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Characteristics of the UCLA cyclotron radiofrequency system
l\UCI.EAR INSTRUMENTS AND METHODS 18,19 (1962) 177-183; NORTH-HOLLAND PUBLISHING CO.
Part I II. Radio Frequency Systems 177-200 CHARACTERISTICS OF THE UCLA CYCLOTRON RADIOFREQUENCY SYSTEM K. R.
MACKE~ZIE,
S. PLUNKETT and E. L.
PETERSE~
Department 0/ Physics, University 0/ California, Los A ngeles Presented by K. R . Mackenzie An account is given of the various interdependent factors that must be considered when dees are placed in the valleys of a sectorfocuscdcyclotron.Thespiral shaped dees and r.f. resonator comprise a half wave structure in which an II % frequency range is achieved by varying the cro ss-section of the inner conductor near the node. A grounded grid oscillator i); coupled
to the resonator by loops and transmission lin es. The shape and weight distribution of the dees is such that they can be open on both sides, and yet can be easily supported at the outer radius. The distribution of heat allows the dees to be sawed from linch Cu plate with cooling supplied by conduction at the outer radius. They are installed by stretching the vacuum tank.
1. Introduction The 50 nrev Spiral Ridge Cyclotron at the University of California, Los Angeles, was designed around the existing 40 ton magnet of a partially constructed 50 MeV synchrocyclotron. The decision to convert it to a sector focused machine was made somewhat short of completion. With a conceptual design using 180° dees, a reduction in energy to around 30 nre\' was expected, and this was considered to be a reasonable exchange of energy for intensity. There was however, a natural reluctance to accept such a reduction in energy, and the "spiral dee in valley r.f. system" seemed to supply a solution which could make this possible"). In round numbers the synchrocyclotron was designed to give 50 nreV with a pole diameter of 4 It., a magnet gap of 4 in. and a field of 19500 gauss. In the conversion to a sector machine of the same energy, these average numbers were retained. The required flutter depends upon spiral angle and magnet gap. It was soon determined that at 50 nreV, the gap could never be made large enough to permit the use of 180° dees unless the voltage was in the neighborhood of a few kV (limited by sparking) . Such a solution was most unattractive because it would require extraordinary accuracy in shimming the magnetic field; the beam would be severely space charge limited; and there is the discouraging piece of data that synchrocyclotrons, which usually operate at about this voltage level, have not been able to realize resonant extraction efficiencies of more than a few percent. Therefore, the
possibility of placing the dees in the valleys was investigated. This decision had a very curious effect on design philosophy. Previous thinking had been concerned with accurately shaping the field in order to allow use of the lowest possible voltage gain per turn and hence permit a smaller gap. With the dees in the valleys, the minimum gap was independent of the dee structure. Hence it was made as small as possible in order to reduce the spiral angle 2 ) and produce a sharp field fall-off for deflection. The minimum value was based on an estimate of the amount of space needed by the internal beam. The other variables, such as valley angular width and flutter, were optimized to allow insertion of dees which would give the highest possible voltage gain per turn, and thus, relax the accuracy needed in shimming the field and ease the problem of extraction. Such an objective dictates that the 'valleys be made as wide as possible. This means the isochronous condition should be realized by reducing the gap rather than by flaring the ridges. Fortunately, for a given flutter, maximum focusing occurs with the valleys wider than the hills, and the optimum is rather broad. A choice was made of 34° hills and 56° valleys, which permitted the use of 45° dees with 5.5° side clearance to the hills"). The transit angle is estimated to average about 50°. The 1) S. H . Plunkett, E. Petersen and K . R. Mac Kenzie, Bull. Amer, Phys, Soc. 6 (1961) 520. 2) D. J. Clark, J. R. Richardson and B. T. " 'right, ~ucI. lnstr. and ",Icth. 18.19 (1962) I. III . RADIO FREQUENCY SYSTE)!S
178
K. R. )IACKENZIE
angular side clearance seems large, but at small radii it represents a minimal spacing. At larger radii, where the spiral angle approaches 45°, the azimuthal side clearance does not represent the true dee to ridge gap. At such radii the ions cross the dees at an angle and the radial component of ac-
et al.
