The beam handling system at the bonn isochronous cyclotron

NUCLEAR

INSTRUMENTS

AND

METHODS

I30

(I975) 335-346;

©

NORTH-HOLLAND

PUBLISHING

CO.

T H E BEAM H A N D L I N G S Y S T E M AT T H E B O N N I S O C H R O N O U S C Y C L O T R O N F. H I N T E R B E R G E R , H. G. E H R L I C H * , K. E U L E R , W. H E H E M E Y E R t , P. v. R O S S E N , B. S C H U L L E R a n d G. W E L P +

P. M E Y E R ,

Institut fiir Strahlen- und Kernphysik der Universitiit Bonn, 5300 Bonn, Germany Received 19 September 1975 The lay-out and the ion-optical structure o f the b e a m h a n d l i n g system at the B o n n I s o c h r o n o u s Cyclotron is described. T h e following b e a m preparation m o d e s are possible with two double m o n o c h r o m a t o r systems: (1) double dispersive with an extremely high m o m e n t u m resolution o f 30 000; (2) double dispersive with an adjustable dispersion m a t c h i n g with a magnetic spectro-

graph; (3) nondispersive, nearly isochronous, variable m o m e n t u m resolution up to 8000, adjustable time o f flight resolution below 0.5 ns; (4) a c h r o m a t i c with a transmission o f 100%. T h e practical experiences in operating the system a n d achieving the design performances are discussed. T h e results o f rigorous test measurements are given.

1. Introduction

momentum resolution and a sharp time structure can be adjusted simultaneously. It is also possible to achieve an achromatic beam transport with a transmission of 100%. Altogether the present system makes it possible to control the geometrical extent, the time structure and the momentum spread of the beam bunches. The lay-out of the complete system is presented in section 2. The ion-optical structure of the beam handling system is described in section 3. The special beam preparation modes of the symmetric and the antisymmetric double monochromator are discussed in sections 4 and 5 respectively. Important aspects of operating the system are discussed in section 6. Some outstanding results regarding the beam preparation are given in section 7. The specification of the ion-optical elements is compiled in the appendix.

The Bonn Isochronous Cyclotron × has been designed for the acceleration of protons, deuterons, 3He- and 4He-ions. The range of the final beam energy after the extraction is 7--15 MeV per nucleon corresponding to the radio frequency range 20--29 M H z of the acceleration system. The accelerator has been in operation since the dedication in 1970. In a series of articles the experiences and developments concerning external ion sources, axial injection, pulse suppression, acceleration modes, beam preparation, beam handling and technical installations shall be described. The present contribution deals with the beam handling system between the accelerator and the experimental areas. The purpose of this paper is (1) to present the design of the beam handling system, (2) to describe the special possibilities of beam preparation, (3) to report the practical experiences and (4) to sketch the beam quality achieved at the experimental areas. The emphasis lies on the ion-optical structures and the problems concerning a defined beam preparation. The subsystems which are most important to achieve a high beam quality are two double monochromator systems of opposite symmetry. The novelty of these double monochromator systems is the installation of six quadrupole lenses. This trick makes a very versatile beam preparation possible. In the double dispersive mode of operation an extremely high momentum resolution can be achieved despite the narrowness of the existing building. In the nondispersive mode a high * N o w at K e r n f o r s c h u n g s a n l a g e Jtilich (KFA), 517 Jfilich, Germany. t N o w at G y m n a s i u m Sankt A u g u s t i n , 5205 St. A u g u s t i n , Germany. + N o w at K r a f t w e r k U n i o n ( K W U ) , 852 Erlangen, G e r m a n y . x Construction: Allgemeine Elektrizit~its-Gesellschaft (AEG).

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2. Lay-out of the beam handling system and experimental areas

The lay-out of the beam handling system is shown schematically in fig. 1. The dipole magnets A0, A1, A2, A3 and A4 are represented by the pole cross sections. The quadrupole magnets are indicated by black rectangles. After the extraction out of the cyclotron the beam can be directed to 9 experimental areas. The experimental areas are designated by the numbers 1 to 8, the high current station is designated by H. The complete assembly is characterized by a very compact design. The cyclotron vault and the experimental areas are 7 m under ground. The walls of the building and of the shielding are also sketched in fig. 1. The high current station H serves for the production of radioactive sources. With the aid of the deflecting magnet A0 this terminal is easily separated from the main beam line. The experimental area 1 is destined for solid state

