Abstract:
A photoelectron linear accelerator for producing a low emittance polarized electric beam. The accelerator includes a tube having an inner wall, the inner tube wall being coated by a getter material. A portable, or demountable, cathode plug is mounted within said tube, the surface of said cathode having a semiconductor material formed thereon.

Description:
GOVERNMENTAL RIGHTS IN INVENTION 
     This invention was made with governmental support under Small Business Innovation Research (SBIR) Contract No. DE-FG03-02ER83401 awarded by the Department of Energy to DULY Research Inc. The government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention provides a photoelectron linear accelerator for producing a polarized electron beam with low emittance. 
     2. Description of the Prior Art 
     Polarized electron beams are a principal investigative tool at a number of major accelerator centers. It has been demonstrated that polarized electrons will be extremely useful in electron position colliders. Current polarized electron beams for accelerators are generated by dc-biased electron guns that utilize gallium arsenide (GaAs) as the photocathode material. The relatively long pulse (on the order of nanoseconds) generated by these sources is rf chopped and bunched in the injector to derive the desired pulse structure, including microbunch number and temporal width, to match the accelerator and experiment requirements. 
     The normalized rms transverse eminance of high charge rf-bunched beams is typically on the order of 10 −4  m. Future colliders require an emittance of ˜10 −8  m in at least one plane. Current designs achieve this extremely low emittance in the vertical plane using an appropriately designed damping ring. Since the photoemitted electrons are rapidly accelerated to relativistic energies by electric fields that are much higher than used in dc guns, the effects of space charge on emittance growth are minimized. Since the initial emittance growth in an rf gun is correlated, this growth can be reversed by placing a solenoidal field immediately after the cathode. An emittance-compensated, rf photoinjector is normally designed to achieve the minimum emittance at a compensation point some distance beyond the solenoid exit. Simulations indicate that emittances as low as 10 −6  m for 1nC of charge per micropulse can be achieved with an rf photoinjector for round beams, although the measured values tend to be slightly larger. 
     Photoinjectors are currently in widespread use and have been proposed as a source of cw unpolarized electron beams for energy recovery linacs (ERL). The gun laser required for an ERL may only be feasible if a GaAs (visible laser) or CsK 2 Sb (green) cathode is utilized. In this case, the plane wave transformer (PWT) injector would have to provide adequate cooling. The cooling requirement is somewhat less stringent in some versions of electron ion colliders, which require polarized electrons, for which the rf frequency of the cw injector can be quite low. 
     The problem for a dc gun is not the gradient on the cathode, which can be fairly high and potentially even as high as the field on the cathode of a PWT gun at extraction. Thus the emittance of the beam exiting a dc gun can be comparable to that exiting an rf gun, but the energy is 5 to 50 times lower. If a short pulse high-charge beam is required, as for a collider, the problem is coupling the still low-energy beam to an accelerating structure before the emittance (both the transverse and especially the longitudinal emittance) grows significantly due to the intense space charge forces. Emittance compensation should in principle work for a dc gun as well as an rf gun, but the problem is the vastly lower energy and thus the effect of the space charge field still remains. 
     What is thus desired is to provide a device for providing a polarized electron beam using an rf gun, the beam having a low emittance. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus to produce a high-quality, polarized electron beam and, in particular, uses an rf photoelectron gun using the PWT photoelectron linear accelerator design, thereby generating a lower emittance beam than available in the prior art. 
     Semiconductors such as binary compounds (and their ternary and quartemary analogs) combining elements from the III and IV columns of the periodic table, for example, gallium arsenide, are proven cathode materials which are used to produce polarized electron beams. A polarized electron beam is produced when such a cathode semiconductor is illuminated by a circularly polarized laser beam. An ultra high vacuum (&lt;10 −11  Torr) condition is provided in order for the semiconductor target to have good quantum efficiency and long lifetime for the production of polarized electrons. 
     The present invention utilizes certain features of conventional dc-biased polarized guns to produce polarized electron beams using an rf gun, in order to dramatically improve the emittance of the beam. A low emittance is desired and is an indication of the good quality of the electron beam. 
     The PWT rf gun design is especially well matched to the features necessary for production of polarized electrons. Specifically, the PWT design has 1) an inherently high vacuum conductance which improves the vacuum, 2) an integrated photocathode inside an rf linear accelerator, and 3) an emmitance compensating beam focusing system which improves the beam quality. 
     Additional features that further improve the operation of the PWT gun for the production of a polarized electron beam include a load-lock for introducing the activated semiconductor coated cathode under ultra-high vacuum conditions into the PWT tube structure, enhancing the inherently superior vacuum pumping potential of the PWT design by enlarging the diameter of the outer cylinder, and coating the interior cylindrical tube wall with a thin-film of residual gas absorbent such as TiZrV. 