out of phase on the fundamental so that adjacent dees are either in or out of phase. The third possibility yields a low gain per turn of 1.6 V o and is quite unattractive. Case (2) seems very complicated especially if variable frequency is desired. However, the engineering exists, since it is similar to a
Fig. I. View of r.f. liners in opposite valleys.
celeration causes an angular phase change which three phase design which was used on the Thomas cancels the apparent phase change due to the spiral. electron models and a 20 inch proton model of the Initially, a requirement for wide frequency range central region at Berkeley 3). If alternate valleys existed and in this respect, the small 45° dees, with are empty, two more possibilities can be considered: their small energy storage, were much superior to (4) dees in opposite valleys excited in phase at 180° dees. This requirement was soon relaxed. A double frequency, yielding a gain of 3V 0 per turn, decision was made not to introduce delays by de- and (5) dees in opposite valleys which oscillate out signing for variable energy at this time. . of phase at the fundamental frequency and yield a Several ways were considered for fitting such gain per turn of 1.8V o' dees into a resonating system. If all four valleys are Scheme number 5 was chosen. It is the simplest used, three possibilities exist: (1) dees in adjacent of all; appears to give sufficient energy gain per valleys, excited in opposite phase at double fre- turn; and leaves 1\\'0 empty valleys for probes, quency, yielding a gain per turn of 6V o ; (2) dees in magnetic channels, ion sources and other devices. adjacent valleys operating with a phase difference In the central region it provides ideal starting of 90° at the fundamental, with a net gain per turn 3) B. H. Smith and K. R. ~IacKenzie. Rev. Sci. Instr. 27 of 3.6V o ; (3) dees in opposite valleys which oscillate (1956) 485.
THE UCLA CYCLOTRON RADIOFREQUENCY SYSTE:lI
conditions. Almost from the beginning it was evident that the tips of the dees could be squared off to look like 1800 dees, so the energy gain could be as high as 4 V 0 for the first few turns. Ncar the source there is very little magnetic focusing and the fastertheionscan be pushed through this region the
179
dees in the valleys. The resemblance to ordinary dees has essentially disappeared, and for lack of a better name they are now called spiral dees . Fig. 3 is an elevation view which shows the internal dimensions of the dees, hills, and valleys. In the center of the machine, the vertical hill
Fig. 2. 45° dees in the valleys.
:~~~;.~~:c~~7:s~~c~:~~n~~t~:::~;~~ns~7p:c::~;~
that the IOns would become confused. After these = I - -~.=... --~~=-=-----= d:cisionswerem~detheinteractio~ofther.f. syst.em ~_ .~)\»\\\\mTIF with the magnetic field configuration was essential- .-~~ . 77~7~ 777r ly settled. The dee in valley design emerged as I / / having many advantages over the 180 system with o~.. f no evident fundamental limitations, such as sparking, vertical or radial defocusing, or need of ex//U///////////////// cessive r.f. power.There were unsolved construction and installation problems of course, which were ,= regarded as new and unusual, rather than difficult. 0
?Zt., 7Zl
2. Dee Design Fig. 1 shows a plan view ofthe ridges, or sectors, with copper lin ers in two of the valleys. Fig. 2 shows
tzL
"
fZL
7//0777////77777777
7
Fig. 3. Two elevation views of the dees and ridges showing spacings and clearances. III. RADIO FREQUEXCY SYSTE:llS
180
K. R. MACKENZIE
spacing is over two inches and hence it is possible for the dees to widen out to 180 0 in this region (see fig. 2). The transition to 45 0 is made rather abruptly in order to provide maximum clearance at the point where the dee crosses over the edges of the hills at small radii. A local reduction of hill width is ex-
et at.
