Passive method for controlling and correcting energy correlations in charged particle beams

Abstract

A technique for controlling and compensating the energy spread of a charged particle beam is provided. This technique is based on a passive dielectric-loaded structure that redistributes the energy within the bunch by means of the wakefield generated in the structure. Cylindrical and planar structure configurations are provided and also means for electrical and mechanical tuning to optimize performance. The instant abstract is neither intended to define the invention disclosed in this specification nor intended to limit the scope of the invention in any way.

Claims

1. A passive electromagnetic resonator used to condition a charged particle beam by redistributing its energy comprising: a passive dielectric cavity that redistributes an energy of a charged particle bunch passing through it by means of a wakefield induced in the structure by said particle bunch, wherein energy is thereby redistributed in such a way as to reduce the energy spread of the bunch by removing the longitudinal energy-position correlation.

2. The passive electromagnetic resonator of claim 1, wherein said passive dielectric cavity further comprises a planar geometry dielectric structure adapted to be tuned to produce an optimized bunch energy spread, wherein the mechanism of tuning is selected from the group consisting of one or more of the following: altering the beam channel aperture in the structure; use of a thin ferroelectric layer incorporated into the dielectric structure to electrically control the effective permeability of the structure; and use of a thin ferroelectric layer incorporated into the dielectric structure to thermally control the effective permeability of the structure.

3. The passive electromagnetic resonator of claim 1, wherein said passive dielectric cavity further comprises a cylindrical geometry dielectric structure that can be tuned to produce an optimized bunch energy spread, wherein the mechanism of tuning is selected from the group consisting of one or more of the following: altering the beam channel aperture in the structure; use of a thin ferroelectric layer incorporated into the dielectric structure to electrically control the effective permeability of the structure; and use of a thin ferroelectric layer incorporated into the dielectric structure to thermally control the effective permeability of the structure.

4. The passive electromagnetic resonator of claim 1 adapted for use in improving the performance of an X-Ray free electron laser.

5. The passive electromagnetic resonator of claim 1 adapted for use in improving the performance of a linear accelerator based system.

6. The passive electromagnetic resonator of claim 2 adapted for use in improving the performance of an X-Ray free electron laser.

7. The passive electromagnetic resonator of claim 2 adapted for use in improving the performance of a linear accelerator based system.

8. A method to compensate and correct the position correlated energy spread of a charged particle beam through the use of conditioning the particle beam by redistribution of energy inside a bunch and thereby eliminating the need for an external power source for the device.

9. An apparatus for practicing the method of claim 8.

10. The passive electromagnetic resonator of claim 3 adapted for use in improving the performance of an X-Ray free electron laser.

11. The passive electromagnetic resonator of claim 3 adapted for use in improving the performance of a linear accelerator based system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The advantages and features of the present invention will become better understood with reference to the following more detailed description and claims taken in conjunction with the accompanying drawings, in which like elements are identified with like symbols, and in which:

(2) FIG. 1 is a diagram of a planar energy compensating structure showing the methods for controlling its parameters;

(3) FIG. 2 is a diagram of a cylindrical energy compensating structure showing the methods for controlling its parameters;

(4) FIG. 3a is a numerical calculation of the wakefield in the energy compensator showing the principle of compensating the energy spread of a charged particle beam;

(5) FIG. 3b shows the longitudinal phase space of a simulated electron beam prior to entering the energy compensator;

(6) FIG. 3c shows the longitudinal phase space of a simulated electron beam at the exit of the energy compensator; and

(7) FIG. 4 is a sketch of an energy compensated particle accelerator concept according to an exemplary embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(8) Briefly described according to a broad embodiment of the present invention, geometric configurations and methods of adjusting the characteristics of the aforementioned device are provided. The best mode for carrying out the invention is presented in terms of its preferred embodiment, herein depicted within the Figures.

(9) 1. Detailed Description of the Figures

(10) A wakefield is the electromagnetic wave emitted by a charged particle passing through a resonant structure. In the case of the dielectric loaded cavities considered here the radiation is emitted via the Cherenkov effect. The radiation couples to the resonant modes of the cavity. In the case of a charged particle bunch the wakefields of the particles add collectively. Particles in the bunch experience different retarding or accelerating forces depending on their relative positions within the bunch. The magnitude and sign of the force on a given particle depends on a number of factors: the total charge of the bunch; the axial distribution of particles; and the mode structure of the cavity.

