A chain drive consisting of a driver sprocket. (21), two driven sprockets. (lo), two fixed idler sprockets. (23), a spring- loaded idler sprocket. (24) and a hardened.
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SHUTTER ELEMENTS
AND FOR
COLLIMATOR THE
Princeton
BEAM
WITH FROM
ROTATING
THE
RADIATION-COOLED
PRINCETON
A.V.
C. C. Foster Palmer Physical Laboratory University, Princeton, New
CYCLOTRON”
Jersey
on the cylinder at its mid-point. A test was performed, using a 1. 5 kW beam from an electron s owed this worked at a power density gun, which 9 and which of 10 kW/cm was consistent with the calculations. As a result of these calculations and this test, such cylinders cooled as indicated have beconic the basic elenrrnts in the pre-analyzing magnet slits and beam shutter assembly and the post-analyzing magnet slits assembly of the Princeton heam transport system
Summary A beam collimator and shutter assembly has been designed and fabricated to define and lnterru,t the high maxinlum area1 power density (30 kW ! cm2) external beam anticipated for the Princeton A. V. F. Cyclotron. Pairs of graphite cylinders which rotate at 300 r/min about parallel axes a precisely adjustable distance apart, define the beam in each of the horizontal and vertical directions. A similar rotating graphite cylinder which is raised and lowered by an air cylinder operating in a fail-safe mode functions as the beam shutter. The power of the beam incident of these cylinders is dissipated by thermal radiation to surrounding water-cooled structures Tests with an electron gun have shown that the slit cylinders can successfully dissipate a beam power of 2. 2 kW with an area1 density power of 10 kW,/cm2 with no indication of deterioration or overheating of critical conlponents, Calculations indica:e power levels up to 5. 0 kW with 5OkW,&& area1 density nlay be tolerated by these slits.
General Description As indicated in Fig 1, pairs&f these graphite cylinders, which rotate about parailel axes and are a precisely adjustable distance apart, define the beam in each of the horizontal and vertiwhich may cal directions. One of these cylinders, be lowered into the beam, functions as the beam shutter. Figure 2 is an exploded view of the preanalyzing magnet slits and shutter assembly. It is seen to consist of five subassemblies; the central vacuum chamber, the vertically defining slit unit (VDSU), horizontally defining slit and shutter unit and cooling units for each of these. The (HDSSU), slit mechanisms in the HDSSU and the VDSU are nearly identical, however, the HDSSU housing has been enlarged to include the beam shutter nlechanism which takes its rotation ironi the same shaft as the slit cylinder. There are only diiferences in detail between the two cooling units, which are of hard-soldered copper construction, and the central vacuum chamber is of standard heliarc welded aluminum construction. A similar asscmbly. but without the shutter mechanism, is used for the post-analyzing magnet slits. As a result of these considerations, the focus of this paper is on the HDSSU of which an assembly drawing is seen in Fig. 3.
Introduction The Princeton A. V. F. Cyclotron will produce high intensity external beams of several particle ty-pes with energies variable up to its magnetic rigidity limit of 450 kG-in. 60 PA of 56 MeV protons will have the maximum beam power of 3. 36 kW. Extracted beams will be focused at the position of a set of pre-analyzing magnet slits by a magnetic yuadrupole doublet into a minimum beam area spot 0. 1 cm wide by 1. 0 cm high. After passing through these slits, a beam niay follow one of three paths which depend on the analyzing magnetic field setting. It may, for high resolution work, be bent through 90° and pass through a set of post-analyzing magnet slits into a switching magnet which deflects it into the appropriate scattering chamber. It may be bent through Zoo into an on-line atomic beam machine 1), or allowed to pass undeflected into a target irradia-. tion facility 2). For each of these paths, uses can be visualized in which the beam will have the maximum area1 power density at the position of the pre-analyzing magnet slits. Therefore, these slits must be capable of withstanding beams with area1 power densities up to 33. 6 kW/cm2. The apparent area1 power density limit for dissipation of heat, by water-cooling with high-velocity thin water filnls, is of the order of 10 kW/cm2 3~ 4). Calculations indicated that a 2. 0 in. diameter hollow7 graphite cylinder 5. 5 in. long rotating about its axis at 300 r/min could dissipate, by radiation to surrounding water-cooled structure and to a water-cooled probe inserted along the axis of the cylinder, up to 50 kW/cm2 incident ;kWori supported in part by the U. S. Atomic gy Comm. and the Higgins SC. Trust Fund.
F.
Dctailetl
Descriptions
The Holding Mechanism. In order to hold the slit cylinders with the necessary precision and to al low for th:ir easy removal, special rollet asscmblies (4) ’ were fabricated. Each collet grips a centerless ground 0. 250 i 0 0001 in diameter calcium stabilized zirconium oxide rod 1. 5 in. long (3) which is‘pinned to the slit cylinder (1) with a graphite pin (2). This rod, because of its low thermal conductivity (about l/450 that of copper), is expected to allow less than 5 W to pass into the collet when 5. 0 kW is incident on the slit cylinder. This power, and any developed by friction, is then conducted to the outside through a spring-loaded beryllium copper sliding electrical contact (5) which is used to monitor the charge collected on the cylinder as well.
