Evidence for beam loading by distributed injection of electrons in a Plasma Wakefield Accelerator.
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Transcript of Evidence for beam loading by distributed injection of electrons in a Plasma Wakefield Accelerator.
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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
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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
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UCLAWhy Rubidium
• Advantage: avoiding– Head erosion– Ion motion
• Disadvantage:– Secondary ionization
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UCLATransformer Ratio
• Beam loading can occur because of a trailing beam or captured background electrons
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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
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UCLAExperimental Setup
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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
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UCLAExcess Charge Correlates with X-ray Yield
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UCLAAbsolute Calibration of Length
Length is absolutely calibrated using experimental results from two ovens
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UCLABeam Loading with the Same Current Profile
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UCLARaw Data and Transformer Ratio
Transformer ratio reduces with excess charge
÷6x2
Foil3 Foil2 Foil1 No Foil
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UCLATransformer Ratio and Excess Charge
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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.
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UCLAReduction in Transformer Ratio
• E- drops because of head erosion• E+ drops at a faster rate than the E- because of beam loading
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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.
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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.
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THE END
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UCLAData Corroboration for the Two Ovens
• Trapped charge has the same form as X-ray Yield
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UCLA
0 50 100 150 200 250 300 3500
0.5
1
1.5
2
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3
3.5
Variation of Min Bunch Width with Emittance In Beam Enve-lope Model
Normalized Emittance
min
(σr)
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UCLADipole radiation on X-rays
Actual blocked radiation Schematically shown blocked radiation