Observation of the Self-Modulation Instability via Time-Resolved Measurements

Abstract

Self-modulation of an electron beam in a plasma has been observed. The propagation of a long (several plasma wavelengths) electron bunch in an overdense plasma resulted in the production of multiple bunches via the self-modulation instability. Using a combination of a radio-frequency deflector and a dipole spectrometer, the time and energy structure of the self-modulated beam was measured. The longitudinal phase space measurement showed the modulation of a long electron bunch into three bunches with an approximately 200 keV/c amplitude momentum modulation. Demonstrating this effect is a breakthrough for proton-driven plasma accelerator schemes aiming to utilize the same physical effect.

Experimental Observation of Plasma Wakefield Growth Driven by the Seeded Self-Modulation of a Proton Bunch

“…The successful and controlled development of the SSM is a necessary requirement to be able to use long proton bunches to drive large amplitude wakefields and to accelerate particles (e + , e − ) in these wakefields. Previous work showed self-modulation resulting in the formation of two [18] or a few microbunches [19]. In [18] the authors claim that the instability grew above seed level, but the argument is based on simulation results.…”

“…Previous work showed self-modulation resulting in the formation of two [18] or a few microbunches [19]. In [18] the authors claim that the instability grew above seed level, but the argument is based on simulation results. Results of the Advanced Wakefield Experiment (AWAKE) show formation of a large number of proton microbunches (up to 100).…”

Experimental Observation of Proton Bunch Modulation in a Plasma at Varying Plasma Densities

“…The plasma density is then ðσ z =σ r Þ 2 , and E wb is σ z =σ r larger than with σ z =ðc=ω pe Þ ≅ ffiffi ffi 2 p . Microbunching of a 22 MeV electron beam was observed recently [13].…”

“…The results presented here with a relativistic ionization front (as opposed to a sharp rising charge distribution as in Refs. [12,13]) show for the first time the seeding and the development of the self-modulation of a long charged particle bunch in a plasma. At long timescales, we observe that the SSM effect starts at the time of the ionizing laser pulse and thus of the ionization front.…”

Observation of High Transformer Ratio Plasma Wakefield Acceleration

“…The results also validate the predictions of the theoretical concept to operate a HTR PWFA in the nonlinear regime for mitigating beamplasma instabilities, which would inhibit the transport of HTR-capable driver bunches. Neither the transverse two stream or self-modulation instability [9,16,18,34] nor the hose instability [17,19] have been observed in the HTR acceleration over multiple instability growth lengths (SMI ∼ 13, hosing ∼ 10) [18], as it is required for PWFA with considerable energy gain. The presented transformer ratio enables plasma wakefield acceleration by 2.3 times higher energies or a corresponding reduction of the driver bunch acceleration length, compared to acceleration with the transformer ratio limit imposed by the fundamental theorem of beam loading.…”