Emittance Growth in Intense Beams Due to Collective Effects at Metallic Boundaries

M.C. Thompson, J.B. RosenzweigUCLA Department of Physics and Astronomy

Joint ICFA Workshop 2002 – Chia Laguna, Sardinia

Motivation

x

I IIIII

Foils are widely used for vacuum isolation in beamlines, especially when plasma based elements are present.

The foil has three phases of interaction with the beam as it passes through:

I : Interaction between the beam and its image charge.

II : Multiple scattering of the beam in the foil material.

III : Interaction between the beam and its image charge.

While the impact of multiple scattering 0 as x 0, the collective effects in regions I and III remain the same regardless of foil thickness.

0.0 20.0p 40.0p 60.0p 80.0p10-2

10-1

100

101

500 pC constant50 MeV (perfectly cold)Sigma r = 250 micronSigma t = 3 psec - 500 fsec

67 amp

100 amp

200 amp

400 amp

Em

ittan

ce x

(mm

-mra

d)

Time

Emittance Growth In a Perfectly Cold Beam as it Emerges from a Perfectly Conducting Foil (2D PIC Simulation)

The Image Charge Interaction

As electron beams are pushed to ever higher current and lower emittance, the beam - image interaction may become increasingly significant source of emittance growth. This is especially true for thin foils for which multiple scattering is small.

This interaction at the metal boundary can be viewed as the near field aspect of CTR.

Future Work

We are at a very early stage in exploring this idea. We plan to continue pursuing it using:

Theoretical analysis based on an expansion of current work in our group on coherent transition radiation produced from rough surfaces.

Improved simulations that model all three phases of the beam foil interaction.

Experiments are under consideration if more sophisticated analyses continue to predict a measurable emittance growth effect.