Colton, E P

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RESULTS FROM FIRST OPERATION OF THE SPALLATIONNEUTRON SOURCE PROTON STORAGE RINGEUGENE P. COLTONLos Alamos National LaboratoryLos Alamos, New Mexico, U.S.A.The proton storage ring (PSR) at Los Alamos is an 800 MeV accumu-lator ring which has recently been constructed at the Clinton P.Anderson Meson Physics facility, LAMPF. l The prime use will befor driving a spallation neutron source for materials scienceapplications. The design average current of 100 ~A is to beachieved by operating the ring at a pulse rate of 12 Hz with astored beam of 5.2 x 1013 protons per pulse. The 800 MeV H- beamfrom the LAMPF linac is first magnetically stripped to HO justoutside the ring lattice and then is injected into the ring as H+by use of a 300 ~g/cm2 carbon foil stripper. During the injectionperiod of 750-1000 ~sec the rf voltage applied to the single 2.8MHz buncher cavity is ramped up linearly from 3 to 15 kV, in orderto counter the longitudinal space charge potential and maintain a270 nsec bunch length. After accumulation of the linac macro-pulse is completed, the beam is immediately extracted in a singleturn using a combination of two stripline kickers and a dc septummagnet.PSR project construction was completed in late March, 1985,and a first low-intensity beam was circulated in the ring on April26. Successful operation of the single-turn extraction system wasachieved at the end of May. The concentration of effort duringthe summer was on understanding PSR operation at low intensities,improving controls software and beam diagnostics, and in wringingout equipment problems.© 1986 Gordon and Breach Science Publishers S.A.6768 ACCELERATOR NEWSA major surprise for PSR tuners was the unexpectedly highlevel of beam losses occuring during the accumulation process,now firmly associated with scattering of the circulating protonsin the injection stripper foil. Machine experiments focussedstrongly throughout the summer on obtaining a clear picture ofthis phenomenon.During a first high-peak current beam test (November 1985)the number of protons accumulated in the ring was raised to 1.6 x1310 ppp, representing an average current of 30 ~A at 12 Hz. PSRwas then run at that average current level for several days toprovide neutron-scattering users with first access to production.Several experiments were successfully carried out, and the usersexhibited strong enthusiasm for the high neutron fluxes availableat their spectrometers.Fast beam loss was initially observed at peak intensitiesslightly above that of this production run. In later high-currenttests; however, it was possible to raise the number of protons13accumulated to 2.7 x 10 ppp (half the design value) by in-creasing the transverse beam size in the machine and the buncherrf amplitude. At the end of the run cycle, the peak intensity inPSR was limited by the intensity of the presently available Hbeam. Ion source improvements now in progress should raise the H13intensity high enough to permit machine experiments up to 4 x 10ppp during the 1986 run cycle.During production at 30 ~A, the beam losses appeared to beabout 5%, which was associated with scattering from multiplepasses of the stored protons through the stripper foil (3%) andthe single-turn extraction process (2%).The machine was operated at the tunes Q = 3.2 and Q = 2.2.x yThe chromaticities (dQ/Q)/(dp/p) were measured to be ~ -1.9 andxS = -1.1; predicted values were ~ = -0.8 and ~ = -1.3. Thisy x yobservation has suggested the possibility of a significant sextu~pole error in the ring lattice.ACCELERATOR NEWS 69Early data showing the fast beam loss is displayed in Fig. 1,an oscilloscope trace of the circulating beam current vs. time in-to the injection cycle. The picture shows a linear ramp up to1.6 x 1013 protons accumulated, followed by rapid loss of morethan half the beam within about 100 ~sec. The remaining beam isthen extracted about 700 ~sec after the start of injection. In-formation obtained from the storage-ring fast-position pickupssuggests that the beam begins to execute coherent transverse mo-tion just prior to the fast beam loss. The amplitude of theoscillation, which appears to start first in the vertical plane,grows with an apparent e-folding time of about 20 ~sec.o 200 400 600 800t {fLsec>FIGURE 1 Oscilloscope trace of PSR current transformer out-put (accumulated current vs. ttme into cycle).Picture shows fast beam loss after reaching 1.6 x1013 ppp, followed by extraction of remaining beamat 700 ~sec.During the present six-month shutdown, the commissioning teamis studying results of the 1985 beam tests and implementing vari-ous improvements to PSR. They hope to uncover the source of thelarger than predicted horizontal chromaticity and the foil-70 ACCELERATOR NEWSscattering losses. These phenomena may be related to one another.Detailed particle tracking through a model PSR lattice that in-cludes non-linear effects and foil scattering falls short of pre-dicting the magnitude of the observed beam loss by about a factorof ten. Several practical steps are being taken to reduce overallbeam losses from scattering, including using thinner stripperfoils, beam collimation in the ring, and opening up restrictiveapertures.Further studies of the possible coherent instability areplanned with improved instrumentation for the next run cycle, whichbegins in July, 1986. The observations made to date are too pre-liminary to warrant initiating development of a fast transversedamper, a device that may, nevertheless, eventually be required to13reach the PSR peak current design goal of 5.2 x 10 ppp. In themeantime, equipment is being added to improve control over radialand longitudinal space-charge distributions and increase the ef-fective working area in betatron tune space, all of which shouldaid in pushing the maximum stored charge level beyond what wasachieved in 1985: These improvements include the addition of har~monies to the fundamental 2.8 MHz buncher waveform, and installa-tion of non-linear magnetic elements in the lattice.REFERENCES1. G. P. Lawrence, R. A. Hardekopf, A. J. Jason, P. N. Clout, andG. A. Sawyer, Los Alamos High Current Proton Storage Ring: AStatus Report, IEEE Trans. Nue. Sei. NS-32, 2662 (1985).

Results From First Operation Of The Spallation Neutron Source proton storage Ring

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