     The present invention thus provides an improved rf photoelectron gun for producing a polarized electron beam with low emittance. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention as well as other objects and further features thereof reference is made to the following description which is to be read in conjunction with the accompanying drawing wherein: 
     FIG. 1 is a schematic diagram of polarized electron PWT photoinjector in accordance with the teachings of the present invention; 
     FIG. 2 is a cross-sectional view along line  2 — 2  of FIG. 1; and 
     FIG. 3 illustrates the load lock system. 
    
    
     DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a schematic diagram of the polarized electron PWT photoinjector  10  of the present invention. 
     The integrated PWT photoelectron linear accelerator  10  which includes photocathode  12  is located directly inside the full accelerating structure and supported on demountable cathode assembly  14 . The PWT linac  10  is a n-mode, standing-wave, linac structure which consists of a series of cylindrical disks  16  forming a disk assembly, each disk  16  being spaced half a wavelength apart, except for the first and last disks which are at a distance about a quarter wavelength from the end plates  18  and  19 . The disk assembly is positioned within the tube, or tank,  26 , and is supported by a water-carrying tube  22 , tube  22  serving both to support and cool disks  16 . A cooling channel  33  is provided to additionally cool the disks  16 . Suspended along the axis of a large cylindrical tank, or tube,  26 , the disk assembly defines a series of open cavities or cells. 
     Unlike conventional disk-loaded structure, the PWT cells have no cavity walls thus providing cell-to-cell coupling. The rf power is coupled into the linac through a small RF coupling iris, or hole,  24  in the tank wall  26 , from the RF port  28 , the rf power exciting a TEM-like mode in the annular region between the tank wall and the disk assembly. 
     An emittance compensating solenoid  32  straddles the front end of the PWT linac  10  beginning at the plane of the photocathode  12 . A bucking magnet  34  extends beyond the linac over the cathode assembly. The combined magnets provide the emittance compensation for the electron beam  30  in the linac  10 . Magnets  32  and  34  are also designed to provide an axial magnetic null on the surface of photocathode  12  so that the electron beam  30  would be minimally disturbed by the magnetic field at low velocities upon its creation at the photocathode  12 . It should be noted that the design of the present invention is scalable to any desired operating frequency, including the L, S and X-bands. 
     FIG. 2 is a cross-sectional view along line  2 — 2  of FIG.  1 . The inner surface of tank wall  26  has a coating  44  of thin-film of residual gas absorbent getter material such as TiZrV, formed thereon. 
     The demountable cathode assembly  14  is operatively engageable with a load lock system  50 . FIG. 3 is a simplified schematic illustrating the load lock system  50 . 
     A semiconductor photocathode, such as a thin GaAs wafer, is mounted onto a grooved plug  52 , connected to the end of the first linear rack  54  of the exchange chamber  58 , isolated via valves  56  and  62 , and pumped down to high vacuum. 
     The end of the first linear rack is inserted into the rear of the plug  52  and made secure via a pair of leaf-spring-loaded sapphire cylindrical rollers. 
     The gun isolation valve  56  of the load lock  50  is opened and the first linear rack advances the plug  52  onto the gun cathode plate  19 . The applied pressure of plug  52  onto the plate is monitored by a torque sensing device  66  mounted on the rotary motion feedthrough  68  of the pinion gear that drives the first linear rack. The motion feedthrough is motorized so that the torque sensor value can be used in conjunction with the motor to keep the applied pressure on plug constant. This can be monitored remotely during photoinjector operation so that the applied pressure may be changed to modify the electrical behavior of the rf seal that is made between the plug  52  and the cathode plate. 
     Occasionally, the photocathode needs cesium metal added to its surface. The motorized feed through  68  of the first linear rack is computer-controlled for remote withdrawal, for touch-up cesium metal addition to the photocathode surface, and for re-insertion of the plug  52  into the gun. To accomplish this, the first linear rack  54  is retracted to a position upstream of the gun isolation valve  56 . The isolation valve  56  is then closed so that no cesium metal vapor may enter the gun during the touch-up operation. A ring of computer-controlled cesium metal vapor dispensers  60 , located internal to the vacuum pipe, are now exposed to the front of the plug  52  and the photocathode surface. Cesium metal vapor is deposited onto the photocathode surface. Following the desposition, isolation valve  56  is re-opened and the first linear rack  54  moves the plug  52  back into the gun. 
     The cathode plug may be completely removed from the gun and the load lock system  50  by retracting plug  52  via the first linear rack  54  to the exchange chamber  58 . Isolation valve  56  is closed to protect the photoinjector in event of vacuum failure. An external transfer chamber is attached to the exchange chamber, pumped down to high vacuum, and the isolation valve  62  is opened for access between chambers. A second linear rack located in the transfer chamber removes the plug from the exchange chamber. New plug-mounted photocathodes may be installed into the load lock in similar manner. 
     While the invention has been described with reference to its preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its essential teachings.