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

Outline • Introduction → Plasma WakeField Acceleration → High Transformer Ratios • PITZ PWFA experiments → Photocathode laser bunch shaping → PITZ gas discharge cell • Demonstration of HTR PWFA • Outlook | HTR PWFA @ PITZ | Gregor Loisch |ARD Alliance Meeting, HH 05.09.2018 | Page 2

Introduction HTR PWFA

Plasma Wakefield Acceleration (PWFA) Principles, characteristics, implications Basic principle • Relativistic driver enters plasma • Pushes plasma electrons away due to space charge • σ z ~ λ p : 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 | HTR PWFA @ PITZ | Gregor Loisch |ARD Alliance Meeting, HH 05.09.2018 | Page 4

High Transformer Ratio (HTR) wakefields Increasing ratio of acceleration to deceleration • Plasma wakefield ~ transformer  Energy-transfer from driver to witness • Fundamental theorem of beamloading: R = E acc /E dec <2 (symmetrical driver, linear theory) • Asymmetrical bunch shapes proposed  R ≤ 2π L driver / λ p HTR in PWFA  λ p ≤ mm  ps-scale bunch shaping  Driver witnesses several periods of wake  instability  operation in (quasi-) nonlinear regime: n b > n p | HTR PWFA @ PITZ | Gregor Loisch |ARD Alliance Meeting, HH 05.09.2018 | Page 5

PITZ PWFA Experiments

PITZ PWFA experiments 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 | HTR PWFA @ PITZ | Gregor Loisch |ARD Alliance Meeting, HH 05.09.2018 | Page 7

PITZ PWFA experiments 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”) Simulation Measurement • 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) | HTR PWFA @ PITZ | Gregor Loisch |ARD Alliance Meeting, HH 05.09.2018 | Page 8

PITZ PWFA experiments 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 <10 13 cm -3 up to 3x10 16 cm -3 • Simple, reliable setup | HTR PWFA @ PITZ | Gregor Loisch |ARD Alliance Meeting, HH 05.09.2018 | Page 9

PITZ PWFA experiments SMI Measurement of self-modulation instability (SMI) principle Self-modulation instability • Transverse modulation of long y-z bunches (L b >> λ p ) projection • Proposed to provide proton driver trains for PWFA (AWAKE@CERN) Self-modulation at PITZ Longitudinal phase space • Proof-of-principle experiments • Modulate flat-top electron bunches • Meanwhile also measured at high density (≤ 3x10 15 cm -3 ) in GDP cell First demonstration  M. Gross et al., Phys. Rev. Lett. 120 , 144802 (2018) | HTR PWFA @ PITZ | Gregor Loisch |ARD Alliance Meeting, HH 05.09.2018 | Page 10

Demonstration of HTR PWFA

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  n b >n p  witness defocused → Maximum simulated TR = ~6 | HTR PWFA @ PITZ | Gregor Loisch |ARD Alliance Meeting, HH 05.09.2018 | Page 12

HTR PWFA Experiments Witness First demonstration of HTR PWFA Tail Time [ps] Head • TR calculated from slice energy gain/loss TR = 𝟓. 𝟕 +𝟑.𝟑 𝟓. 𝟕 −𝟏.𝟖 • Plasma density of ~2 x 10 13 cm -3 Time [ps] • HTR also observed at other densities • Simulations show TR of 4.3 • ~70% of witness particles lost Time [ps] Phys. Rev. Lett. 121 , 064801 (2018) | HTR PWFA @ PITZ | Gregor Loisch |ARD Alliance Meeting, HH 05.09.2018 | Page 13

Outlook Improvements, further possible experiments Simulation of bunch slice parameters • New photocathode laser under way (talk of C. Koschitzki, this session) Ṡ olc fan filter bunch shaping SLM based bunch shaping • 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 | Page 14

for your attention! Contact Gregor Loisch Deutsches PITZ-T Elektronen-Synchrotron gregor.loisch@desy.de +49 33762 7-7185 www.desy.de