Sequence Of Relativistic Electron Bunches Research Articles

ELABORATION OF THE PLASMA-DIELECTRIC WAKEFIELD ACCELERATOR WITH A PROFILED SEQUENCE OF DRIVER ELECTRON BUNCHES

A theoretical and experimental study of wakefield excitation by a profiled sequence of relativistic electron bunches in the plasma-dielectric structure, the parameters of which provide the conditions for the excitation of a small decelerating field for all driver bunches with simultaneous growth with the number of bunches of the accelerating total wakefield was carried out. Theoretically, the transformation ratio was found for the parameters of the experiment as the ratio of the total wakefield of the sequence to the field that decelerates driver bunches. In the performed experiments, the total wakefield was measured by the microwave probe. The magnitude of the decelerating field is determined by the shift of the maximum of the spectrum measured by the magnetic analyzer before and after wakefield excitation in the structure. The obtained transformation ratio increases with the number of bunches in the sequence and significantly exceeds this one for a nonprofiled sequence.

DEPENDENCE OF WAKEFIELD EXCITATION IN PLASMA BY NON-RESONANT SEQUENCE OF ELECTRON BUNCHES ON THEIR LENGTHS

The present paper describes the results of numerical simulation (using 2d3v code LCODE) of the regime, when the wakefield is excited at maximum growth rate in the plasma by a nonresonant sequence of relativistic electron bunches. As a result, the wakefield increases approximately in steps. The paper gives the parameters, at which this regime is achieved. It is shown that for smaller bunch radii, the amplitude of the excited wakefield is larger. At long lengths of the bunches, the amplitude of the wakefield is larger, in contrast to the excitation by the resonant sequence of bunches.

HOMOGENEOUS FOCUSING FIELD FOR SHORT RELATIVISTIC ELECTRON BUNCHES IN PLASMA

Plasma wake lens in which all short relativistic electron bunches of sequence are focused identically and uniformly is studied analytically and by numerical simulation. For two types of lenses necessary parameters of focused sequence of relativistic electron bunches are formulated. Verification of these parameters is performed by numerical simulation.

BBU instability in rectangular dielectric resonator

Studies were made into the arise and an evolution of the beam breakup (BBU) instability in a rectangular dielectric resonator under excitation by a sequence of relativistic electron bunches. The dielectric resonator is a section of a rectangular waveguide loaded by the two dielectric slabs (located along the wide side) and closed by the metal grids. A material of the slabs is teflon. A thickness of the slabs is 8.2 mm, a wide size and a length of the slabs are equal to the resonator wide size and length respectively. The metal grids are transparent for the charged particles and nontransparent for the excited electromagnetic fields. The wavelength of the LSM2,1,12 (Longitudinal Section Magnetic) operating mode is 53.2 mm. The electron energy of bunches is 4.5 MeV , the charge of each bunch is 6.4 nC, the bunch repetition period is equal to twice the wavelength of the LSM2,1,12 mode. By the use of numerical PIC simulations, the charge losses of electron bunches on the dielectric plates were investigated as the bunches were displaced relative to the cavity axis. It is found that the charge losses on the dielectric slabs due to the BBU instability do not exceed 5%. When the bunch repetition period is changed to a multiple of another eigenfrequency (e.g., the LSM1,1,12 mode), the charge losses of drive bunches do not change appreciably.

WAKEFIELDS AND DRIVE ELECTRON BUNCHES DYNAMICS IN THREE-ZONE DIELECTRIC RESONATOR AT ASYMMETRICAL INJECTION

A numerical study of the wakefield excitation in a rectangular dielectric-loaded resonator by a sequence of relativistic electron bunches in the case of non-axial injection is carried out. The effect of the shift of injected bunches on the particle dynamics, as well as on the spatial distribution of the components of the bunch-excited fields, is studied. The current losses of a sequence of relativistic electron bunches due to the particles deposition on the surface of the dielectric is determined.