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investigations and correlated problems as observed via the channeling and blocking effect. The primary requirement of these experiments is a highly collimated parallel beam. A multipurpose 50 cm vacuum scattering chamber for scattering experiments with charged particles has been installed at the experimental area 2. For such experiments the beam momentum resolution should lie in the order of 5000. Then the total energy resolution is primarily determined by the surface barrier detectors. However, the monochromator system should not have a fixed but rather an adjustable energy resolution in order to meet the varying experimental requirements with respect to beam intensity and momentum resolution. The 45°-deflecting magnet A4 can be used as a "clean-up" magnet in experiments where charged reaction products must be separated under 0 ° and 180 ° from the direct beam. For such experiments the beam must be sharply confined so that the beam intensity of the halo around the beam is negligibly small. The scattering chamber of the experimental area 3 is provided for experiments with the polarized beam. As a speciality this scattering chamber can be rotated around the beam axis by an arbitrary angle. A polarized beam transport system should meet the requirement of a high transmission. Therefore besides a variable adjustment of the monochromator resolution also the achromatic beam transport with 100% transmission should be possible. Altogether a great variety of different beam preparations is needed at the experimental areas 1, 2 and 3. It will be shown that it is possible to obtain a momentum resolution in the order of 30000 in the double dispersive mode and a variable momentum resolution of less than 8000 in the nondispersive mode of the symmetric double monochromator system which feeds the "C-way". The achromatic and the nearly isochronous beam preparation can also be obtained. A split-pole magnetic spectrograph (Enge-type) ~) has been mounted at the experimental area 4. The full application of the magnetic spectrograph as energyloss spectrometer is achieved by the facility of matching the dispersion of the antisymmetric double monochromator system with that of the spectrograph. At target station 4a a 33 cm vacuum scattering chamber has been installed for high resolution studies of narrow resonances via excitation functions. For these measurements the momentum resolution of the preceding double monochromator system should be as high as possible. It will be shown that it is possible to obtain a momentum resolution of at least 30 000.

The target station 5 is provided for the production of intense neutron beams. A nondispersive beam transport with 100% transmission is possible between the accelerator and target station 5. At the experimental area 6 a multipurpose 50 cm vacuum scattering chamber has been installed which is identical to the scattering chamber at the experimental area 2. Besides charged particle reactions also particleinduced X-ray studies are performed at this station. The beam momentum resolution lies between 2000 and 8000 depending on the slit adjustment of the monochromator systeem between SXI and SX3 which is normally used for the beam preparation. The beam collimation for X-ray studies is achieved with the aid of the entrance slits SX1, SX2, SY1 and SY2 which are far away form the target area. The beam line from target station 6 downstream is free for movable experiments. The target area 7 is especially suited for particle-y reactions. A particular low background has been achieved by the additional concrete walls around the target area. The collimation of the beam is achieved with the slits SXI, SYI, SX2 and SY2 as in the case of target area 6. From the analyzing slit SX3 on the beam transport is diaphragm-free. Therefore a very low background can be achieved at the experimental area 7. An on-line j3-spectrometer of the orange type has been installed at target station 8. There a low background and well collimated beams are also needed. The beam momentum resolution lies between 2000 and 8000 just as in the case of the beam line leading to the experimental area 6.

3. The ion-optical structure of the beam handling system The complete beam handling system can be divided into 3 sections: (1) matching of the primary beam onto the entrance slits of the beam preparation system; (2) beam preparation; (3)beam transport from the beam preparation system and focusing onto the target. 3.1. THE MATCHINGSYSTEM The matching system consists of two quadrupole lenses Q1 and Q2, one switching magnet A0 and four steering units which are not shown in fig. 1. The primary beam which has to be matched onto the slit assemblies SXI, SYI and SX2, SY2 can roughly be characterized in the following manner: horizontally nearly parallel, vertically divergent with an effective source 1.7 m before the lens Q1. For the high resolution modes of the beam preparation system it is necessary

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to get a very sharp horizontal focus at the entrance slit SXI. Therefore the distance between the horizontally focusing quadrupole Q2 and the entrance slit SXI is kept very narrow. Altogether three different settings of the lens pair Q I-Q2 are used yielding the following modes: (a) a sharp horizontal focus (less than 1 ram) and a weak vertical focus (between 2 and 3 mm) at the slit assemblies SX1, SY1 ; (b) a weak horizontal focus (2-3 mm) and a weak vertical focus (about 4 mm) at the slit assembly SX2, SY2; (c) a parallel beam with a horizontal and vertical divergence of about _ 0.7 mrad. 3.2.

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tively. The horizontal plane is identical to the magnetic midplane of the system. One can distinguish two double monochromator systems with different symmetries. The S-way double monochromator begins at the entrance slit SX1 and ends at the exit slit SX1S. The arrangement of the elements with respect to the middle slit SX4 is antisymmetric. The C-way double monochromator begins at the entrance slit SX1 and ends at the exit slit SX1C. The arrangement of the elements with respect to the middle slit SX3 is symmetric. According to this scheme the following elements are identical within the framework of linear ion-optics:

T H E ION-OPTICS OF THE BEAM PREPARATION

Antisymmetric double monochromator (S-way):

SYSTEMS

The decisive part of the beam handling system is the monochromator system with respect to the preparation of definite beam properties. Beam preparation means the ion-optical transformation of the phase space distribution into special shapes with the aid of ionoptical elements and the limitation of the phase space distribution with the aid of slit assemblies. The complex of the beam preparation system is separately shown in fig. 2. Concerning the slit marking the coordinates X and Y are related to the extent perpendicular to the beam axis in the horizontal and vertical plane respec-

entrance slits SXI, SYl quadrupole Q3 quadrupole Q4 analyzing magnet A1 quadrupole Q5

-

- exit slits SX1S, SY1S; - quadrupole Q3S; - quadrupole Q2S; analyzing magnet A3; - quadrupole Q1S.

