Speaker
Falk Braunmueller
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P2.2013.pdf
Investigations on the seeded self-modulation in a long proton bunch using
coherent transition radiation measurements
F. Braunmueller for the AWAKE collaboration
Max Planck Institut for Physics Munich, Föhringer Ring 6, 80805 München, Germany
AWAKE is a proof-of-principle experiment at CERN for testing proton-driven plasma wake-
field acceleration over a 10 m laser-ionized plasma [1]. This novel acceleration method promises
strong acceleration (∼GeV/m) of electrons over long distances with a single drive bunch and a
single, long plasma [2]. It is planned to accelerate a witness electron bunch in the plasma wake-
field of a long proton bunch that is transformed into a train of microbunches by Seeded Self-
Modulation (SSM) [3]. We present frequency measurements of the modulated proton bunch
exiting the plasma, as well as investigations on the physics of SSM using these measurements.
The self-modulated proton bunch is expected to be modulated at the plasma frequency, which
can be varied in the range of 90-300 GHz, and the modulation is expected to have a duration
of 300-700 ps. A waveguide-integrated heterodyne diagnostic for coherent transition radiation
(CTR) was designed to precisely measure the modulation frequency of the modulated proton
pulse through the frequency of its CTR-pulse [4]. In this contribution, we first describe the mea-
surement principle and the experimental set-up of AWAKE, and of the diagnostic in particular.
Moreover, we show how the frequency-measurement of this diagnostic can be used for investi-
gating the nature of the SSM. We confirm that the modulation frequency indeed coincides with
the expected plasma frequency, as predicted by theory, by measuring modulation frequency as
a function of vapour density, that is ionized by a short laser pulse. In a second measurement, the
modulation process in the presence of a linear density gradient is analysed. Using both the pre-
cise frequency-measurement of the heterodyne CTR-diagnostic and a streak-camera capturing
the Optical Transition Radiation (OTR) emitted by the protons, the modulated bunch exiting the
plasma can be studied in detail. From this, one can draw conclusions about the SSM-interaction
inside the plasma, and inside the changing plasma density. Using these measurements, we will
try to obtain a better understanding of important processes governing the SSM, which is impor-
tant for optimizing and analysing the upcoming electron acceleration experiments.
References
[1] P. Muggli et al., Plasma Physics and Controlled Fusion 60(1), 014046 (2017)
[2] A. Caldwell et al., Phys. Plasmas 18, 103101 (2011)
[3] N. Kumar, et al., Physical Review Letters 104, 255003 (2010).
[4] F. Braunmueller et al., Nuclear Instruments and Methods A, DOI 10.1016/j.nima.2018.02.080 (2018)
