Benchmarking of 3D space charge codes using direct phase space measurements from photoemission high voltage dc gun

Abstract

We present a comparison between space charge calculations and direct measurements of the transverse phase space of space charge dominated electron bunches from a high voltage dc photoemission gun followed by an emittance compensation solenoid magnet. The measurements were performed using a double-slit emittance measurement system over a range of bunch charge and solenoid current values. The data are compared with detailed simulations using the 3D space charge codes GPT and PARMELA3D. The initial particle distributions were generated from measured transverse and temporal laser beam profiles at the photocathode. The beam brightness as a function of beam fraction is calculated for the measured phase space maps and found to approach within a factor of 2 the theoretical maximum set by the thermal energy and the accelerating field at the photocathode.

Improved Space Charge Modeling for Simulation and Design of Photoinjectors

“…There are substantial differences in time and spatial scales between the overall photoinjector accelerating structure and the photo-emitted electron bunch making detailed time-dependent simulations of bunch formation and propagation difficult. In fact, simulations of bunch evolution are often performed separately from simulations of the accelerating structure [1]. With sub-millimeter/picosecond bunches and tens-of-centimeter/GHz RF cavities the space/time scales are disparate enough to make simulations difficult but close enough that the long-time scale cannot be treated as DC, especially because of the low electron velocities during the initial bunch formation and acceleration process.…”

Fast wire scanner for intense electron beams

“…6). Peaks (1), (2), and (3) are used for determining the distance between adjacent wire images and for subsequently calculating the wire's speed. magnification of our optical system.…”

“…Such interceptive measurements of beam emittance are required since these intense beams tend to be space-charge rather than emittance dominated. Therefore, in order to properly access the transverse phase space, the slits or the pepper-pot openings must sample the full charge per bunch beam at a much reduced repetition rate, converting the space charge dominated beam to emittance dominated beamlets that carry information on both beam position and divergence [2]. Whenever the entirety of beam physics can be explained by single bunch dynamics, such interceptive methods can be adequate.…”

Operational experience with nanocoulomb bunch charges in the Cornell photoinjector

“…New radiation shielding was added around the EMS so that emittance measurements could be performed at energies above roughly 7 MeV [7], allowing for neutron production. The dc gun has now been processed up to 395 kV, but all other features of its design remain the same [9,10]. Similarly, the buncher has been processed up to 90 kV, to allow for adequate bunching at high charge.…”

“…For this paper, a NaKSb cathode with a mean transverse energy (MTE) of 140 AE 10 meV and a large (> 1 cm) active area with a nearly uniform quantum efficiency (QE) around 5% was used [11]. A more detailed description of the beam line and EMS can be found in [7,12] and [9,13] respectively.…”