Symmetric double monochromator (C-way): entrance slits SX1, SYl quadrupole Q3 quadrupole Q4 analyzing magnet A1 quadrupole Q5

-

exit slits SX1C, SY1C; quadrupole Q2C; quadrupole Q 1C; analyzing magnet A2; quadrupole Q6.

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It should be mentioned that the quadrupole lens Q6 does not belong to the S-way double monochromator. Concerning the beam lines leading to the experimental areas 6 to 8 the first half of the C-way double monochromator is normally used for the beam preparation. It is also possible to use the first half of the S-way double monochromator. The speciality of the present beam preparation system is the installation of three quadrupole lenses in a simple monochromator system. As a consequence a great variety of optical modes can be adjusted by remote control of the strength as well as the polarity of the respective quadrupoles. The optical modes can be distinguished according to the projection of monoenergetic rays onto the horizontal and vertical plane along the beam axis. Considering these projections one can distinguish discrete modes which correspond to the following imaging from the entrance to the exit of a system: point-to-point, point-to-parallel, parallel-to-point, parallel-to-parallel. Altogether 16 discrete optical modes can be adjusted, that means 4 horizontal modes times 4 vertical modes. For a given discrete optical mode it is possible to vary the dispersion continuously within certain limits. This possibility is schematically illustrated in fig. 3. Among

the great variety of possible modes several modes have been selected which meet the different requirements of beam preparation. The selection of a certain mode was determined by the negligibility of overfocusing effects. The numerical calculations have been done with the computer program TRANSPORT2). Details of the ion-optical calculations can be found in ref. 3. The most important modes of the symmetric as well as the antisymmetric double monochromator will be sketched in sections 4 and 5. 3.3. FOCUSINGONTO THE TARGET The beam transport fi'om the exit slit of the respective beam preparation system onto the target area is achieved with the quadrupole lenses along the beam lines. The arrangement of these quadrupole lenses has been dictated by the necessity of conforming to the existing floor plan. For every target area a standardized ion-optical adjustment has been developed with the aim to avoid overfocusing effects as much as possible. Most of the experiments where angular distributions of charged reaction products are measured require a beam spot on the target of horizontally less than 1 mm and vertically less than 2 mm. This requirement can be easily fulfilled at the target areas 2, 3, 4 and 6 where such experiments are performed. At the target areas

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Fig. 2. The beam preparation system.

Fig. 3. Variation of the dispersion: (a) increased dispersion, (b) zero angular dispersion yielding an achromatic mode of the symmetric double monochromator, (c) zero position dispersion yielding an achromatic mode of the antisymmetric double monochromator.

BONN ISOCHRONOUS CYCLOTRON 1, 7 and 8 the beam can be focused onto spots with a horizontal and vertical extent of less than 1 mm. Such beam spots are especially suited for certain experiments where the beam must be brought through annular surface barrier detectors without touching circular diaphragms. At the target area 5 a beam spot with a diameter of about 10 mm can be produced. Under certain circumstances it can be advantageous to change the size of the beam spot on the target. This can easily be achieved at the target areas 2, 3 and 8 with an appropriate adjustment of the quadrupole lenses Q3C to Q6C, Q5C to Q9C and Q1D to Q5D respectively. To some extent it can also be achieved at the target areas 6 and 7 with the quadrupole lenses Q6 to Q2D and Q6 to Q2E respectively. However, the size of the beam spot at the target areas 1, 4 and 4a can only be varied by breaking the symmetry of the preceding double monochromator system. Then the quadrupole lenses Q1C to Q4C and Q2S to Q5S respectively are combined for the variation of the beam spot at the target areas 1, 4 and 4a.

4. Beam preparation modes of the symmetric C-way double monochromator 4.1. THE DOUBLEDISPERSIVEMODE In order to make full use of the symmetric double monochromator regarding momentum resolution the double dispersive mode must be adjusted where the horizontal mode exhibits a point-to-parallel imaging from the entrance slit SX1 to the middle slit SX3. Then one gets a point-to-point imaging from the entrance slit SX1 to the exit slit SX1C with a one-toone magnification. The momentum resolution depends on the quadrupole setting. This dependence is discussed in section 5. It is possible to obtain the very high momentum resolution of 30 000 with a slit width of 0.8 mm at SXI and SX1C. 4.2. THE NONDISPERSIVEMODE The symmetric C-way double monochromator is nondispersive if the horizontal imaging from the entrance slit SX1 to the middle slit SX3 is a point-topoint imaging. A m o n g the variety of such modes one standard mode has been chosen which exhibits smooth and narrow envelopes horizontally as well as vertically. This nondispersive double monochromator adjustment has the advantage that the momentum resolution can be controlled without destroying the time structure of the beam bunches. For a slit width of 0.80 mm at the entrance slit SX1 and 0.65ram at the middle slit SX3 one gets a momentum resolution of 6500 (FWHM).