think in terms of dees which are cooled in part by radiation, but primarily by conduction to the sup, porting member, or dee stem, (see fig. 4). One quarter inch hard drawn copper plate appeared to have both adequate heat conductivity and mechs, nical strength, and hence the dees were cut from
Fig. 4. Half wave structure and spiral dees. Note expandable hinged sections in the node region. The long slot through the
straight section allows insertion of probes.
changed for an increase in hill thickness in order to further increase this clearance (see figs. 2 and 3). Since these dees must be "open" on all sides except the outer radius, they must be structurally supported in cantilever fashion from outside the pole area. The weight distribution in the triangular shape is particularly helpful and the depth of the valleys is sufficient to provide some extra support if needed. Ther.f. currents flow radially on the tapered dee surfaces and decrease very rapidly toward the center. Practically all the heating occurs within a few inches of the outer radius. This permits one to
this material and simply bolted to the water cooled stems. In spite of their peculiar and humorous shape, these dees are probably the simplest ever constructed for a cyclotron of moderate size. With the contemplated power level of 50 kW it is estimated that the dees will never reach annealing temperatures, hence they should retain their shape and alignment. Beam scrapers in the unused valleys should prevent heating by ion impact except near the source, where radiation cooled slits or pullers made from refractory materials should be able to dissipate the bombardment energy.
THE UCLA CYCLOTRON RADIOFREQUENCY
3. Dee Support Structure One main problem was finding a convenient method of installation, since the dees will not pass through the one hill inch gap. Separately removable grounded dee stems on opposite sides were considered but were not favored for the following reasons: (1) both sides of the machine would be cluttered with d. structures; (2) they would have to be separately tuned; and (3) sweeping fields or a booster oscillator would have to be used, because the dees could not be biased. Ungrounded biased dees were heavily favored because the problem of getting rid of discharge loading and multipactoring is not a negligible one"). The simplest ungrounded structure is a half wave system, (see fig. 4) and it was chosen after determining that it was possible to construct it in upper and lower halves, which would separately pass through the hill gap. They were bolted together at the dee using special long handled wrenches. Since the r.f. current flows independently on the top and bottom dee stem surfaces, the connection inside the vacuum chamber is primarily mechanical and handles only the small unbalance current which is needed to keep upper and lower halves of the dee at the same potential. Somewhat later a much simpler solution was accidentally discovered. Because of the close proximity of the magnet coils to the gap, it is impossible to install the vacuum chamber as a unit, without raising the upper pole, yoke and coils. Since the lids of the vacuum chamber are held by long bolts through the poles, it is first necessary to unscrew these bolts before the raising operation. On one occasion this was forgotten, and it was discovered that the vacuum chamber lids would stretch as much as 2 in. without breaking the gasket seals around the edge. A fraction of this travel permits easy installation of the dees, so this is the method now in use. Fig. 5 shows a plan view of the dees and dee stems within the vacuum enclosures. Since the stems add capacity to ground, the current increases rapidly from the dee end of the stem to the edge of the vacuum chamber. Consequently, the stems were increased in width in order to keep the power 4) B. H. Smith, Nuc!. Instr, and Meth. 18, 19 (1962) 184.
SYSTE~r
181
Fig. 5. Plan view of resonator and schematic representation of oscillator transmission lines and coupling loops. Loops arc shown displaced horizontally for clarity but are actually placed vertically, one above the other as shown in fig. 6. Cylindrical insulators shown near the hinges are used for support. The flat structure is clamped between pairs of insulators at the points indicated. The resonator is skewed in order to keep the pump and fittings within the outside dimensions of the magnet.