X:Numbers numbers
Ener
622
in parenthesis, of Fig. 3.
(4) are
indicating
Each toilet of a slit pair is held in a carriage (6) in which it can rotate freely. A ground a1un:lr.a spacer (7) accurately positions the collet in the inner races of the pair of collet bearings (8) while insulating them from thermal and electrical currents. A threaded ring (9) loads the upper bearing to remove undesirable play and allow the use of cheaper, more readily available hearings. A driT;c sprocket (IO) is attached to the insulating sleeve. Two carriages are mounted to a pair of parallel steel guide rods (11) by ball bushings (12) which positions them accurately in two directions while allowing their separation to be continuously and accurately varied by the positioning mechanism. The mounting plate (13) for these rods shields the slit mechanisms from thermal radiation and accurately positions it in the vacuum housing (14),
ter is inside of the vacuum enclosure. The drive mechanism must run continuous1 when there is beam, in a vacuum of 10v5 to lO- x1 mm of Hg in a flux of fast neutrons. Therefore, a low vaporradiation-resistant lubricant must be pressure, used. Results of tests of lubricants for hall bearings run under load in vacuum near a 6oCo source, ere reported by F J. Claws and W. C. ;fifTn; 33) , mdlcate that a silicone grease, Versilube F-50. 9) could be used in the present application. Tests of this lubricant under actual use conditions have not been performed as yet. Cooling Structures. The water-cooled probes and surrounding water-cooled structures are hardsoldered onto copper vacuum closure plates as seen in Fig. 2. In the event of slit cylinder brcakthe surrounding structures will catch the age, radioactive fragments and facilitate their removal. Tests and calculations indicate that l/4 of the power incident or a slit cylinder is removed through the probe.
The Positioning Mechanism. A rack (16) mounted ro each carriage engages a common anti-backlash pinion (15). Linear motion of one cart is, thereby-, accrlrately coupled to the other carriage. A push rod (17) transmits the motion of the slit positionIcg mechanism through a metal bellows vacuum iced-through (18) This pL:sh rod nioves 0. 5 mm per revolution of the special gear (19) which is tllrned by a 200 step,/revolution stepping motor (LO). A single step corresponds to 2. 5 x 10e3 mm and a 60 step per second rate moves the slits over thp fllll seperation range of 1 0 to 12. 7 mm in less than 4.5 s.
The Beam Shutter. A slit cylinder (1) is held in a collet assembly (29) similar to those of the slit mechanism, which is insulated by an alllmina sleeve (22) and slides through roller bearings. When it is in the down position, the slit cylinder interrupts the beam and a clutch (30) engages to rotate the slit at 300 r/min. The up-down motion is provided by an air-cylinder which is coupled to the shutter mechanism hy a metal bellows feedthrough (31). If air pressure or electrical power for the air-cylinder is lost, the shutter automatically drops.
‘The Drive Mechanism. A chain drive consisting of a driver sprocket (21), two driven sprockets (lo), two fixed idler sprockets (23), a springloaded idler sprocket (24) and a hardened steel chain (25) is used to ensure positive rotation of the slit cylinders while allowing the necessary positioning flexibility. The driver sprocket (21) and the shutter driver sprocket (26) are fixed to a c:)r.rllon shaft which is connected through a univvrsal couplinl (27) to a magnetic rotary vacuum k. This feed-t&rough, whicil is fc~e&throrlgh momtcd to the HDSSU vacuum cover (28), is connocted by a S ft. long flrxihle shaft to a variable speed I). C. motor which rotates the slits at 300 r/min. This motor 1s molunted outside of the local rsciiati,,n shielding for protection from nuclear radlatiion. Hecause of the possibility that vibrations will develop as the chain drive wears and the a drive mechanism using gears chain strttchos, has been designed and is being developed.
Slit Cylinder .,&(aterial __-slit cylinders are made of molded A’L’J graphite lo) , which has relatively low vapor prrssure at high temperatures, low thern~al expansion coefficients and does nG;t form long-lived radioactive isotopes in high fluxes at fast neutrons. However, the range of charged particles in graphite IS relatively long. Therefore. the bctam. after collimation by two parallel 2 in diameter graphite slit cylinders, will have a large number of energydegraded slit-scattered particles which have passed through the edges of the slits. This effect is particularly important frl fluxes near the slits and the deteri6, 7) it was oration of elastomers in sllch fluxes decided :hat all vacuunl seals would be static so that rrletal seals, which arc radiation resistant, could 1,~ cased Therefore, Il,etal bellows were l;srd to transmit linear niotion and magnetic rotary fred-throilghs were used to transmit rotary motion through the wall of the vacuml enclosure. Inrliu111 wire