(11) Because of the position-dependent wakefield force within the bunch, the initial energy distribution in the bunch will be modified as it passes through the cavity according to its position within the bunch. (The particles are assumed to be sufficiently relativistic so that the relative positions of the particles do not change during the passage of the bunch through the cavity.)

(12) Thus by appropriate choice of cavity parameters, the energy distribution along the bunch can be controlled. In particular, undesirable energy-position correlations present initially in a charged particle bunch can be suppressed.

(13) In order to accomplish the teachings and benefits of the present disclosure, an appropriate design methodology for an energy silencer compensator needed to be developed. Because of the previously mentioned complex dependence of the design on beam and structure parameters, simulation software was developed to model the entire process of the particle beam passing through a rectangular or cylindrical dielectric cavity. Analytic solutions to Maxwell's equations were obtained for both geometries. The code then evaluates the analytic expressions for the forces and uses the numerical results to advance a group of macroparticles with the desired initial spatial and energy distribution through the dielectric device geometry being evaluated.

(14) An important feature of an energy silencer is to provide a dielectric energy compensator that has he capability to adjust the parameters of the cavity to optimize its performance. This is made possible through adjusting the effective permittivity of the dielectric by incorporating temperature or electric-field sensitive ferroelectric layers into it. Additionally, the planar geometry also permits the option of adjusting the cavity by mechanically changing the aperture of its beam channel.

(15) Referring to FIG. 1, a diagram of the planar (rectangular) geometry energy correction cavity concept is shown. A movable conducting wall (A) is fitted to move inside a rectangular conducting channel (B) of length L. Mounted in the channel and movable wall are parallel dielectric (d1) and ferroelectric (d2) slabs running the length of the channel. The beam to be compensated travels down the adjustable gap (g) between the dielectric slabs. The interior ferroelectric slabs (d2) are optional but can be used as an additional means of tuning the structure. The permittivity of the ferroelectric is a function of the temperature, so the wakefield frequency spectrum of the structure can be changed by external thermal control of the cavity. A DC electric field across the ferroelectric slabs can be applied using electrodes thinner than the skin depth over the range of wakefield frequencies present to adjust the permittivity and hence tune the structure.

(16) Next, referring to FIG. 2, is a diagram of a cylindrical geometry energy compensation cavity. Concentric dielectric cylinders (a,b) are inserted into a conducting sleeve (c). A cylindrical beam channel (d) is located on axis. Since in this geometry there is no method to conveniently tune the structure by adjusting the gap, a layer of ferroelectric (a) is present to permit either thermal or electrical tuning as in the aforementioned planar geometry.

(17) Referring now to FIG. 3A, a sketch of the principle of operation of the device is provided in which the longitudinal wakefield as a function of position along the particle bunch is sketched in blue. The decelerating field in this case increases nearly linearly along the bunch; thus in this case particles to the back of the bunch are decelerated more than those at the head, providing compensation for a linear and increasing position energy correlation as shown in FIG. 3B.

(18) FIG. 3C shows the results of passing the beam of FIG. 3B through an optimized energy compensator. With the exception of a small number of particles at the head and tail of the bunch, the energy correlation is removed and the bunch energy distribution flattened.

(19) 2. Operation of the Preferred Embodiment

(20) In operation, FIG. 4 shows generally a block diagram of a possible application of the energy compensator in a linear accelerator. Particles produced in a source (A) are injected into a linear accelerator (B). Callout E1 shows the longitudinal phase ellipse of the particle bunch exiting the linear accelerator, possessing an undesirable tilt or equivalently an energy chirp. After passing through an appropriately designed and adjusted energy compensator (C), the energy chirp is removed and the phase ellipse has been rotated to remove the tilt (Callout E2). The corrected beam then is transported to the device (D) (e.g. an undulator) requiring an unchirped beam.

(21) The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents. Therefore, the scope of the invention is to be limited only by the following claims.