Abstract:
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.

Description:
The United States of America may have certain rights to this invention under Management and Operating Contract DE-AC05-060R23177 from the United States Department of Energy. 
    
    
     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 attached  FIG. 1  wherein 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 magnet  12  may (though is not necessarily will) is used to adjust transverse electron drive beam  16  properties, but does not influence bunch length. In  FIG. 1 , an injected electron drive beam  16  is injected into linear accelerator  18  to produce an accelerated electron beam  20 . 
     After passage through wiggler  12 , the transport system again may (though not necessarily will) be used to adjust electron drive beam  16  transverse properties, but the electron bunch remains longitudinally short until it reaches the first bending magnet  24  of 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 dipole  10  upstream of wiggler  12  and initial bending dipole  24  downstream of wiggler  12  are spatially adjacent to the mirrors  26  and  28  that define the optical cavity  30 . 
     In addition to producing coherent radiation via interaction with wiggler  12  magnetic 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 dipoles  10  and  24 . The CSR radiation from dipole  10  onward propagates down the vacuum system, and poses little operational impediment, but the radiation from dipole  24  onward propagates onto adjacent downstream mirror  28  of optical cavity  30 . The resultant radiation power load can be in excess of tens or hundreds of Watts, is of asymmetric distribution on mirror  28 , and is largely absorbed by many of the materials used for such mirrors. This leads to thermal distortion of mirror  28 , 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 dipole  24  onward that propagates onto adjacent downstream mirror  28  of optical cavity  30 . 
     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 mirror  28  of optical cavity  30 . 
     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. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a conventional energy resolving Linac in accordance with the prior art. 
         FIG. 2  is a schematic representation of an energy resolving Linac incorporating a THz radiation management system in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     We have determined that the introduction of a magnetic electron beam orbit chicane  24  between wiggler  12  and downstream initial bending dipole  24 /downstream mirror  28  alleviates the effect of propagated radiation distorting downstream mirror  28 . Such a chicane  34  is shown schematically in  FIG. 2 ; it can consist either of four similar dipoles  36  (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 mirror  28  at a distance well displaced from downstream mirror  28 , and b) that the subsequent electron beam transport possess a significant momentum compaction at any downstream point in proximity to downstream mirror  28 . 
     With the chicane in place, the bunch is short at the first dipole  36 A of the chicane system  34 , 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 mirror  28 , 2) suppressed in intensity by 1/r 2  (r=r 2 /r 1 , where r 2  the dipole  36 A to mirror  28  separation and r 1  the dipole  24  to mirror  28  separation) at mirror  28 , relative to its power density in the prior art topology shown in  FIG. 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 dipole  24 . 
     As a consequence of these palliative measures, the THz thermal loading of downstream mirror  28  is dramatically alleviated, mirror distortion reduced, and FEL power is significantly enhanced. We note that the farther the initial bend at dipole  36  is from downstream mirror  28 , 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 cavity  30  axis, thereby directing the THz radiation away from mirror  28 , 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 mirror  28  or in any magnetic bending dipole  24  adjacent or in proximity to downstream mirror  28 . 
     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 dipole  24  that might otherwise be propagated onto adjacent downstream mirror  28  of optical cavity  30 . 
     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.