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A weaker momentum resolution can be rapidly adjusted by remote control of the middle slit SX3. The transformation properties concerning the time structure are discussed with the aid of fig. 4. The path length of a trajectory depends on the starting conditions. The only relevant path length differences are caused by trajectories which start with different horizontal angles (full lines in fig. 4) and different momenta (dashed lines in fig. 4) with respect to the central trajectory. The path length differences due to a different horizontal starting position at the entrance slit SX1 can be neglected due to the extremely narrow slit width of less than 1 mm. It should be noted that the path length differences along the straight beam lines can also be neglected. The only relevant path length differences arise within the analyzing magnets. The great advantage of the nondispersive mode is the fact that the angle dependent path length difference of the first monochromator is compensated to zero by the opposite effect in the second monochromator. Particles which follow an outer trajectory in the first monochromator cross the central trajectory at the middle slit SX3 and follow an inner trajectory in the second monochromator and vice versa (see full trajectories in fig. 4). The momentum dependent path length differences mainly arise in the second monochromator (see dashed trajectories in fig. 4) yielding a negative correlation between the time of flight and the momentum deviation. The great advantage of this negative correlation is the

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fact that it compensates to some degree the corresponding positive correlation between the time of flight and the velocity which occurs during the transfer from the accelerator to the target. In other words particles with higher velocities are bent on a longer path length than particles with lower velocities. The standard nondispersive mode of the C-way double monochromator compensates the spreading effect which arises along a flight path of 21 m, that is the distance between the entrance slit SX 1 and the target area 2. According to the same scheme the angle and momentum dependent time of flight differences are also compensated to some degree at the beam line leading to the experimental area 7 due to the 45 ° bending of the second magnet A2. 4.3. THE ACHROMATIC MODE The achromatic mode is a special nondispersive adjustment with a vanishing spatial and angular dispersion. It is especially suited for the transfer of beams with a high momentum spread. The achromatic mode can be adjusted with a horizontal point-to-point imaging between the entrance- and the middle slit. Due to the spatial dispersion at the middle slit the momentum acceptance can precisely be adjusted as in the nondispersive mode [see fig. 3, case (b)]. 5. Beam preparation modes of the antisymmetric S-way double monochromator

5. l. VERY HIGH MOMENTUMRESOLUTIONMODE The antisymmetric combination of two monochromator systems with a point-to-point imaging in the magnetic midplane yields a double dispersive system. The ultimate momentum resolution depends on the quadrupole setting. It can easily be shown 2) that for a given horizontal divergence the first-order

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momentum resolution is the greater the greater the magnetic flux enclosed by the corresponding horizontal envelope in the region of the analyzing magnet. According to this rule the momentum resolution increases with the linear dimensions of the system. However, the present system had to be very compact especially in the region between the entrance slit SXI and the analyzing magnet A1 (see fig. 1). In order to achieve a high momentum resolution and a high compactness at the same time the quadrupole lenses are used to produce large horizontal evelopes in the region of the analyzing magnets. A very high resolution mode is obtained if all six quadrupoles of the antisymmetric double monochromator are horizontally defocusing. The strong pole face rotation of the analyzing magnets increase the horizontal defocusing. These effects are illustrated in fig. 5. The envelope marked by full lines starts at SXI with a horizontal divergence of _+ 10 mrad. This starting condition has been assumed during the design. In practice the horizontal divergence is limited to a value of _+ 1.5 mrad with the aid of the horizontal slit SX2. The reason that this limitation is possible without loss of beam intensity is discussed in section 7. During the short distance between the entrance slit SXI and the first analyzing magnet the horizontal envelope experiences three strong defocusing actions yielding a very enlarged horizontal envelope in the region of the analyzing magnet. The point-to-point imaging between the entrance and the middle slit assemblies has a horizontal magnification of - 1 . 0 and a vertical magnification of +1.9 with a vertical cross-over in the analyzing magnet. The strong vertical overfocusing is attenuated by matching the primary beam so that the vertical waist lies gehind the entrance slit SYI in the region of the second slit SY2. The firstorder momentum resolution of the half-system is 15 000 (fwhm) for a horizontal width of 0.8 m m of the entrance- and the middle slit. The antisymmetric combination yields a first-order momentum resolution of 30 000 (fwhm). In order to minimize the second order aberrations which affect the momentum resolution the pole faces of the analyzing magnets have slight convex curvatures (see specifications in the appendix). A serious secondorder problem of an analyzing system is the discrepancy between the magnetic field distribution assumed during the design and the magnetic field distribution measured after the construction of the magnets. In the case of the present system a satisfactory solution of this problem has been found which will be described in sections 6 and 7.

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5.2.

