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Hollow Channel Plasma Wakefield Experiments at FACET-II
Spencer Gessner CERN October 17th, 2016
2
Outline
• Results from FACET
• Improved “First Generation” experiments at FACET-II • Improved plasma source and plasma profile measurements • Understanding the effect of the channel shape on wakefields • Exploration of the non-linear regime
• “Second Generation” experiments at FACET-II • True hollow channel plasmas • Beam break-up mitigation with external focusing
Results from FACET
Positron Beam Gold Mirror with hole
Kinoform
Laser YAG Screen
Dipole Spectrometer
LANEX Screen
Plasma Channel Imaging Quadrupoles
4
Creation of Meter-Scale Plasmas
m = k?r +m�
X [µm]
Y [µ
m]
−300 0 300
−300
0
300
−400 −200 0 200 4000
10
20
30
40
50
60
Y [µm]
Inte
nsity
[arb
]
Z = 30 cmZ = 60 cmZ = 90 cm
I(r, z) / kzzJ2m(k?r)
S. Gessner et. al. Nat. Comm. 7, 11785 (2016)
Beam-Based Measurements of the Channel Shape
X Kick
X [7m]-500 0 500
Y [7
m]
-500
0
500IP2A Y Kick
X [7m]-500 0 500
Y [7
m]
-500
0
500
Total Kick
X [7m]-500 0 500
Y [7
m]
-500
0
500
X Kick
X [µm]-500 0 500
Y [µ
m]
-500
0
500Y Kick
X [µm]-500 0 500
Y [µ
m]
-500
0
500
Total Kick
X [µm]-500 0 500
Y [µ
m]
-500
0
500
Simplified Model Data
Beam deflection measurements are used to demonstrate the shape of the hollow plasma channel formed by the high-order Bessel laser pulse.
6
Mapping the Longitudinal Wakefield
Bunch Separation [µm]50 100 150 200 250 300 350 400
∆E
[MeV
]
-50-40-30-20-10
01020
WitnessDrive
We observed for the first time the acceleration of positrons in the hollow channel plasma wakefield. We were able to map out the longitudinal wakefield by varying the separation between drive and witness beams. Carl Lindstrøm is performing a similar analysis for the transverse wakefield.
Witness Drive
Beam Loading
Beam loading is an important consideration when trying to optimize the gradient and efficiency. The average drive-to-witness efficiency in this experiment was 18% with an average transformer ratio of 1.8.
Drive Charge ×1091.4 1.5 1.6 1.7 1.8 1.9 2
Driv
e ∆
E [M
eV]
-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
Witness Charge ×1080 1 2 3 4
Witn
ess ∆
E [M
eV]
5
10
15
20
25
30
35Slope = -1.5 MeV/108 particles Slope = -4.0 MeV/108 particles
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Modeling the Plasma Boundary
Density
Z [µm]0 100 200 300 400 500
R [µ
m]
0
50
100
n 0
-1
-0.5
0
0.5
1
Density
Z [µm]0 100 200 300 400 500
R [µ
m]
0
50
100
n 0
-0.1
-0.05
0
0.05
0.1
Longitudinal Field
Z [µm]0 100 200 300 400 500
R [µ
m]
0
50
100
E z (GV/
m)
-0.1
0
0.1
Density
Z [µm]0 100 200 300 400 500 600
R [µ
m]
0
50
100
150
n 0
-0.1
-0.05
0
0.05
0.1
Density
Z [µm]0 100 200 300 400 500 600
R [µ
m]
0
50
100
150
n 0
-1
-0.5
0
0.5
1
Longitudinal Field
Z [µm]0 100 200 300 400 500 600
R [µ
m]
0
50
100
150
E z (GV/
m)
-0.1
0
0.1
PLASMA ELECTRON DENSITY PLASMA ELECTRON DENSITY
TOTAL CHARGE DENSITY TOTAL CHARGE DENSITY
EZ FIELD EZ FIELD
Simulations performed in OSIRIS 2D on UCLA Hoffman2 Cluster
Improved 1st Gen Experiments
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Gas Source Plasmas
Action Items: - Investigate peak intensity
compatible with transmitting focusing optic (UCLA, CU Boulder)
- Develop reflective high-order Bessel optics
Low Intensity High Intensity
Observation of double-wall structure
Gas source plasmas have advantages over lithium vapor sources:
• Direct observation of hollow channel profile • Can use a high-charge drive beam
Gas sources are difficult to ionize and self-phase modulation effects the laser profile.
11
Interferometry with Hollow Channels
Beam-based measurements of plasma shape are limited:
• Convolves beam shape with channel shape • Requires O(1000) shots to reconstruct
channel shape
Interferometric measurements can reconstruct plasma channel shape on a shot by shot basis.
J. Fan, et. al. PRE 2000
Action Items: - Demonstrate channel shape
measurements with interferometry prior to implementation at FACET-II (UCLA, UT Austin, UC Boulder)
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Exploration of Different Channel Shapes
13
Exploration of the Non-Linear Regime
2nd Generation Experiments
How do you form a True Hollow Channel Plasma?
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Laser ablation of a capillary:
Acoustic modulation of a gas:
Action Item: - We need to search for a
solution by looking in fields not directly related to PWFA.
How do you suppress the BBU?
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The growth length of the BBU is a few cm for FACET and FACET-II parameters. An external focusing lattice requires a comparable betatron wavelength.
Action Items: - Develop theory of hollow channel
wakefield in strong external fields. - Investigate if strong PMQ triplets or
solenoids can be used in conjunction with the hollow channel plasma.
J. K. Lim, et. al. PR-STAB 2005
RadiaBeam
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Conclusion
FACET was the only facility in the world which could provide a positron beam, high power laser, and plasma source. All of these components were needed to successfully demonstrate the acceleration of positrons in a hollow channel plasma. FACET-II provides these same tools and more. We may need to look outside the accelerator physics community to solve the problem of generating a truly hollow channel plasma.
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