Tackling instabilities in a plasma wakefield accelerator

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Image credit: AWAKE *

Plasma wakefield accelerators are widely regarded as a promising alternative to conventional RF-based accelerators due to their ability to operate at much higher accelerating gradients (1 – 100 GeV/m).

The AWAKE Experiment at CERN has demonstrated that a proton beam can be used to drive a strong wakefield in a plasma which, in turn may be harnessed to accelerate a witness bunch of electrons.

The QUASAR Group, led by Professor Carsten P Welsch, is developing a set of novel beam diagnostic techniques that are necessary to monitor and optimise the unique properties of the proton and electron beams involved in AWAKE.

However, AWAKE and other plasma accelerator projects have to overcome a series of obstacles like the development of instabilities in the plasma.

One of these instabilities is ‘hosing’, which occurs, e.g., when a seed electron bunch and the long proton bunch are misaligned. The development of hosing could impose a limit on the distance a bunch can propagate in plasma, disrupting the driving wakefields and, therefore, the quality of the witness bunch.

In spite of its detrimental impact on the acceleration process and the fact that it has been observed in a number of plasma wakefield experiments, very few fundamental characteristics of hosing (other than its possible occurrence) were deduced.

Tatiana Nechaeva is a scientist currently working at DESY, who was previously been part of the AWAKE team at Max Planck Institute for Physics in Munich and received her PhD thanks to her work on hosing instabilities.  Tatiana is the lead author of an article on hosing that has just been accepted for publication in Physical Review Letters.

In this Letter, Tatiana and her colleagues from the AWAKE collaboration demonstrate that hosing of a long, relativistic proton bunch propagating in an over-dense plasma can be induced by the misalignment of the initial wakefields driven by a short electron bunch, and thus observed in a reproducible way.

The results, obtained in the AWAKE experiment, show that hosing develops in the plane of misalignment, while self-modulation occurs simultaneously in the perpendicular plane, at frequencies close to the plasma electron frequency. The hosing and self-modulation were induced by the same initial wakefields. Therefore, both processes were reproducible.

Patric Muggli, spokesperson for the AWAKE collaboration says: “it is remarkable that such a tenuous plasma can affect and almighty proton so much and that we can observe it so reproducibly.

The development of hosing was sensitive to the misalignment direction, and its growth depended on misalignment extent and on proton bunch charge. The authors found good general agreement between the hosing observed and a theoretical model, despite differences between assumptions of the model and the experimental conditions.

Hosing is a detrimental process, because it can lead to the breakup of the bunch, limiting acceleration length. The wakefield structure of the drive bunch undergoing hosing is not axisymmetric and evolves all along the bunch, making the possibility of injection into these wakefields somewhat questionable. Concerning the witness bunch, hosing may lead to a decrease in its quality.

The results presented in this paper are not exclusive to AWAKE acceleration scheme, but rather have a general character, and might therefore be interesting for other acceleration schemes.

More information:

T. Nechaeva et al. (AWAKE Collaboration), “Hosing of a long relativistic particle bunch in plasma”, Physical Review Letters 132, 075001 (2024).



*Experimental reconstruction of the 3D proton bunch charge density distribution in the case of hosing and self-modulation developing simultaneously. The image shows the isometric view (a), as well as the two projections (x,t) and (y,t) [(b) and (c), respectively]. This image shows the experimental results obtained by performing an analogue of a CT scan across the proton bunch distribution.