Speaker
Patric Muggli
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P2.2002.pdf
Detailed measurements of the time structure of a self-modulated proton
bunch exiting a plasma in AWAKE
P. Muggli, for the AWAKE Collaboration
Max Planck Institute for Physics, Munich, Germany
and CERN, Geneva, Switzerland
The self-modulation of a long (>1 cm), relativistic, charged particle bunch in a dense
plasma (>1014 cm−3 ) offers the possibility to resonantly drive wakefields to large amplitude
(>1 GV/m) [1]. A witness electron (or positron) bunch can then be externally injected in the
wakefields and accelerated. Because long proton bunches can carry large amounts of energy,
the acceleration process can be sustained over long distances and the accelerated bunch can
reach very large energies along a single plasma [2]. The witness bunch must be deterministi-
cally placed in the accelerating and focusing phase of the wakefields. The wakefields can be
seeded, for example, by an ionization front created by a short laser pulse traveling together
with the proton bunch, as in the AWAKE experiment [3]. The wakefields used for acceleration
are those after the growth of the seeded modulation process has saturated, some meters into the
plasma and approximately one rms bunch length behind the seed point, where they peak. It is es-
sential to understand the variation of the phase of the wakefields with respect to the seed points
and with respect to variations of the parameters of the incoming proton bunch. The phase of the
wakefields is very difficult to measure. However, there is a one-to-one correspondence between
the wakefields and the bunch structure driving the wakefields. We can therefore measure the
time structure of the bunch exiting a first plasma, which is used as self-modulator of the drive
bunch. We show that, despite variations in the bunch input parameters, the time structure of the
bunch to be used to drive wakefields in a second plasma, the accelerator itself, is reproducible
to within a fraction of a wakefields period (∼8 ps). Since a witness electron bunch would be
generated on a photo-cathode or in a laser wakefield accelerator by a replica of the ionizing
pulse used for seeding, these measurements show that the witness bunch can in principle be
deterministically placed at the proper phase of the wakefields in the accelerator plasma, even
when many periods behind the seed point (e.g., ∼100 at a plasma density of 7×1014 cm−3 and
with σz =12 cm).
References
[1] N. Kumar et al., Phys. Rev. Lett. 104, 255003 (2010)
[2] A. Caldwell et al., Phys. Plasmas 18, 103101 (2011)
[3] P. Muggli et al., Plasma Physics and Controlled Fusion, 60(1) 014046 (2017)
