Demonstration of High Transformer Ratio Plasma Wakefield - - PowerPoint PPT Presentation

Demonstration of High Transformer Ratio Plasma Wakefield Acceleration

Proof-of-Principle Plasma Acceleration Experiments at PITZ Gregor Loisch Hamburg ARD Alliance New Beams and Accelerators Meeting Hamburg, 05.09.2018

Page 2 | HTR PWFA @ PITZ | Gregor Loisch |ARD Alliance Meeting, HH 05.09.2018 |

Outline

• Introduction

→ Plasma WakeField Acceleration → High Transformer Ratios

• PITZ PWFA experiments

→ Photocathode laser bunch shaping → PITZ gas discharge cell

• Demonstration of HTR PWFA
• Outlook

Introduction HTR PWFA

Page 4

Plasma Wakefield Acceleration (PWFA)

| HTR PWFA @ PITZ | Gregor Loisch |ARD Alliance Meeting, HH 05.09.2018 |

Principles, characteristics, implications Basic principle

• Relativistic driver enters plasma
• Pushes plasma electrons away due to

space charge

• σz ~ λp: plasma electrons oscillate around

immobile ions

• Trailing witness accelerated in wakefields

PWFA features → Very high fields achievable (~50 GV/m demonstrated) → Wakefields have strong transverse components  focusing & defocusing → Long. & Transv. fields 90° phase shifted  acceleration on slope of E-field

Page 5

High Transformer Ratio (HTR) wakefields

| HTR PWFA @ PITZ | Gregor Loisch |ARD Alliance Meeting, HH 05.09.2018 |

Increasing ratio of acceleration to deceleration

• Plasma wakefield ~ transformer  Energy-transfer from

driver to witness

• Fundamental theorem of beamloading: R = Eacc/Edec <2

(symmetrical driver, linear theory)

• Asymmetrical bunch shapes proposed

 R ≤ 2π Ldriver/λp HTR in PWFA

• λp ≤ mm  ps-scale bunch shaping
• Driver witnesses several periods of wake 

instability  operation in (quasi-) nonlinear regime: nb > np

PITZ PWFA Experiments

Page 7

PITZ PWFA experiments

| HTR PWFA @ PITZ | Gregor Loisch |ARD Alliance Meeting, HH 05.09.2018 |

Experimental environment

• < 25 MeV bunch energy
• High brightness
• Bunch charges 1 pC - 2000 pC
• Various diagnostics
• Emittance
• Longitudinal profile (TDS)
• Longitudinal phase space
• Etc. …
• Beam time for Accelerator R&D
• Flexible bunch shapes

Page 8

PITZ PWFA experiments

| HTR PWFA @ PITZ | Gregor Loisch |ARD Alliance Meeting, HH 05.09.2018 |

Photocathode laser-based bunch shaping

• Bunch shaping by photocathode laser

pulse shaping

• Originally for flattop bunches
• Shaping by adding 14 Gaussian quasi-

pulses (“Ṡolc fan filter”)

• Powerful but complicated tuning
• Witness bunch by splitting pulse before

pulse shaper

• Efficient way of bunch shaping

compared to cutting shapes from high charge symm. beams (C. Joshi, AAC 2018)

• G. Loisch et al., https://doi.org/10.1016/j.nima.2018.02.043, NIMA (2018)

Simulation Measurement

Page 9

PITZ PWFA experiments

| HTR PWFA @ PITZ | Gregor Loisch |ARD Alliance Meeting, HH 05.09.2018 |

Argon gas discharge plasma cell

• Provides plasma medium
• Gas discharge in ~1 mbar Argon
• 10 mm diameter, 100 mm plasma

channel length

• 2 µs, ~300 A peak current pulses
• µm-thick polymer electron beam windows
• Densities <1013 cm-3 up to 3x1016 cm-3
• Simple, reliable setup

Page 10 First demonstration  M. Gross et al., Phys. Rev. Lett. 120, 144802 (2018)

PITZ PWFA experiments

| HTR PWFA @ PITZ | Gregor Loisch |ARD Alliance Meeting, HH 05.09.2018 |

Measurement of self-modulation instability (SMI) Self-modulation instability

• Transverse modulation of long

bunches (Lb >> λp)

• Proposed to provide proton driver

trains for PWFA (AWAKE@CERN) Self-modulation at PITZ

• Proof-of-principle experiments
• Modulate flat-top electron bunches
• Meanwhile also measured at high

density (≤ 3x1015 cm-3) in GDP cell

SMI principle y-z projection Longitudinal phase space

Demonstration of HTR PWFA

Page 12

HTR PWFA Simulations

Beam transport to and through plasma

• Driver bunch slices have different focus

points (↔ different space charge at cathode)

• Witness has different focal point than

driver

• Inhomogeneous focusing enhances SMI

→ Driver focus as tight as possible & lower plasma density  nb>np  witness defocused → Maximum simulated TR = ~6

| HTR PWFA @ PITZ | Gregor Loisch |ARD Alliance Meeting, HH 05.09.2018 |

Page 13

HTR PWFA Experiments

First demonstration of HTR PWFA

• TR calculated from slice energy gain/loss
• Plasma density of ~2 x 1013 cm-3
• HTR also observed at other densities
• Simulations show TR of 4.3
• ~70% of witness particles lost

| HTR PWFA @ PITZ | Gregor Loisch |ARD Alliance Meeting, HH 05.09.2018 |

TR = 𝟓. 𝟕+𝟑.𝟑 𝟓. 𝟕−𝟏.𝟖

Tail Head Witness

• Phys. Rev. Lett. 121, 064801 (2018)

Time [ps]

Time [ps] Time [ps]

Page 14

Outlook

Improvements, further possible experiments

• New photocathode laser under way (talk of
• C. Koschitzki, this session)
• Transverse & longitudinal bunch

shaping allows better control of slice parameters

• Direct control (fast & more accurate shaping)
• Control slice parameters (homogeneous

focusing)

• Higher charge with preserved shape
• Shorter & focused witness
• (further plasma cell development)

Final goal: readiness of photocathode bunch shaping for high energy accelerator

| HTR PWFA @ PITZ | Gregor Loisch |ARD Alliance Meeting, HH 05.09.2018 |

Simulation of bunch slice parameters

Ṡolc fan filter bunch shaping SLM based bunch shaping

Contact Deutsches Elektronen-Synchrotron www.desy.de

Gregor Loisch PITZ-T gregor.loisch@desy.de +49 33762 7-7185

for your attention!