INCREASE OF EFFICIENCY OF ACCELERATION OF THE TEST ELECTRON BUNCH BY SEQUENCE OF ELECTRON BUNCHES IN THE RECTANGULAR DIELECTRIC RESONATOR AT PLASMA FILLING

A result of numerical simulation of acceleration of the test electron bunch following a sequence of relativistic electron bunches in the rectangular dielectric resonator when filling the drift channel with plasma of different density is provided. We have found out that at change of plasma density the linear growth of energy gain of test bunch electrons is observed. It is connected with structure ordering of longitudinal accelerating electric field in the resonator and also with increase in its amplitude. It is due to that at periodic injection of drive bunches on resonator axis the areas with the increased plasma electrons density are formed.

OPTIMAL RESONANT ASYMPTOTICS OF WAKEFIELD EXCITATION IN PLASMA BY NON-RESONANT SEQUENCE OF ELECTRON BUNCHES

Resonant asymptotics of wakefield excitation in plasma by non-resonant sequence of relativistic electron bunches has been numerically simulated. It has been shown that in resonant asymptotics at optimal parameters the wakefield is excited with the maximum growth rate and the amplitude of the excited wakefield is the largest.

Femtosecond response time measurements of a Cs2Te photocathode

Success in design and construction of a compact, high-brightness accelerator system is strongly related to the production of ultra-short electron beams. Recently, the approach to generate short electron bunches or pre-bunched beams in RF guns directly illuminating a high quantum efficiency semiconductor photocathode with femtosecond laser pulses has become attractive. The measurements of the photocathode response time in this case are essential. With an approach of the interferometer-type pulse splitter deep integration into a commercial Ti:Sa laser system used for RF guns, it has become possible to generate pre-bunched electron beams and obtain continuously variable electron bunch separation. In combination with a well-known zero-phasing technique, it allows us to estimate the response time of the most commonly used Cs2Te photocathode. It was demonstrated that the peak-to-peak rms time response of Cs2Te is of the order of 370 fs, and thereby, it is possible to generate and control a THz sequence of relativistic electron bunches by a conventional S-band RF gun. This result can also be applied for investigation of other cathode materials and electron beam temporal shaping and further opens a possibility to construct wide-range tunable, table-top THz free electron laser.

Investigations of the concept of a multibunch dielectric wakefield accelerator

Theoretical and experimental investigations of the physical principles of multibunch dielectric wakefield accelerator concept based on the wakefield excitation in the dielectric structure by a sequence of relativistic electron bunches are presented. The purpose of the concept is to enhance the wakefield intensity by means of the multibunch coherent excitation and wakefield accumulation in a resonator. The acceleration of bunches is achieved at detuning of bunch repetition frequency relative to the frequency of the excited wakefield. In such a way the sequence of bunches is divided into exciting and accelerated parts due to displacing bunches into accelerating phases of wakefield excited by a previous part of bunches of the same sequence. Besides the change of the permittivity and loss tangent of dielectrics under the irradiation by 100MeV electron beam is studied.

Excitation of wake fields by lengthy electron bunches in a dielectric resonator

Theoretical research and numerical simulation of the wake-field excitation in a dielectric resonator by a regular sequence of relativistic electron bunches is carried out. A nonlinear system of equations for the multimode excitation of the wake field in the dielectric resonator is derived. The numerical simulation is carried out, and the dependences of the maximum amplitude of the longitudinal electric field on the bunch length and charge are investigated and physically interpreted.

Synchronization of wakefield modes in the dielectric resonator

Multimode excitation of a wakefield in a planar dielectric resonator by a regular sequence of relativistic electron bunches is studied analytically and numerically. It is shown that, if the wakefield being excited consists of a large number of radial modes, the fields from the regular sequence of the bunches add together. Conditions under which this sequence provides synchronization of the dielectric resonator modes are found. Acceleration of a test bunch in the wakefield of the bunch sequence is studied.