Fig. 6. Elevation view of the external vacuum chamber, showing cross-section of the expandable inner conductor. a and b represent the coupling loops. The upper and lower hinged sections are drawn to shown the extremes of motion. In reality they move apart together. III. RADIO FREQUENCY
SYSTE~IS
182
K. R. :-IACKENZIE
density low. At the edge of the vacuum tank the stems take on a more or less "box" shape as shown in figs. 5 and 6. Fig. 6 shows a cross section of the external vacuum chamber which is made as large as consistent with the dimensions of the rest of the 2
....
0
"0 >
I
0
.:>:
2
.:
2
0' 0
Plate
Line 4
5
6
~
6
mete rs Tube
Cathode
~ 0
>
0
2 Tube
2.
s
4
Distance, mete r s
Fig. 7. Transmission line voltages for 20 kV on dees, plotted from tube to vacuum feed through bushings.
machine. This allows maximum surface area on the inner conductor. The chamber doubles as a pump manifold. Since wide frequency variation is no longer required, a modest range is accomplished by varying the cross section of the inner conductor as shown in fig. 6. The total change is 29 to 26 Mc/s which permits a larger energy variation than is possible with the magnetic field. This frequency control is useful in allowing the oscillator to be tuned to the optimum value of the field. In order to permit dee bias to be applied, the whole support structure is mounted, near its center of gravity, on insulators as shown in fig. 5. These are short alumina cylinders with a til wall thickness, 21" long and 3" in diameter. They run uncooled in the vacuum at about half dee voltage. The half wave structure consists of a water cooled copper skin over a dural frame, sawed from flat stock. The movable sections of the frame (see fig. 4) pivot at the widest portion of the stems to reduce the current density on the flexible copper hinges. Bias is applied through d. chokes at the node. 4. The Oscillator The resonator is excited through transmission
et al.
lines by a grounded grid oscillator similar to instalIations onformer f.m.rnachines"). The tube employed for the present pulsed operation is an Eimac 3W5000A with 5 kW plate dissipation. Fig. 5 shows a schematic representation of the coupling lines and the oscillator box. In reality, the two coupling loops are placed vertically, one above the other as shown in fig. 6. The separation discourages parasitic oscillation at the line frequencies. Fig. 7 shows the voltage distribution along the transmission lines. The oscillator is powered by a conventional threephase full wave rectifier unit using mercury vapor tubes with rough primary saturable reactor control. A series regulator tube, for fine control, is at present being used only for pulsing. 5. Performance The system is now in operation on a pulse basis to reduce radiation and activation during tune up, with approximately 50 kV on each dee to ground. The limitation is apparently sparking at the tips of the dees where the vertical clearance is a minimum of approximately one half inch. Future variations in the center region may increase this clearance and make it possible to hold more voltage r.f. power in the pulse is 30 kW. A new oscillator is planned, using a Machlett 6426, which will allow c.w. operation at 50 kW of d. power and permit the peak dee voltage to be raised to 75 kV or more on each dee. A new and more rigid support structure, designed to take advantage of the newly discovered simple installation procedure, is being fabricated. Eventually, it is expected that precise control of the d. level will be required, and this will be done with the series regulator tube in the power supply, which is also a Machlett 6426. Acknowledgements The authors extend thanks for advice, help and encouragment from J. R. Richardson, B. T. Wright, D. Hollister, the cyclotron crew, and the physics machine shop and its late supervisor E. Griffiths. 5) K. R. MacKenzie, Rev. Sci. Instr. 22 (1951) 302.
THE UCLA CYCLOTRON RADIOFREQUENCY SYSTE)I
183
DlSCUSSIO:-i
Speaker addressed: K. R. l\IAcKENZlE (UCLA) Questioll by B. H. SmTH (LRL): When you say you have enough power for 50 or 75 k\V, that's under pulse conditions, isn't it? Answer: I neglected to mention that. Right now we are
pulsing with a small 5-k\V tube. We are going to install a 40-k\V tube, and the total installed power is about 80 k\V. We should be able to go appreciable higher than 50 kW. COllllllellt by J. R. RICItARDSOX (UCLA): Perhaps I should point out that these proposed modifications are included in the cost summary which I gave yesterday. (Laughter).
