is shown in Figure 1. The ring itself is a strong focusing The new design of the Duke storage ring lattice was race-track with two 34 meter long straight sections. The south initiated in February of 1991 and was completed in October straight section lattice is designed to optimize FEL operation the same year [1,2]. The circumstances leading to the design with 7 to 28 m long FELs. The north straight section is used of the ring's novel modified second-order achromatic lattice is for injection and installation of the RF system and synchrotron presented elsewhere [3]. Our experience with the Duke radiation insertion devices. The main parameters of the Duke storage ring has shown that this lattice is very "tolerant". The storage ring are listed in Table 1. use of precise magnetic measurements for control [4,5] in All dipoles on the Duke storage ring, including those in combination with precise alignment [6] was the key to the injection chicane, are fed by one 560 kW PEI power trouble-free commissioning. supply, while all quadrupoles have individual power supplies. This feature provides flexibility for the lattice design. II. DUKE STORAGE RING Arcs: Each arc is divided into ten strong focusing FBDB The unique “third generation” 1 GeV Duke storage ring is (FODO) cells to provide low natural emittance of the electron designed to drive UV and soft X-ray FELs as well as to beam. Details of the design are described in [3]. produce high brightness synchrotron radiation from the South straight section: The lattice of this section bending magnets and insertion devices. The facility is comprising 8 quadrupoles has bilateral symmetry with 3.5 m comprised of the 1 GeV storage ring, the 280 MeV linac- horizontal and vertical β -functions at its center. This lattice injector and linac-to-ring (LTR) channel. is optimized to facilitate operation of the 8 m long OK-4 UV/VUV FEL [7]. We also designed an alternative lattice for *Work supported by ONR grant #N00014-94-1-0818 this straight section to accommodate a future 26 m FEL [8].

Table I. Designed Parameters of Duke Storage Ring ----------------------------------------------------------------------Operating energy [GeV] 0.25 - 1.0 Ring circumference [m] 107.46 Arc and straight section length [m] 19.52; 34.21 Revolution frequency [MHz] 2.7898 RF frequency [MHz] 178.547 Number of dipoles and quadrupoles 40; 64 Betatron tunes, Qx and Qy 9.111, 4.180 Orbit compaction factor, α 0.0086 Natural chromaticities, Cx and Cy -10.0, -9.78 Compensated values, Cx and Cy +0.1; +0.1 Acceptances [mm mrad], Ax and Ay 56.0, 16.0 Energy acceptance, ?E/E, of ring >±5.0% limited by existing RF ±2.8% Maximum β-functions [m], x and y 13.6, 21.3 Maximum η-function [m] 0.245 ----------------------------------------------------------------------North straight section comprising 14 quadrupoles, is a more diverse lattice: it provides optimal conditions for the 3.75 m NIST undulator (soft X-ray spontaneous source), RFcavity and injection. Ring RF system comprising a RF cavity, a circulator, and Duke a 55 kW QEI power amplifier, is described in [9]. Storage Ring injection system includes a 280 MeV linac, a LTR channel, achromatic vertical chicane, and ferrite kicker. Injection is in the horizontal plane to accommodate insertion devices with small vertical gaps. The 280 MeV Linac- injector comprising a microwave electron gun, 11 SLAC accelerator sections, and low and high energy spectrometers, was commissioned in October 1994. The linac description can be found elsewhere in these proceedings [13]. A chopper (the gun kicker) installed after the microwave gun forms 25-50 nsec electron bunch trains which define the filling pattern (5 to 10 buckets) in the ring. The linac delivered about 2-2.3 nC of the charge per shot, which transfers to 5-6 mA of average current in the ring. The ring has one injection kicker (instead of designed three). The ring kicker is described in [10]. The timing system, originally developed at Stanford and adjusted to the new RF frequency at Duke, provides synchronization of the linac and storage ring pulsed systems, namely the gun and ring kickers. The injection chicane, comprised of three 9° dipoles and a Lambertson type septum magnet, provides a 60 cm vertical bump of the electron trajectory. The chicane dipoles are identical to those on the ring and are fed by the same power supply. This arrangement matches the energies of the ring and chicane which is also used as a spectrometer. The last 3 meters of the chicane employ a vacuum pipe with 8 x 12 mm inside cross-section for differential pumping between the storage ring and linac .

