Jorge Vieira

 

Beam breakup suppression

 

​

The hosing or beam breakup instability leads to an exponential amplification of the initial transverse centroid displacements of a relativistic charged particle bunch in a wakefield structure. This instability can fully destroy a relativistic bunch no matter how small the initial centroid perturbations are. It is therefore a major concern in conventional and advanced accelerator techniques.

​

Several strategies were developed to mitigate beam breakup in conventional accelerators, which now use the so called BNS damping technique to stabilize the propagation of relativistic bunches in for arbitrarily long propagation distances. In order to mitigate beam breakup, BNS introduces betatron resonance detuning elements along the accelerator.

​

Beam breakup was discovered in plasma accelerators by D. Whittum and co-workers in the early 90's [whittum'91]. His conclusions have been seed as a source of major concern for successful plasma wakefield acceleration research programs world-wide. Despite predicting the existence of hosing in plasma acceleration, no experimental proof was ever found of this instability in plasma wakefield acceleration experiments at SLAC, for example.

We discovered that plasma accelerators can intrinsically provide betatron detuning mechanisms akin to BNS damping. We have first identified these mechanisms in the context of the AWAKE experiment at CERN [vieira'14]. In AWAKE, we found that, under certain conditions, the variation of the focusing force along the long proton bunch could provide an intrinsic detuning mechanism that damps hosing.

​

In addition to AWAKE, which employs long proton bunches to drive plasma accelerators, other experimental programs, hosted at SLAC (FACET II program [facetii]) and at DESY (flashforward program [flashforwad]) are devoted instead to plasma wakefield accelerators driven by short electron bunches. We found similar hosing damping mechanisms also hold for in the context of plasma wakefield accelerators in these experiments. Here, the variations of the driver energy can induce a betatron detuning mechanism that damps hosing. We also found that it is possible to enhance betatron detuning by introducing a correlated and uncorrelated energy spread on the drive bunch [mehrling'17].

​

Our work [vieira'14,mehrling'17] motivated a further round of theoretical and computational research on intrinsic hosing stabilization mechanisms in plasma accelerators. Variation of the focusing fields in quasi-linear regimes can also effectively suppress beam breakup [lehe'17]. Strong variations of the focusing fields could also be achieved in the blowout regime, and suppress hosing in one betatron wavelength [ossa'18]. Stabilization mechanisms have also been found in conditions where the motion of background plasma ions is important, as when the accelerated bunch charge density is extremely high [an'17,mehrling'18].

 

The movie below shows an example of an Osiris simulation illustrating the hosing instability in the context of experiments at DESY [flashforward]. The simulation and the animation were performed by our co-author Alberto Martinez de la Ossa  (DESY).

​

Please find more about this work in the paper here T. Mehrling et al, Phys. Rev. Lett. 118, 174801 (2017).