III. RADIO FREQUENCY SYSTE)IS
Part I II. Radio Frequency Systems 177-200 CHARACTERISTICS OF THE UCLA CYCLOTRON RADIOFREQUENCY SYSTEM K. R.
MACKE~ZIE,
S. PLUNKETT and E. L.
PETERSE~
Department 0/ Physics, University 0/ California, Los A ngeles Presented by K. R . Mackenzie An account is given of the various interdependent factors that must be considered when dees are placed in the valleys of a sectorfocuscdcyclotron.Thespiral shaped dees and r.f. resonator comprise a half wave structure in which an II % frequency range is achieved by varying the cro ss-section of the inner conductor near the node. A grounded grid oscillator i); coupled
to the resonator by loops and transmission lin es. The shape and weight distribution of the dees is such that they can be open on both sides, and yet can be easily supported at the outer radius. The distribution of heat allows the dees to be sawed from linch Cu plate with cooling supplied by conduction at the outer radius. They are installed by stretching the vacuum tank.
1. Introduction The 50 nrev Spiral Ridge Cyclotron at the University of California, Los Angeles, was designed around the existing 40 ton magnet of a partially constructed 50 MeV synchrocyclotron. The decision to convert it to a sector focused machine was made somewhat short of completion. With a conceptual design using 180° dees, a reduction in energy to around 30 nre\' was expected, and this was considered to be a reasonable exchange of energy for intensity. There was however, a natural reluctance to accept such a reduction in energy, and the "spiral dee in valley r.f. system" seemed to supply a solution which could make this possible"). In round numbers the synchrocyclotron was designed to give 50 nreV with a pole diameter of 4 It., a magnet gap of 4 in. and a field of 19500 gauss. In the conversion to a sector machine of the same energy, these average numbers were retained. The required flutter depends upon spiral angle and magnet gap. It was soon determined that at 50 nreV, the gap could never be made large enough to permit the use of 180° dees unless the voltage was in the neighborhood of a few kV (limited by sparking) . Such a solution was most unattractive because it would require extraordinary accuracy in shimming the magnetic field; the beam would be severely space charge limited; and there is the discouraging piece of data that synchrocyclotrons, which usually operate at about this voltage level, have not been able to realize resonant extraction efficiencies of more than a few percent. Therefore, the
possibility of placing the dees in the valleys was investigated. This decision had a very curious effect on design philosophy. Previous thinking had been concerned with accurately shaping the field in order to allow use of the lowest possible voltage gain per turn and hence permit a smaller gap. With the dees in the valleys, the minimum gap was independent of the dee structure. Hence it was made as small as possible in order to reduce the spiral angle 2 ) and produce a sharp field fall-off for deflection. The minimum value was based on an estimate of the amount of space needed by the internal beam. The other variables, such as valley angular width and flutter, were optimized to allow insertion of dees which would give the highest possible voltage gain per turn, and thus, relax the accuracy needed in shimming the field and ease the problem of extraction. Such an objective dictates that the 'valleys be made as wide as possible. This means the isochronous condition should be realized by reducing the gap rather than by flaring the ridges. Fortunately, for a given flutter, maximum focusing occurs with the valleys wider than the hills, and the optimum is rather broad. A choice was made of 34° hills and 56° valleys, which permitted the use of 45° dees with 5.5° side clearance to the hills"). The transit angle is estimated to average about 50°. The 1) S. H . Plunkett, E. Petersen and K . R. Mac Kenzie, Bull. Amer, Phys, Soc. 6 (1961) 520. 2) D. J. Clark, J. R. Richardson and B. T. " 'right, ~ucI. lnstr. and ",Icth. 18.19 (1962) I. III . RADIO FREQUENCY SYSTE)!S
178
K. R. )IACKENZIE
angular side clearance seems large, but at small radii it represents a minimal spacing. At larger radii, where the spiral angle approaches 45°, the azimuthal side clearance does not represent the true dee to ridge gap. At such radii the ions cross the dees at an angle and the radial component of ac-
et al.