Vacuum system: The ring has stainless steel vacuum chambers with smooth transitions. Synchrotron radiation absorbers are located in the arcs. However, we are presently using temporary end-of-arc vacuum chambers without absorbers and with sudden jumps of the vacuum pipe crosssection. These chambers do not allow operation at full energy with full current. They are also the main source of longitudinal impedance for the ring. More than fifty vacuum pumps are distributed around the storage ring. Overall vacuum in the ring is (2-8)*10-10 torr without electron beam. Vacuum in the arcs climbs up to (1.82.8)*10-8 torr with a 100 mA beam at 280 MeV, and up to (3.6-4)*10 -8 torr with a 4 mA beam at 1 GeV. Vacuum is sufficiently good to provide 4 hours lifetime at 1 GeV with 4 mA beam current. The alignment system is described in these proceedings [6]. Duke Ring diagnostic system comprises the following systems: DCCT for average current measurements with 1 ?A resolution (made in BINP); four remotely controlled screens for one-turn tracking; four end-of-arc synchrotron radiation ports equipped with TV cameras, photo-multipliers and a dissector with 20 psec resolution. The tune measurement system is described elsewhere[11]. Duke Storage Ring control system includes advanced intelligent functions for control of lattice modifications, tune and chromaticity controls, magnet normalization, and energy ramp. This system is well described in [4,5].

III . COMMISSIONING OF THE RING
Commissioning of the Duke storage ring proceeded very smoothly and successfully. We did not experience any problems tracking the beam through the LTR, the chicane, and the storage ring from the first shot without use of any correctors. There was no problem storing the beam and ramping the energy from 230 MeV to the design energy of 1 GeV. Later, the beam was ramped to 1.1 GeV as well. This success was achieved with the use of very simple diagnostics. Table II. Measured Parameters of Duke Ring ----------------------------------------------------------------------Operating energy [GeV] 0.2 - 1.1 Revolution frequency [MHz] 2.7898 RF frequency [MHz] 178.547 Betatron tunes, Qx and Qy 9.118 4.145 Natural chromaticities, Cx and Cy -10.0, -9.78 Compensated values, Cx and Cy +0.1; +0.1 Acceptances [mm mrad], Ax and Ay >56.0, >16.0 Energy acceptance, ?E/E, of ring ±6.0% Closed orbit (no correction) x and y, mm <±5, <±4 Deviation of β-functions , x and y <±20% η-function [m] in straight sections < .005

Table III. Electron Beam Parameters ----------------------------------------------------------------------Design Measured Beam current, mA 100 115 Emittance (@1 GeV) 1 Horizontal ,m*rad 18.10-9 16-19.10-9 Vertical,m*rad 1.10-9 <1 .10-9 2 , ps Bunch length 33 < 70 ----------------------------------------------------------------------The commissioning of the Duke ring has resoundingly demonstrated the effectiveness of the lattice, control system and alignment. Table II comprises the main measured parameters of the ring with the designed objectives. Stacking the electron beam with 100% efficiency using one kicker (instead of the designed three) was achieved because of the large dynamic aperture of the ring. The measured acceptance of the ring is in very good agreement with physical aperture of the ring [14]. The measured parameters of the electron beam in the Duke storage ring are summarized in Table III. In addition, two unusual effects were observed during commissioning: capture of 20% of the electrons injected outside the RF separatrix (with higher energy) and stable capture of electrons in a -30 dB level side-band of the RF frequency.

