174 research outputs found
Physics of beam self-modulation in plasma wakefield accelerators
The self-modulation instability is a key effect that makes possible the usage
of nowadays proton beams as drivers for plasma wakefield acceleration.
Development of the instability in uniform plasmas and in plasmas with a small
density up-step is numerically studied with the focus at nonlinear stages of
beam evolution. The step parameters providing the strongest established
wakefield are found, and the mechanism of stable bunch train formation is
identified.Comment: 13 pages, 15 figure
Effect of beam emittance on self-modulation of long beams in plasma wakefield accelerators
The initial beam emittance determines the maximum wakefield amplitude that
can be reached as a result of beam self-modulation in the plasma. The wakefield
excited by the fully self-modulated beam decreases linearly with the increase
of the beam emittance. There is a value of initial emittance beyond which the
self-modulation does not develop even if the instability is initiated by a
strong seed perturbation. The emittance scale at which the wakefield is twice
suppressed with respect to the zero-emittance case (the so called critical
emittance) is determined by inability of the excited wave to confine beam
particles radially and is related to beam and plasma parameters by a simple
formula. The effect of beam emittance can be observed in several discussed
self-modulation experiments.Comment: 6 pages, 10 figures, 1 tabl
Excitation of two-dimensional plasma wakefields by trains of equidistant particle bunches
Nonlinear effects responsible for elongation of the plasma wave period are
numerically studied with the emphasis on two-dimensionality of the wave. The
limitation on the wakefield amplitude imposed by detuning of the wave and the
driver is found.Comment: 4 pages, 4 figure
Self-modulation instability of a long proton bunch in plasmas
An analytical model for the self-modulation instability of a long
relativistic proton bunch propagating in uniform plasmas is developed. The
self-modulated proton bunch resonantly excites a large amplitude plasma wave
(wake field), which can be used for acceleration of plasma electrons.
Analytical expressions for the linear growth rate and the number of
exponentiations are given. We use the full three-dimensional particle-in-cell
(PIC) simulations to study the beam self-modulation and the transition to the
nonlinear stage. It is shown that the self-modulation of the proton bunch
competes with the hosing instability which tends to destroy the plasma wave. A
method is proposed and studied through PIC simulations to circumvent this
problem which relies on the seeding of the self-modulation instability in the
bunch
Plasma Wakefield Acceleration with a Modulated Proton Bunch
The plasma wakefield amplitudes which could be achieved via the modulation of
a long proton bunch are investigated. We find that in the limit of long bunches
compared to the plasma wavelength, the strength of the accelerating fields is
directly proportional to the number of particles in the drive bunch and
inversely proportional to the square of the transverse bunch size. The scaling
laws were tested and verified in detailed simulations using parameters of
existing proton accelerators, and large electric fields were achieved, reaching
1 GV/m for LHC bunches. Energy gains for test electrons beyond 6 TeV were found
in this case.Comment: 9 pages, 7 figure
Natural noise and external wake field seeding in a proton-driven plasma accelerator
We discuss the level of natural shot noise in a proton bunch-driven plasma
accelerator. The required seeding for the plasma wake field must be larger than
the cumulative shot noise. This is the necessary condition for the axial
symmetry of the generated wake and the acceleration quality. We develop an
analytical theory of the noise field and compare it with multi-dimensional
simulations. It appears that the natural noise wake field generated in plasma
by the available at CERN super-protons-synchrotron (SPS) bunches is very low,
at the level of a few 10 kV/m. This fortunate fact eases the requirements on
the seed. Our three dimensional simulations show that even a few tens MeV
electron bunch precursor of a very moderate intensity is sufficient to seed the
proton bunch self-modulation in plasma.Comment: 5 pages, 5 figure
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