out of phase on the fundamental so that adjacent dees are either in or out of phase. The third possibility yields a low gain per turn of 1.6 V o and is quite unattractive. Case (2) seems very complicated especially if variable frequency is desired. However, the engineering exists, since it is similar to a
Fig. I. View of r.f. liners in opposite valleys.
celeration causes an angular phase change which three phase design which was used on the Thomas cancels the apparent phase change due to the spiral. electron models and a 20 inch proton model of the Initially, a requirement for wide frequency range central region at Berkeley 3). If alternate valleys existed and in this respect, the small 45° dees, with are empty, two more possibilities can be considered: their small energy storage, were much superior to (4) dees in opposite valleys excited in phase at 180° dees. This requirement was soon relaxed. A double frequency, yielding a gain of 3V 0 per turn, decision was made not to introduce delays by de- and (5) dees in opposite valleys which oscillate out signing for variable energy at this time. . of phase at the fundamental frequency and yield a Several ways were considered for fitting such gain per turn of 1.8V o' dees into a resonating system. If all four valleys are Scheme number 5 was chosen. It is the simplest used, three possibilities exist: (1) dees in adjacent of all; appears to give sufficient energy gain per valleys, excited in opposite phase at double fre- turn; and leaves 1\\'0 empty valleys for probes, quency, yielding a gain per turn of 6V o ; (2) dees in magnetic channels, ion sources and other devices. adjacent valleys operating with a phase difference In the central region it provides ideal starting of 90° at the fundamental, with a net gain per turn 3) B. H. Smith and K. R. ~IacKenzie. Rev. Sci. Instr. 27 of 3.6V o ; (3) dees in opposite valleys which oscillate (1956) 485.
THE UCLA CYCLOTRON RADIOFREQUENCY SYSTE:lI
conditions. Almost from the beginning it was evident that the tips of the dees could be squared off to look like 1800 dees, so the energy gain could be as high as 4 V 0 for the first few turns. Ncar the source there is very little magnetic focusing and the fastertheionscan be pushed through this region the
179
dees in the valleys. The resemblance to ordinary dees has essentially disappeared, and for lack of a better name they are now called spiral dees . Fig. 3 is an elevation view which shows the internal dimensions of the dees, hills, and valleys. In the center of the machine, the vertical hill
Fig. 2. 45° dees in the valleys.
:~~~;.~~:c~~7:s~~c~:~~n~~t~:::~;~~ns~7p:c::~;~
that the IOns would become confused. After these = I - -~.=... --~~=-=-----= d:cisionswerem~detheinteractio~ofther.f. syst.em ~_ .~)\»\\\\mTIF with the magnetic field configuration was essential- .-~~ . 77~7~ 777r ly settled. The dee in valley design emerged as I / / having many advantages over the 180 system with o~.. f no evident fundamental limitations, such as sparking, vertical or radial defocusing, or need of ex//U///////////////// cessive r.f. power.There were unsolved construction and installation problems of course, which were ,= regarded as new and unusual, rather than difficult. 0
?Zt., 7Zl
2. Dee Design Fig. 1 shows a plan view ofthe ridges, or sectors, with copper lin ers in two of the valleys. Fig. 2 shows
tzL
"
fZL
7//0777////77777777
7
Fig. 3. Two elevation views of the dees and ridges showing spacings and clearances. III. RADIO FREQUEXCY SYSTE:llS
180
K. R. MACKENZIE
spacing is over two inches and hence it is possible for the dees to widen out to 180 0 in this region (see fig. 2). The transition to 45 0 is made rather abruptly in order to provide maximum clearance at the point where the dee crosses over the edges of the hills at small radii. A local reduction of hill width is ex-
et at.