DISPERSION

MATCHING

OF

THE

S-WAY

DOUBLE

MONOCHROMATOR

In order to make full use of a magnetic spectrograph 4) it is necessary to match the dispersion of the monochromator with the dispersion of the spectrograph. Then the combination monochromator-spectrograph represents an energy-loss spectrometer. This condition is achieved if rays of different momentum are dispersed on the target so that the corresponding rays of the reaction products go to just the same focus on the focal plane of the spectrograph. The dispersion which is needed for this matching depends on the kinematics of the reaction, the scattering angle, the rotation of the target and the location along the focal plane. Therefore a great range of possible dispersion adjustments is required. A continuous variation of the dispersion can easily be achieved with the aid of the quadrupole lenses of the antisymmetric double monochromator. The optical mode from the entrance- to the middle slit is horizontally a point-to-point imaging and vertically a parallel-to-point imaging. The speciality of the point-to-point imgaging from the entrance slit assembly SX1, SY1 to the target is the fact that the horizontal and vertical magnification is practically not affected by the variation of the dispersion. The horizontal magnification is 0.546 +0.002, the vertical magnification is 1.28 +0.04. The dispersion on the target can be varied between 1.5 mm < Ax/(lOOOAp/p) <_ 15 ram. If the dispersion of the beam preparation system is correctly matched a sharp image of the horizontal slit SXI is formed in the focal plane of the spectrograph which is independent of the beam momentum spread. With the aid of the middle slit SX4 one can still control the accepted momentum spread in order to avoid nonlinear effects~ The ultimate resolution is given by the width of the horizontal slit SXl. For instance a line width of the elastic and inelastic scattering of 0.14 mm can be achieved with an SX1 slit width of 0.8 ram. The corresponding momentum resolution is p/Ap -- 10 000 (fwhm) on the high momentum side of the broad range spectrograph. 5.3. THE NONDISPERS1VEAND ACHROMATICMODE The nondispersive mode of the antisymmetric double monochromator can only be achieved with a horizontal point-to-parallel imaging between the entrance- and the middle slit. The achromatic mode can also be adjusted with a nondispersive mode between the entrance- and the middle slit [see fig. 3, case (c)]. In both cases a defined momentum selection at the middle slit is not possible in opposition to the nondispersive and achro-

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matic modes of the symmetric C-way double monochromator.

6. Operation 6.1.

SETTING OF THE DIPOLE AND QUADRUPOLE MAGNETS

After the selection of a certain mode of beam preparation the actual adjustment of the beam handling system only depends on the magnetic rigidity of the beam. The adjustment of the dipole and quadrupole magnets is defined by the energizing current. The magnetic field of the analyzing magnets is additionally measured with the aid of N M R probes yielding a precision of better than 5 x 10 - 6 . The stability of the power supplies is 1 × 10 -4 for the quadrupole magnets and 5 x 10-6 for the analyzing magnets. In order to achieve a precise realization of the calculated ion-optical modes setting tables of the quadrupole adjustments as a function of the magnetic rigidity, i.e. particle energy and particle kind, have been calculated. Especially the strength of the quadrupoles in the monochromator system must be adjusted as precisely as possible. For instance in order to achieve the very high momentum resolution the uncertainty of the quadrupole setting must be less than 1%. The quadrupole setting tables have been generated with a computer program taking into account the following input information. The effective length and the field gradient of every quadrupole type have been measured with an uncertainty of + 5 x ] 0 - 3 (ref. 5). The field gradient-current calibration curve has been fitted with a third-order polynomial. The potentiometer-current calibration curve of the power supplies has also been parametrized with the same precision. The measured field inhomogeneity of the analyzing magnets has been fitted by a second-order polynomial after dividing each magnet into 11 sectors. The fringing field distribution of the analyzing magnets has also been measured and parametrized6). Using the measured effective lengths of the quadrupole magnets and the parametrized field distributions of the analyzing magnets the ion-optical modes have been recalculated with the computer program T R A N S P O R T 2) in order to obtain the correct quadrupole adjustments. A special advantage of this recalculation is the fact that the first-order effects caused by the measured inhomogeneity of the analyzing magnets are corrected by slightly modified quadrupole strengths. The output of the quadrupole setting table is organized in such a manner that the quadrupole settings of a complete

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beam line are given as a function of the particle energy (and particle kind). The step size is less than 4%0. 6.2. REPRODUCIBILITYOF THE DIPOLE AND QUADRUPOLE ADJUSTMENT In order to get reproducible adjustments of the dipole and quadrupole magnets a very simple standardized program has been developed which has also been observed during the magnetic field measurements. The current adjustment starts with the maximum current of the power supply. The overshoot is 30% for beams with the maximum magnetic rigidity of 11.2 k G . m . The overshoot time is at least 2rain in the case of the dipole magnets and 20 s in the case of the quadrupole magnets. In order to achieve a quick and trouble-free adjustment every power supply has been equipped with a switch which allows the selection of either a tunable or a fixed potentiometer as current reference. The fixed potentiometer yields the maximum current of the power supply. Before and during the overshoot time the operator can precisely adjust the tunable potentiometer according to the setting table. After the overshoot time the current reference of the power supply is switched from the fixed to the tunable potentiometer yielding the preselected current within the regulation time of the power supplies which is less than 2 s. The quadrupole lenses are immediately ready for operation. The asymptotic field value of the dipole magnets is reached after 5 min. The reproducibility of this procedure is better than I × 10 -4 in the case of the quadrupole magnets and 1 x l0 -5 in the case of the dipole magnets. The standardized switch-on procedure in connection with the quadrupole setting tables makes a rapid adjustment of the complete beam handling system within 5-10 rain possible. 6.3. MATCHING AND COLLIMATION In order to match the primary beam onto the monochromator system the horizontal and vertical phase ellipses have been measured before installing the beam handling system7). These emittance measurements revealed a high beam quality of the extracted beam. At deuteron currents up to 5/xA the measured emittance is less than 3~z m m - m r a d horizontally as well as vertically. Horizontally the beam is nearly parallel with a weak horizontal waist 2.7 m outside of the cyclotron and an horizontal divergence of ± l . 0 mrad. Vertically the beam is divergent with an effective source 1.7 m before the quadrupole Q1 and a divergence of ± 2.3 mrad. Using this information the setting of the