Kurkin at BINP, Novosibirsk; E. Forest, and J. Bengsston at LBL. This project would not be possible without the wise guidance and support of Dr. Howard Schlossberg and his colleagues.

VI. REFERENCES
[1] V.Litvinenko, Y.Wu, "New Lattice for the Duke Storage Ring", Duke FEL Lab. Report, October 1991 [2] Y.Wu, V.N.Litvinenko, J.M.J.Madey, "Lattice and Dynamic Aperture of the Duke FEL Storage Ring", in Proceedings of the 1993 Particle Accelerator Conference, Washington D.C., p. 218; Y.Wu, V.N.Litvinenko, E.Forest, J.M.J.Madey, "Dynamic Aperture Study for the Duke Storage Ring", Nucl. Instr. and Meth., A331 (1993) 287 [3] V.N.Litvinenko, Y.Wu, B.Burnham, J.M.J.Madey, S.H.Park, "Performance of Achromatic Lattice with Combined Function Sextupoles at Duke Storage Ring", These Proceedings. [4] B.Burnham, V.Litvinenko, Y.Wu, "Application of Precision Magnetic Measurements for Control of the Duke Storage Ring", These Proceedings. [5] Y.Wu, B.Burnham, V.Litvinenko, "The Duke Storage Ring Control System", These Proceedings. [6] M.Emamian, N.Hower, Y.Levashov, "Alignment of the Duke FEL Storage Ring", These Proceedings. [7] V.Litvinenko, J.Madey, N.A.Vinokurov "UV-VUV FEL Program at Duke Storage Ring with OK-4 optical klystron", Proceedings of the 1993 Particle Accelerator Conference, Washington D.C., p. 1442 [8] Y.Wu, V.N.Litvinenko, J.M.J.Madey, "Study of undulator influence on the dynamic aperture for the Duke Storage Ring", Nulc. Instr. and Meth., A341 (1994) 363 [9] P.Wang, G.Kurkin, P.Morcombe, Y.Wu, "RF System for Duke 1 GeV Storage Ring", These Proceedings. [10] R.Sachtshale, C.Dickey, P.Morcombe, "Pulsed Power Technology and Accelerators", These Proceedings. [11]V.Litvinenko, B.Burnham, N.Hower, P.Morcombe "Duke Storage Ring Tune Measurements System using Razor Blade and Photo-multiplier", These Proceedings. [12] Y.Wu, "A Report on the 250 MeV and 1 GeV Linac Lattice and Alignment Tolerances", Duke FEL Lab. Report, January 11, 1993 [13] P.O'Shea et al., "Accelerator Archaeology - The Resurrection of the Stanford MKIII Electron Linac at Duke", These Proceedings. [14] Y.Wu, V.N.Litvinenko, B.Burnham, "Experimental Study of the Duke Storage Ring Dynamic Aperture", These Proceedings.

IV. FUTURE PLANS
We plan the following upgrades of the Duke storage ring: 1) smooth end-of-arc vacuum chambers with absorbers to attain the full capabilities of the Duke ring; 2) electronics for the 60 existing strip-line BPMs; 3) reduced injection bunch duration to 5 nsec for single bucket filling; 4.) two additional kickers to simplify injection. Near term plans include installation and commissioning of the UV/VUV OK-4 FEL and the NIST undulator.

V. ACKNOWLEDGMENTS
We want to thank all those who contributed to the design, installation and commissioning of the Duke storage ring system: all staff from the Duke FEL laboratory, our colleagues from Novosibirsk Institute of Nuclear Physics and Lawrence Berkeley Laboratory. We would like to thank personally C. Pat, R. Cataldo, J. Faircloth, H. Goehring, S. Goetz, M. Johnson, L. Kennard, H. Mercardo, J. Meyer, O. Qakeley, J. Patterson, and R. Taylor at Duke FEL Laboratory; S. Mikhailov, Yu. Levashov, and G.
1 Defined by accuracy of measurements 2 3.5 mA, GeV, RF @300kV, required confirmation.


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