think in terms of dees which are cooled in part by radiation, but primarily by conduction to the sup, porting member, or dee stem, (see fig. 4). One quarter inch hard drawn copper plate appeared to have both adequate heat conductivity and mechs, nical strength, and hence the dees were cut from
Fig. 4. Half wave structure and spiral dees. Note expandable hinged sections in the node region. The long slot through the
straight section allows insertion of probes.
changed for an increase in hill thickness in order to further increase this clearance (see figs. 2 and 3). Since these dees must be "open" on all sides except the outer radius, they must be structurally supported in cantilever fashion from outside the pole area. The weight distribution in the triangular shape is particularly helpful and the depth of the valleys is sufficient to provide some extra support if needed. Ther.f. currents flow radially on the tapered dee surfaces and decrease very rapidly toward the center. Practically all the heating occurs within a few inches of the outer radius. This permits one to
this material and simply bolted to the water cooled stems. In spite of their peculiar and humorous shape, these dees are probably the simplest ever constructed for a cyclotron of moderate size. With the contemplated power level of 50 kW it is estimated that the dees will never reach annealing temperatures, hence they should retain their shape and alignment. Beam scrapers in the unused valleys should prevent heating by ion impact except near the source, where radiation cooled slits or pullers made from refractory materials should be able to dissipate the bombardment energy.
THE UCLA CYCLOTRON RADIOFREQUENCY
3. Dee Support Structure One main problem was finding a convenient method of installation, since the dees will not pass through the one hill inch gap. Separately removable grounded dee stems on opposite sides were considered but were not favored for the following reasons: (1) both sides of the machine would be cluttered with d. structures; (2) they would have to be separately tuned; and (3) sweeping fields or a booster oscillator would have to be used, because the dees could not be biased. Ungrounded biased dees were heavily favored because the problem of getting rid of discharge loading and multipactoring is not a negligible one"). The simplest ungrounded structure is a half wave system, (see fig. 4) and it was chosen after determining that it was possible to construct it in upper and lower halves, which would separately pass through the hill gap. They were bolted together at the dee using special long handled wrenches. Since the r.f. current flows independently on the top and bottom dee stem surfaces, the connection inside the vacuum chamber is primarily mechanical and handles only the small unbalance current which is needed to keep upper and lower halves of the dee at the same potential. Somewhat later a much simpler solution was accidentally discovered. Because of the close proximity of the magnet coils to the gap, it is impossible to install the vacuum chamber as a unit, without raising the upper pole, yoke and coils. Since the lids of the vacuum chamber are held by long bolts through the poles, it is first necessary to unscrew these bolts before the raising operation. On one occasion this was forgotten, and it was discovered that the vacuum chamber lids would stretch as much as 2 in. without breaking the gasket seals around the edge. A fraction of this travel permits easy installation of the dees, so this is the method now in use. Fig. 5 shows a plan view of the dees and dee stems within the vacuum enclosures. Since the stems add capacity to ground, the current increases rapidly from the dee end of the stem to the edge of the vacuum chamber. Consequently, the stems were increased in width in order to keep the power 4) B. H. Smith, Nuc!. Instr, and Meth. 18, 19 (1962) 184.
SYSTE~r
181
Fig. 5. Plan view of resonator and schematic representation of oscillator transmission lines and coupling loops. Loops arc shown displaced horizontally for clarity but are actually placed vertically, one above the other as shown in fig. 6. Cylindrical insulators shown near the hinges are used for support. The flat structure is clamped between pairs of insulators at the points indicated. The resonator is skewed in order to keep the pump and fittings within the outside dimensions of the magnet.