quadrupole lenses Q I and Q2 has been calculated which is appropriate for the various beam preparation modes. The normal mode of operation is characterized by a horizontal waist of less than 1 mm at the horizontal slit SX1 and a vertical waist of less than 2 mm at the vertical slit SY1. The geometrical acceptance is defined by the setting of the horizontal slits SXI and SX2 and the vertical slits SYI and SY2. The acceptance is not elliptical. The slit setting plays an important role with respect to the preparation of a defined momentum resolution and a defined geometrical collimation of the beam. Most of the experiments require a defined collimation of the beam. This collimation is easily achieved with the aid of the slits SXI, SX2, SY1 and SY2 at the entrance of the beam preparation system. Since these defining slits are located in the cyclotron vault, the target areas are well shielded from the produced neutron- and ?-radiation. The slit setting is performed by remote control. The procedure to use the slit assemblies for the beam collimation has the advantage that a rapid adjustment of every wanted collimation is possible. 6.4. BEAM STEERING

Besides matching the relative phase space distribution of the primary beam the absolute direction of the beam axis should be fixed independent of accelerator and quadrupole focusing conditions. The beam can be horizontally and vertically deflected and translated with the aid of four steering units which are located between the accelerator and the beam preparation system. The steering units are not shown in fig. I. Due to the restricted space the vertical steering units are combined with the quadrupole magnets Q1 and Q2. Concerning the horizontal steering a short but powerful horizontal steering magnet has been installed between the exit port of the cyclotron and the quadrupole lens QI. The second horizontal steering unit is combined with the deflecting magnet A0. The beam axis is defined by the positionning of the slit assemblies SXI, ST2, SYI and SY2. The central position of these slit assemblies corresponds to the central axis of the ion-optical system. After a rough adjustment of the beam axis with t!~e deflecting magnet A0 the beam axis is precisely regulated onto the central axis of the beam preparation system with the aid of an electronic feedback system. The beam intensity impinging on the slit edges is used to adjust the steering magnets so that the difference of opposite slit edge currents is zero. Besides the advantage of a rapid operation variations of the primary beam direction due

BONN ISOCHRONOUS

to time instabilities of the cyclotron or changes of the accelerator mode are automatically compensated due to the fast response of the electronic feedback system. The transport of the beam through the beam preparation system is free from steering magnets. The beam axis is defined by the alignment of the elements and the adjustment of the analyzing magnets. The fine adjustment of the beam position on the target can be performed with a horizontal and vertical steering unit which has been installed just before every target area. 6.5. BEAM DIAGNOSTICSAND BEAM CONTROL Phase ellipse measurements for the beam development are performed with the horizontal slits SX1 and SX2 and the vertical slits SY1 and SY2 using the double slit method. The slit width and the slit position can be independently adjusted by remote control. The precision of this adjustment is _+0.01 ram. The transmitted beam is measured with the stopper ST1G (see fig. 1). The momentum distribution of the primary beam is measured with the aid of either the horizontal slit SX3 or the horizontal slit SX4 using the dispersion of the first half of the symmetric or the antisymmetric double monochromator. In the region of the beam preparation system the beam is horizontally and vertically controlled with the aid of the slit assemblies. No other beam diagnostics are used in the normal mode of operation although phosphorescent beam viewers are provided. The adjustment of the analyzing magnets is supervised by measuring the intensity impinging on the slit edges and the transmitted intensity. The horizontal slits at the end of the double monochromator systems may be used to remove the low energy tail of particles which are scattered at the adges of the middle slit. A clean-up function is achieved if the beam passes through the exit slits without striking the slit edges. Then unwanted particles accompanying the main beam as a halo are extremely suppressed. The adjustment of the beam spot size and position on the target is optically controlled with a phosphorescent beam viewer using a closed circuit television system. 7. Results

The main parts of the beam handling system have been operated since 1971. During this time the different beam preparation modes were tested and slight modifications of the initial design have been established in order to improve the beam handling. Due to the practical experience a quick and reliable beam handling has been

343

CYCLOTRON

developed s). The whole system is now in full operation. The most critical point of the beam preparation is the installation of six quadrupole lenses in a double monochromator system. This conception has been applied for the first time with the aim to obtain a great flexibility as well as a very high momentum resolution. The easy handling of such a complex system is only possible with the aid of precise setting tables. Therefore the ion-optical parameters of the dipole and quadrupole magnets have carefully been measured after the fabrication. The most interesting and critical test of whole the assembly was the experimental determination of the momentum resolution. 7.1. MOMENTUMRESOLUTION To this end we measured the very sharp 12C+p resonance at 14.231 MeV several times. This famous resonance has also been used at other laboratories 9) in order to test the final momentum resolution. In the present case the quadrupole lenses of the antisymmetric double monochromator system were adjusted to yield a m o m e n t u m resolution between 26 600 and 40 000 depending on the width of the object and analyzing slits which were varied between 0.6 and 0.9 ram. The horizontal divergence has been limited to + 1.5 mrad with the slit SX2 in order to reduce the second-order aberrations which are caused by the measured field inhomogeneity of the analyzing magnets. This strong limitation of the horizontal acceptance is possible without loss of intensity due to the fact that the primary

