Abstract
For the Compact Linear Collider Test Facility (CTF) at CERN a new rf gun with a laser driven photocathode is under construction. The new rf gun will replace the present 112 cell gun and will consist of 212 cells accelerating the beam to a momentum of 7.0 MeV/c with an electric field strength of 100 MV/m. The strong space-charge forces at low beam energy caused by the high charge density of the electron bunches are contained by radial and longitudinal rf focusing in the gun. The rf gun under construction has been optimized by MAFIA beam simulations for an injector assembly comprising a second accelerating rf structure and an intermediate solenoid magnet correcting the beam divergence of the 212 cell gun. The beam loading of the rf gun, by a train of 48 bunches with 21 nC charge each, causes a strong energy decay accompanied by an increase of the flight time for the bunches with lower energy. These effects can be corrected by slightly shifting the acceleration frequency of the gun. The experimental results obtained with the 112 cell gun and booster section presently in operation are reported.
Highlights
At the Compact Linear Collider Test Facility (CTF) at CERN, an rf photoelectron gun with 11/2 cells of the Brookhaven type [1] has been in operation since 1990
The new 3 GHz photoelectron gun will produce a bunch train of 48 ultrashort electron bunches of 21 nC charge each with a bunch length of 8 ps fwhm or shorter
It has been checked by beam simulations for a linear decay of the accelerating field strength from 125 to 98 MV/m and for a linear phase advance from 20_ to 32_ between the first and last bunch that the flight time from the photocathode to the gun exit is stabilised to within ± 1 ps and the energy droop is reduced to 16.2 > γ > 13.9 The linear phase advance is obtained by a frequency offset of the gun by ∆f = + 1.95 MHz
Summary
At the Compact Linear Collider Test Facility (CTF) at CERN, an rf photoelectron gun with 11/2 cells of the Brookhaven type [1] has been in operation since 1990. The 11/2 cell gun together with the booster section and the intermediate solenoid magnet have produced an electron bunch train of 48 bunches with a momentum of 9.5 MeV/c and a total electric charge of 450 nC. Since the high intensity beam of 450 nC reduces the stored rf energy of the gun by half , the beam velocity slows down as the peak field strength declines from 100 MV/m for the first bunch to 70 MV/m for the last bunch. In order to produce a higher bunch charge in the gun and to reduce the beam loading factor k'0 of the cavity, the iris diameter for the new gun has been increased to 40 mm. The electric field strength on the photocathode must be as high as possible, Ec=100 to 130 MV/m, in order to accelerate the high charge electron bunches to the highest possible energy, γ > 4. The beam characteristics of the 21/2 cell gun are summarized in table 2
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