The beam matrix of a picosecond long slice of an electron bunch was measured. A short slice is selected out of an energy chirped beam by a slit in a dispersive region. The emittance is measured using the quadrupole scan technique. We observe the process of emittance compensation of the beam by repeating the measurement for various values of the compensating solenoid and for several slices.[S0031-9007(96)00194-9] PACS numbers: 29.27.Fh, 41.85.Si Less than a decade ago, Carlsten [1] proposed that the observed emittance growth due to linear components of space charge forces in a photocathode RF electron gun can be compensated. This emittance growth is due to a misalignment of the phase space ellipses of short longitudinal sections of the electron beam bunch (slices). His compensation technique uses a solenoid lens to produce a laminar-flow beam-waist downstream of the gun. The space-charge interaction in the beam waist rotates the slice ellipses differentially to counter the original rotation and to produce alignment downstream of the waist. As shown since in numerous experiments [2][3][4], this technique resulted in a large improvement in the brightness of electron beams so significant for e 1 e 2 linear colliders and short wavelength free-electron lasers.In the past, emittance compensation has been observed by measuring the reduction of this total projected emittance. We present here the first measurement of the relative rotation of the slice ellipses leading to emittance compensation. We do this by dissecting the electron bunch longitudinally into slices on a picosecond time scale. The emittance and orientation of the phase space ellipse are measured for each individual slice by the quadrupole magnet scan technique. We repeat this measurement for a number of field settings of the solenoid, and show that the phase space ellipse of the slices indeed rotate relative to each other as the solenoid field is changed, and go through near alignment at a particular solenoid field.The measurement was done with the one-and-half cell S-band laser-photocathode RF gun of the BNL Accelerator Test Facility. Figure 1 is a schematic of the experiment. The beam is generated at the photocathode by a 10-ps (FWHM) long UV laser pulse. The electron bunch, with a charge of 0.4 nC, is rapidly accelerated in the twin cells of the gun's RF cavity by a 89 MV͞m peak electrical field to an energy of 4 MeV. The beam emerging from the gun is focused by a solenoid magnet and injected into a 2856 MHz linear accelerator (linac). A laminar-flow beam waist is formed in the linac, during acceleration. The acceleration continues until the spacecharge forces become negligibly small, freezing the resultant phase space distribution. The linac consists of two sections. The phase of the second section is controlled independently by a motor controlled phase shifter. The high energy beam is bent horizontally by a 20 ± dipole magnet and a small slice in energy is selected by a slit. A quadrupole magnet following the slit is scanned in current and the vertic...
The longitudinal accelerating field E, has been measured as a function of azimuthal angle in the full cell of the cold test model for the 1.6 cell BNL/SLAC/UCLA #3 S-band RF Gun using a needle rotation / frequency perturbation technique [l]. These measurements were conducted before and after symmetrizing the full cell with a vacuum pump out port and an adjustable short. Two different waveguide to full cell coupling schemes were studied. The dipole mode of the full cell is an order of magnitude less severe before symmetrization for the &coupling scheme. The multi-pole contribution to the longitudinal field asymmetry are calculated using standard Fourier series techniques. The Panofsky-Wenzel theorem [2] is used in estimating the transverse emittance due to the multipole components of E t .
Design and operation of a 50 MeV Electron LinearAccelerator utilizing a low emittance ( 7 c -5 to 10 mm-mrad) radio-frequency gun operating at an output energy of 5 MeV and a charge of 1 nC is described.Design calculations and early radio-frequency measurements and operational experience with the electron gun utilizing a dummy copper cathode in place of the proposed photocathode emitter are given.
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