12C(p,p)12C (~..AB168 4000

0

2000

3OOO4

'

i# ~

14.230

lZ,.i40

EL~,B (MeV) Fig. 6. The measured excitation function o f the 1 2 C + p resonance at 14.231 MeV. The energy variation was achieved by applying an electrostatic potential on the target which was automatically varied in discrete steps. The resonance excursion has a m a x i m u m - t o - m i n i m u m ratio of 2.3.

344

F. H I N T E R B E R G E R

beam is characterized by a very narrow phase ellipsoid of especially high beam intensity. This "hot core" has small extensions, not only with respect to the momentum deviation and the pulse length but also with respect to the horizontal phase ellipse. The typical proton beam currents were 30 nA at a momentum resolution of 30 000. The primary beam currents at the entrance of the double monochromator were typically 2/aA. The differential excitation function measured at extreme backward angles always showed a resonance excursion with a maximum to minimum ratio between 2.1 and 2.3 (see fig. 6) which was never reached before. The resonance excursion was very insensitive against a variation of the beam momentum resolution. Therefore we have measured the momentum spectrum of the analyzed beam directly using the split-pole magnetic spectrograph at target area 4. We obtained an increased spectrograph analyzing power of p/Ap > 100 000 using a special slit detection system with a position resolution of 10/am. For this test measurement the ion-optics and the analyzing slits have been adjusted to yield a momentum resolution p/Ap = 29 400___ 1600 (fwhm). The uncertainty corresponds to the uncertainty of the absolute width of the analyzing slits. Details of this measurement will be published elsewherel°). The measured beam spectrum exhibited a triangular shape with a momentum resolution p/Ap=28800-+lO00 (fwhm) in agreement with the ion-optical value. This agreement is a sensitive confirmation of the ion-optical parameters, the calibration curves and the expectation based on the ion-optical calculations. Using the experimentally determined beam momentum resolution which corresponds to an energy width of 0.91 keV in the c.m.-system l Z C + p we started an analysis of the resonance which revealed that (l) a Doppler broadening due to the lattice vibrations of the ~2C target nuclei in the order of 0.81 keV must be taken into account and (2) the natural width of the resonance is not 0.82 keV as previously assumed but equals (1.1 _+0.09) keV (ref. 1 l). With the broad range magnetic spectrograph an overall momentum resolution of 4200 has been achieved 12) using the dispersion matching of the double monochromator system. The momentum resolution of the beam has been varied between 5000 and 2000 without loss of the overall resolution. 7.2. TIME RESOLUTION The time transformation properties of the beam preparation system have not yet been tested thoroughly. The time resolution which has been achieved in pulsed

et al.

beam experiments at target area 7 has been mainly determined by the detector (ref. 13). The measured overall time spread ranged from 1 ns to 5 ns. At target area 2 an overall time resolution of 340 ps (fwhm) has been achieved during test measurements of the time structure of the primary beam. Thereby the isochronous mode of the symmetric C-way double monochromator has been used without limitation of the angle- and momentum spread of the primary beam. From these measurements it can be concluded that the preparation of a very high time resolution can be achieved with the present system. Using the unique correlation between time- and momentum spread which is automatically given by the if-acceleration of an isochronous cyclotron with single turn extraction it should be possible to prepare beam bursts of less than I00 ps by cutting out unwanted parts with the monochromator slit SX3. 7.3.

INTENSITY TRANSMISSION

Using the versatile nondispersive modes of the C-way a very simple and quick variation of beam intensity versus momentum resolution can be achieved with the monochromator slit SX3 without destroying the sharp focus at the exit of the double monochromator and the target. A variation of the momentum resolution from 5000 to 1000 yields an increase of the horizontal beam extent at the exit slit from 1.0 to 1.2 mm. The transmission factor that is the ratio between analyzed and primary beam depends on the momentum spread of the primary beam which ranges between 1/2000 and 1/500 (fwhm). Typically a transmission factor of 0.2 can be achieved at a monochromator momentum resolution of 4000 (fwhm) and primary beam currents up to 5 #A. If a transmission of 100% is required the achromatic mode is preferable. Both the symmetric and the antisymmetric double monochromator system have successfully been operated in the achromatic mode yielding a transmission of up to 100%. 8. Conclusion

The test results as well as the accomplished and the running experiments demonstrate that a versatile beam preparation can be achieved with the beam handling system of the Bonn lsochronous Cyclotron. The installation of six quadrupole lenses in a double monochromator makes the adjustment of a great variety of double dispersive, nondispersive and achromatic modes possible. A set of standardized ionoptical modes has been found which are especially suited to meet the experimental requirements. It is

