Magnetic chicane for terahertz management

The introduction of a magnetic electron beam orbit chicane between the wiggler and the downstream initial bending dipole in an energy recovering Linac alleviates the effects of radiation propagated from the downstream bending dipole that tend to distort the proximate downstream mirror of the optical cavity resonator.

FIELD OF THE INVENTION

The present invention relates to the management of terahertz (THz) radiation and more particularly to the use of a magnetic chicane to achieve such management.

BACKGROUND OF THE INVENTION

Conventional Energy Recovering Linac (ERL)-based Free Electron Lasers (FELs) employing optical cavity resonators use a combination of off-crest acceleration and magnetic bunching to produce the high peak currents required for FEL operation. This imposes on the system a requirement for merging and separating the drive electron beam and the FEL optical mode. This is typically done with a topology as shown in attachedFIG. 1wherein the final dipole of the upstream magnetic recirculator/bunching system is the site of the final bunch length compression. Subsequent transport to a light-producing wiggler magnet12may (though is not necessarily will) is used to adjust transverse electron drive beam16properties, but does not influence bunch length. InFIG. 1, an injected electron drive beam16is injected into linear accelerator18to produce an accelerated electron beam20.

After passage through wiggler12, the transport system again may (though not necessarily will) be used to adjust electron drive beam16transverse properties, but the electron bunch remains longitudinally short until it reaches the first bending magnet24of the energy recovery recirculation/energy compression system of the ERL.

In this system topology, the system geometric footprint is dominated by the optical cavity length D; in the prior art, the final magnetic bending dipole10upstream of wiggler12and initial bending dipole24downstream of wiggler12are spatially adjacent to the mirrors26and28that define the optical cavity30.

In addition to producing coherent radiation via interaction with wiggler12magnetic fields, the tightly bunched electron beam used in high power FELs also produces coherent synchrotron radiation (CSR) in the THz spectral regime via its interaction with the bending fields in magnetic dipoles10and24. The CSR radiation from dipole10onward propagates down the vacuum system, and poses little operational impediment, but the radiation from dipole24onward propagates onto adjacent downstream mirror28of optical cavity30. The resultant radiation power load can be in excess of tens or hundreds of Watts, is of asymmetric distribution on mirror28, and is largely absorbed by many of the materials used for such mirrors. This leads to thermal distortion of mirror28, rendering it astigmatic and consequently limiting the power that can be generated by and extracted from the FEL.

There thus remains a need to control or eliminate the radiation from dipole24onward that propagates onto adjacent downstream mirror28of optical cavity30.

OBJECT OF THE INVENTION

It is therefore an object of the present invention to provide a mechanism that controls or eliminates radiation from the initial magnetic dipole onward that propagates onto adjacent downstream mirror28of optical cavity30.

SUMMARY OF THE INVENTION

The introduction of a magnetic electron beam orbit chicane between the wiggler and the downstream initial bending dipole of an energy recovering Linac alleviates the effects of radiation propagated from the downstream bending dipole that tend to distort the proximate downstream mirror of the optical cavity resonator.

DETAILED DESCRIPTION

We have determined that the introduction of a magnetic electron beam orbit chicane24between wiggler12and downstream initial bending dipole24/downstream mirror28alleviates the effect of propagated radiation distorting downstream mirror28. Such a chicane34is shown schematically inFIG. 2; it can consist either of four similar dipoles36(as depicted) or of a generic symmetric three-dipole chicane bending through angles α, −2α, and α, or of any other appropriate beamline design geometry. The only fundamental requirements are: a) that the “chicane” (or any other beam transport system) bend the short electron bunch away from the optical mode, i.e. downstream mirror28at a distance well displaced from downstream mirror28, and b) that the subsequent electron beam transport possess a significant momentum compaction at any downstream point in proximity to downstream mirror28.

With the chicane in place, the bunch is short at the first dipole36A of the chicane system34, and CSR radiation is thus produced at this location. This radiation is, however, 1) directed away from, and thus displaced in transverse position by the time it forward propagates to the location of downstream mirror28, 2) suppressed in intensity by 1/r2(r=r2/r1, where r2the dipole36A to mirror28separation and r1the dipole24to mirror28separation) at mirror28, relative to its power density in the prior art topology shown inFIG. 1, and 3) transport through the remainder of the chicane significantly increases the electron bunch length (by virtue of the momentum compaction of the chicane), thereby dramatically suppressing CSR production at initial bending dipole24.

As a consequence of these palliative measures, the THz thermal loading of downstream mirror28is dramatically alleviated, mirror distortion reduced, and FEL power is significantly enhanced. We note that the farther the initial bend at dipole36is from downstream mirror28, the better the suppression will be. Again, the geometry need not be that of a classic “magnetic chicane”, it need only bend the beam away from the optical cavity30axis, thereby directing the THz radiation away from mirror28, and should supply momentum compaction to the wiggler-to-energy recovery transport, so that the bunch is not short when it is in spatial or temporal proximity to downstream mirror28or in any magnetic bending dipole24adjacent or in proximity to downstream mirror28.

Magnetic chicanes have generally benign and well understood transverse and longitudinal focusing properties, and are thus readily integrated with both preexisting wiggler-to-energy recovery system transport systems and the transverse and longitudinal matching manipulations used in the energy recovery/energy compression systems of typical ERLs. The orientation of the chicane is not constrained; either horizontal or vertical bending can be used, as can, in principle, out-of-principal-plane bending or any other realizable combination of bend orientations provided that other system constraints are appropriately satisfied.

There has thus been described a method and apparatus that alleviate the effects of radiation emitted by initial bending dipole24that might otherwise be propagated onto adjacent downstream mirror28of optical cavity30.

As the invention has been described, it will be apparent to those skilled in the art that the same may be varied in many ways without departing from the spirit and scope of the invention. Any and all such modifications are intended to be included within the scope of the appended claims.