UCLA

Evidence for beam loading by distributed injection of electrons in a Plasma Wakefield

Accelerator.

Presented by Navid Vafaei-NajafabadiAdvisor: Chan Joshi

On behalf of E200 Collaborators

UCLAAuthors

• (UCLA) N. Vafaei-Najafabadi, K. A. Marsh, C. E. Clayton, W. An, W. Lu, W. B. Mori, C. Joshi

• (SLAC) E. Adli, S. Corde, M. Litos, S. Li, S. Gessner, J. Frederico, M.J. Hogan, D. Walz, J. England, J. P. Delahaye

• P. Muggli

UCLAWhy Rubidium

• Advantage: avoiding– Head erosion– Ion motion

• Disadvantage:– Secondary ionization

UCLATransformer Ratio

• Beam loading can occur because of a trailing beam or captured background electrons

UCLARequired Charge for Beam Loading

nb>np

Pencil beamLong beam Λ~1

Src: M. Tzoufras, PRL (2008)

Our beam’s Characteristicsnp=2.7x1017,N=1.3x1010,σr=3 μm, σz=35 μm

Q>110 pC

UCLAExperimental Setup

UCLABetatron Oscillations in Plasma

• X-ray yield depends on the number of oscillations• If the trapped charge is a function of interaction length, we

are prominently trapping Rb II.

ArRbIIArII

RbI

ArI

Dark Current Region

Rb

RbII/ArII

RbII

ArAr

Evolution of beam envelope

UCLAExcess Charge Correlates with X-ray Yield

UCLAAbsolute Calibration of Length

Length is absolutely calibrated using experimental results from two ovens

UCLABeam Loading with the Same Current Profile

UCLARaw Data and Transformer Ratio

Transformer ratio reduces with excess charge

÷6x2

Foil3 Foil2 Foil1 No Foil

UCLATransformer Ratio and Excess Charge

UCLASimulations Show Beam Loading

• Simulation parameters: 2.7x1017, εN=250 mm-mrad• Simulations where Rb II or Ar are turned off, showed little to

no beam loading.

UCLAReduction in Transformer Ratio

• E- drops because of head erosion• E+ drops at a faster rate than the E- because of beam loading

UCLAConclusions

• Distributed injection of electrons occurs due to beam induced ionization of Rb+

• These electrons can beam load the wake and reduce the accelerating field.

• Quantified this effect by correlating the increase of Rb II electrons with energy gain and loss of the different slices of the drive electron bunch as the plasma length is varied.

• PIC simulations shows that this correlation is indeed the cause of the reduction of the transformer ratio due to the beam loading effect.

UCLAFuture Work

• This has given rise to mixed gas experiment to produce decent quality and high energy beam

• Will write a long paper once this is accepted.

THE END

UCLAData Corroboration for the Two Ovens

• Trapped charge has the same form as X-ray Yield

UCLA

0 50 100 150 200 250 300 3500

0.5

1

1.5

2

2.5

3

3.5

Variation of Min Bunch Width with Emittance In Beam Enve-lope Model

Normalized Emittance

min

(σr)

UCLADipole radiation on X-rays

Actual blocked radiation Schematically shown blocked radiation