Measurements of Intra-beam Scattering at Low Emittance in the Advanced Light Source

The beam emittance at the interaction point of linear colliders is expected to be strongly influenced by the emittance of the beams extracted from the damping rings. Intra-beam scattering (IBS) potentially limits the minimum emittance of low-energy storage rings, and this effect strongly influences the choice of energy of damping rings [1]. Theoretical analysis suggests that the NLC damping rings will experience modest emittance growth at 1.98 GeV, however there is little experimental data of IBS effects for very low-emittance machines in the energy regime of interest. The Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory is a third-generation synchrotron light source operating with high-intensity, low-emittance beams at energies of approximately 1 2 GeV, and with emittance coupling capability of 1% or less. We present measurements of the beam growth in three dimensions as a function of current, for normalized natural horizontal emittance of approximately 1 10 mmmrad at energies of 0.7 1.5 GeV, values comparable to the parameters in an NLC damping ring. Using a dedicated diagnostic beamline with an x-ray scintillator imaging system, measurements of the transverse beamsize are made, and bunch length measurements are made using an optical streak camera. Emittance growth as a function of bunch current is determined, and compared with preliminary calculation estimates. 1 ALS MACHINE PARAMETERS 1.1 General Parameters The Advanced Light Source (ALS) is a third generation synchrotron radiation facility at Lawrence Berkeley National Laboratory, California. The machine, of circumference 196.8 m, operaties typically between 1.5 to 1.9 GeV, with 1 2 mA per bunch in approximately 300 bunches in multibunch mode, with bunch length typically 15 ps at 1.5 GeV. Up to 35 mA per bunch may be stored in two-bunch mode. Bunch volume is increased through the use of harmonic cavities in typical operations, in order to improve the Touschek lifetime. The RF system is 500 MHz, and momentum compaction 1.6 x 10. Beam parameters for energies at which IBS studies were performed are shown in table 1. Figure 1 shows the triplebend achromat (TBA) cell, with lattice functions. For user operations, the machine is typically operated with emittance coupling of 3 6 %, to increase the Touschek beam lifetime. The machine is capable of much smaller coupling, and values below 1% may readily be achieved. Correction of closed orbit distortion and vertical dispersion is achieved through use of the 96 BPM's, 94 horizontal correctors, and 72 vertical correctors. BPM's have approximately 1 micron resolution (multi-turn, 190 averages in approximately 0.5 sec.) Beam-based alignment is used in most quadrupole locations, and a closed-orbit distortion of approximately 50 μm is typical in the arcs, and <10 μm in the straights. Table 1: ALS parameters Energy / GeV 0.7 1.0 1.5 Natural energy spread / x 10 2.8 4 6 Normalized natural emittance / 10 m rad 1.0 2.9 10 Damping times, x,y,E / ms 147, 216, 128 52, 74, 44 15, 22, 13 Figure 1: ALS cell and lattice functions. SLAC-PUB-11750 Contributed to 18th International Conference on High-Energy Accelerators (HEACC 2001), 26-30 March 2001, Tsukuba, Japan 1.1 Machine set-up The machine is set-up by correcting the orbit including RF frequency, setting tunes to optimal values determined from known non-linear dynamics, setting chromaticities, and iterating on tune and orbit corrections. Orbit correction is to quadrupole centers as determined by beambased alignment. A model of the lattice is generated from an orbit response matrix using the code LOCO [2,3]. Residual beta-beating can be corrected to less than 1% rms, and is typically 3%. Figure 2 shows residual beta-beat measured at 1.5 GeV without additional correction, with an rms value of 1.3% in the horizontal and 2.2% in the vertical. Measured dispersion is shown in figure 3, showing a typical residual vertical dispersion of 5.2 mm rms. Emittance measurements made with an x-ray beamline (see section 2.2) agree to better than 5% with calculated natural emittances. Figure 2: Measured ß-beat. Figure 3: Measured dispersion. 2 DIAGNOSTICS 2.1 Diagnostic beamline 3.1 We use an x-ray beamline (designated 3.1) dedicated for use as an imaging diagnostic, with a source point in the first dipole magnet of a TBA cell. The dipole bend angle is 10°, and the beamline source point is downstream of the center tangent. At this point ßy varies rapidly, ßx is smoothly varying with distance along the orbit, and the dispersion is relatively small. Lattice functions at the source point are approximately ßx = 0.4 m, ηx = 0.03m, ßy = 19 m, ηy = -0.01 m. Dispersion is readily determined at the source point by measuring beam centroid motion as a function of rf frequency. The beamline location in an arc cell is shown in figure 4. Figure 4: Diagnostic beamline 3.1. The imaging optics are shown in figure 5. The optical system is a Kilpatrick/Baez mirror system, which produces a 1:1 image of the source on a bismuth germinate crystal. At 1.5 GeV, the optical resolution of the system determined by lifetime and beamsize measurements is less than 10 μm. At this energy or the correction factor is not significant for smallest beamsizes measured. Resolution at lower energies requires further study. Carbon neutral density filters are used to avoid saturation of the optics and to block visible light. An optical microscope magnifies the visible light image from the crystal scintillator onto a 640 x 480 pixel CCD camera, with pixel resolution of approximately 4 μm. Figure 5: Diagnostic beamline optics CCD output is read into processing software and a 2-D Gaussian fit is derived for the x,y beam distribution. Since the betatron axes are not in the horizontal and vertical planes at the source point, the fitting routine must find these axes before deriving σx and σy. This effect is significant since the axes may be rotated by up to 30°, 3.1