Fig. 6. Elevation view of the external vacuum chamber, showing cross-section of the expandable inner conductor. a and b represent the coupling loops. The upper and lower hinged sections are drawn to shown the extremes of motion. In reality they move apart together. III. RADIO FREQUENCY
SYSTE~IS
182
K. R. :-IACKENZIE
density low. At the edge of the vacuum tank the stems take on a more or less "box" shape as shown in figs. 5 and 6. Fig. 6 shows a cross section of the external vacuum chamber which is made as large as consistent with the dimensions of the rest of the 2
....
0
"0 >
I
0
.:>:
2
.:
2
0' 0
Plate
Line 4
5
6
~
6
mete rs Tube
Cathode
~ 0
>
0
2 Tube
2.
s
4
Distance, mete r s
Fig. 7. Transmission line voltages for 20 kV on dees, plotted from tube to vacuum feed through bushings.
machine. This allows maximum surface area on the inner conductor. The chamber doubles as a pump manifold. Since wide frequency variation is no longer required, a modest range is accomplished by varying the cross section of the inner conductor as shown in fig. 6. The total change is 29 to 26 Mc/s which permits a larger energy variation than is possible with the magnetic field. This frequency control is useful in allowing the oscillator to be tuned to the optimum value of the field. In order to permit dee bias to be applied, the whole support structure is mounted, near its center of gravity, on insulators as shown in fig. 5. These are short alumina cylinders with a til wall thickness, 21" long and 3" in diameter. They run uncooled in the vacuum at about half dee voltage. The half wave structure consists of a water cooled copper skin over a dural frame, sawed from flat stock. The movable sections of the frame (see fig. 4) pivot at the widest portion of the stems to reduce the current density on the flexible copper hinges. Bias is applied through d. chokes at the node. 4. The Oscillator The resonator is excited through transmission
et al.
lines by a grounded grid oscillator similar to instalIations onformer f.m.rnachines"). The tube employed for the present pulsed operation is an Eimac 3W5000A with 5 kW plate dissipation. Fig. 5 shows a schematic representation of the coupling lines and the oscillator box. In reality, the two coupling loops are placed vertically, one above the other as shown in fig. 6. The separation discourages parasitic oscillation at the line frequencies. Fig. 7 shows the voltage distribution along the transmission lines. The oscillator is powered by a conventional threephase full wave rectifier unit using mercury vapor tubes with rough primary saturable reactor control. A series regulator tube, for fine control, is at present being used only for pulsing. 5. Performance The system is now in operation on a pulse basis to reduce radiation and activation during tune up, with approximately 50 kV on each dee to ground. The limitation is apparently sparking at the tips of the dees where the vertical clearance is a minimum of approximately one half inch. Future variations in the center region may increase this clearance and make it possible to hold more voltage r.f. power in the pulse is 30 kW. A new oscillator is planned, using a Machlett 6426, which will allow c.w. operation at 50 kW of d. power and permit the peak dee voltage to be raised to 75 kV or more on each dee. A new and more rigid support structure, designed to take advantage of the newly discovered simple installation procedure, is being fabricated. Eventually, it is expected that precise control of the d. level will be required, and this will be done with the series regulator tube in the power supply, which is also a Machlett 6426. Acknowledgements The authors extend thanks for advice, help and encouragment from J. R. Richardson, B. T. Wright, D. Hollister, the cyclotron crew, and the physics machine shop and its late supervisor E. Griffiths. 5) K. R. MacKenzie, Rev. Sci. Instr. 22 (1951) 302.
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Speaker addressed: K. R. l\IAcKENZlE (UCLA) Questioll by B. H. SmTH (LRL): When you say you have enough power for 50 or 75 k\V, that's under pulse conditions, isn't it? Answer: I neglected to mention that. Right now we are
pulsing with a small 5-k\V tube. We are going to install a 40-k\V tube, and the total installed power is about 80 k\V. We should be able to go appreciable higher than 50 kW. COllllllellt by J. R. RICItARDSOX (UCLA): Perhaps I should point out that these proposed modifications are included in the cost summary which I gave yesterday. (Laughter).
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