Method for energy dither of a particle beam

Abstract

A method for varying the wavelength of a free electron laser (FEL) by applying an energy dither to the charged particles supplying the FEL. Bunches of charged particle beams are accelerated by cavities that are operated at a harmonic of the bunch repetition rate. The method involves adding one or more secondary radiofrequency accelerator cavities after the primary beam transport and near the wiggler to apply a fluctuation between individual bunches with a pseudo-random distribution. The secondary radiofrequency accelerator cavities provide fine variations of the beam energy about a nominal operating point. Operating a free electron laser (FEL) with a 1% change in the electron beam energy via the added secondary cavities will result in a 2% wavelength variation of the FEL output.

Claims

1. A method for varying the output wavelength of a free electron laser by changing the energy of the input particle beam, comprising: a. providing an accelerator producing a charged particle beam including bunches of charged particles having a primary bunch repetition frequency at a primary accelerator cavity frequency or a subharmonic thereof; b. providing a wiggler to periodically laterally deflect the charged particle beam; c. adding one or more secondary radiofrequency accelerator cavities after the accelerator and before the wiggler; d. setting the frequency of each added secondary radiofrequency accelerating cavities to a non-integer harmonic or subharmonic of the primary bunch repetition frequency; and e. adjusting the accelerating voltage of each of the secondary cavities to provide a desired variation in the energy of the bunches which make up the charged particle beam, whereby the variation in the output wavelength of the free electron laser includes a distribution equal to a multiple of the energy distribution of the charged input particle beam.

2. In a free electron laser (FEL) system including an input particle beam, a method for varying the output wavelength of the FEL output beam by changing the energy of the FEL input particle beam, comprising: a. providing a primary accelerator cavity for supplying the input particle beam and a primary beam transport for transporting the input particle beam, wherein the input particle beam has energy with a nominal operating point and a primary accelerator bunch frequency; b. providing a wiggler to periodically deflect the beam of particles inside the input particle beam; c. inserting one or more secondary radiofrequency accelerator cavities between the primary beam transport and the wiggler; d. setting the frequency of each secondary radiofrequency accelerator cavity to a non-integer harmonic or subharmonic of the primary accelerator bunch frequency; and e. dithering the particle beam energy of the FEL input particle beam by operating the secondary radiofrequency accelerator cavities at the non-integer harmonic or subharmonic of the primary accelerator bunch frequency, said dithering varying the particle beam energy of the FEL input particle beam about the nominal operating point and varying the wavelength of the FEL output beam.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a plot of the energy distribution of a particle beam subject to an applied energy dither according to the present invention.

(2) FIG. 2 is a schematic depicting the layout of a free electron laser illustrating the principles of the present invention.

DETAILED DESCRIPTION

(3) The current invention includes a method for applying an energy dither to a charged particle beam in order to provide fine variations of the beam energy about a nominal operating point.

(4) According to the method of the present invention, one or more secondary radiofrequency accelerator cavities are added near the wiggler after the primary beam transport. The secondary radiofrequency accelerator cavities are used for fine variations of the beam energy about a nominal operating point.

(5) In a practical application, with reference to FIG. 2, depicting a free electron laser 12 supplied with charged particles from a primary accelerator 20, one or more secondary radiofrequency accelerator cavities 10a and 10b are added to a free electron laser (FEL) 12 near the wiggler 14 after the primary beam transport 16. The wiggler 14 includes a group of magnets 15 with alternating poles longitudinally arranged along a narrow gap. The frequency of the secondary accelerating cavity may be chosen as a harmonic or sub-harmonic of the primary accelerator bunch frequency, in which case the variation in energy output will be a small number of values reproducible in time. If a more random variation is desired, the cavity frequency or frequencies can be chosen as non-integer harmonic, harmonics or sub-harmonics of the bunch frequency and the output will vary in a pseudo-random manner over a range set by the cavity accelerating gradients and frequencies. A wide range of energy distributions can be obtained depending on these parameters. An approximate 1% change in the electron beam energy via the secondary cavity 10 will result in a 2% wavelength variation of the FEL output 18.

(6) As an example, if one were to use 320 MHz for the bunch frequency, a secondary cavity 10a operating at 833.23 MHz with an accelerating voltage of 0.88 MeV, a secondary cavity 10b operating at 823.0 MHz with an accelerating voltage of 1.0 MeV, and sample the resultant beam variation over a 1 ms period; one would obtain 7745 energies with the distribution functions shown in FIG. 1.

(7) The method of the present invention, using cavities with frequencies that are non-integer harmonics of the bunch repetition rates, allows one to vary the wavelength of the FEL output beam by changing the energy of the input particle beam, which is valuable in industrial applications of FELs and other charged particle beams. The method described herein is applicable to any charged particle beam, including electron beams and proton beams, and is also applicable to any architecture for producing the beam including free electron lasers, synchrotrons, cyclotrons, and linear accelerators. The meaning of various terms used herein are as follows:

(8) 1) The term primary accelerator bunch frequency as used herein refers to the primary frequency applied to group electrons such that they occupy a small longitudinal space relative to the wavelength of the accelerating RF frequency. The CEBAF bunch length is about 0.5 mm. The number of electrons per bunch can be calculated from the beam current, the electron charge (1.6021773310.sup.19 C), and the bunch frequency (nominally 1497 MHz).

(9) 2) The term wiggler refers to a group of magnets with alternating poles longitudinally arranged along a narrow gap in order to bend a particle beam in a sinusoidal path to generate synchrotron light.

(10) 3) The term radiofrequency accelerator cavity refers to a closed volume structure, e.g. cube, cylinder, sphere, ellipsoid, that resonates at frequencies whose wavelengths are half integer multiples of the dimensions. At the right frequency, a resonant field can build up to store thousands (copper cavity) or millions (superconducting cavity) of times more energy than when off resonance. This is the fundamental foundation for accelerating structures that can develop gradients equivalent to millions of volts.

(11) 4) The term primary beam transport as used herein refers to the system used to control the particle beam path and the energy properties of the beam in order to transport the beam to the wiggler.

(12) The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiments herein were chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.