BONN ISOCHRONOUS

possible to achieve an extremely high momentum :cesolution in the order of 30 000. On the other hand it iis possible to achieve a sharp time structure and a high momentum resolution simultaneously. The dispersion matching with a magnetic spectrograph can be freely .adjusted according to experimental requirements. The intensity transmission which is coupled with the momentum spread of the primary beam and the momentum resolution of the monochromator can be arbitrarily adjusted according to experimental requirements without destroying the sharp focus at the target. The achromatic beam transport with a transmission of 100% is possible along the symmetric and the antisymmetric double monochromator. The problem of a rapid, precise and reproducible adjustment of the ion-optics has been solved with the aid of setting tables which have been calculated after the precise determination of the ion-optical parameters and the calibration curves. The authors thank Prof. T. Mayer-Kuckuk for initiating this work and for his continuous interest. They wish to acknowledge the fruitful cooperation with the collaborators of the Allgemeine Elektrizit~its Gesellschaft (AEG). They want to appreciate the extraordinary designs and constructions of H.D. Rosendaal. The important contributions made by M. Agena, Dr O. Beer and J. Ollhoff and the staffs of the accelerator technician and electrical shops are gratefully acknowledged. Helpful discussions with Prof. J. Ernst, J. Reich and Dr H. Wahl and the support of Prof. C. Mayer-B6ricke are gratefully mentioned. The authors are much indebted to Dr K. L. Brown and his collaborators for making available the computer program TRANSPORT2). The numerical calculations have been done with the IBM computers of the Kernforschungsanlage Jfilich and the Gesellschaft ffir Mathematik und Datenverarbeitung in Bonn. Appendix

Specification of the ion-optical elements. 1) Analyzing magnets: A0 - homogeneous field< 12kG, pole gap = 4 cm, pole width > 36 cm; AI - e n t r a n c e pole-face r o t a t i o n = 5 0 °, curvature =0.585 m - l ; bending angle = 90 °, radius = 1.25 m ; exit pole-face rotation = 1.2°, no curvature; A2 - entrance pole-face rotation = 1.2°, no curvature;

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345

bending angle = 90 °, radius = 1.25 m; exit pole-face rotation = 5 0 °, curvature = 0.588 na -1 ; bending angle = 45 °, radius = 1.966 m; exit pole-face rotation = 50 °, no curvature; bending angle = 22.5 °, radius = 3.562 m; exit pole-face rotation = 27.5 °, no curvature; A3-

entrance pole-face rotation = 1.2°, curvature = 0.455 m -1 ; bending angle = 90 °, radius = 1.25 m; exit pole-face rotation = 5 0 °, curvature = 0.455 m - 1.

2) Deflecting magnets: A0 - homogeneous field < 12 kG, pole gap = 4 cm, diameter of the effective field boundary = 26.4 cm. A4-

homogeneous field < 19.8kG, pole gap = 4 c m , diameter of the effective field boundary = 47.5 cm.

3) Quadrupole magnets: Q1, Q2, Q3C, Q4C, Q4S, Q5S, Q1E, Q2E -effective length = 2 2 . 6 4 c m , aperture radius = 2.525 cm, field gradient <2.0 kG/cm; 03, Q2C, O3S l e n g t h = 2 2 . 7 6 c m , aperture radius = 2.525 cm, field gradient < 2.2 kG/cm;

-effective

Q4, Q 1C, QZS -effective l e n g t h = 2 4 . 3 0 c m , aperture radius = 4.025 cm, field gradient < 0.54 kG/cm. Q5, Q6, Q5C, Q6C, Q1S, QI D, Q2D length =25.0 cm, aperture radius = 4.025 cm, field gradient < 1.6 kG/cm.

- effective

Q3D, Q5D, Q7C, Q9C length = 9.9 cm, aperture radius = 3.18 cm, field gradient < 1.6 kG/cm;

- effective

Q4D, Q8C l e n g t h = 1 8 . 5 4 c m , aperture = 3.18 cm, field gradient < 1.6 kG/cm

-effective

radius

References

1) j. E. Spencer and H. A. Enge, Nucl. Instr. and Meth. 49 (1967) 181. 2) K. L. Brown and S. K. Howry, SLAC Report 91 (Stanford, 1970). 3) F. Hinterberger, ISKP Bericht 1970/I (Bonn, 1970).

346

F. H t N T E R B E R G E R et al.

4) B. L. Cohen, Rev. Sci. Instr. 33 (1962) 85; B. L. Cohen, Rev. Sci. Instr. 30 (1959) 415. 5) H. G. Ehrlich, Diplom thesis (Bonn, 1971). 6) G. Welp, Diplom thesis (Bonn, 1971). 7) p. v. Rossen, Diplom thesis (Bonn 1971). s) F. Hinterberger, ISKP Bericht 1973/1 (Bonn, 1973). 9) M. J. Le Vine and P. D. Parker, Phys. Rev. 186 (1969) 1021 ; G. M. Temmer, B. Teitelmann, R. Van Bree and H. Ogata, Proc. Intern. Conf. on Nuclear structure, Tokyo (1967) p. 318; R. E. Hintz, F. B. Selph, W. S. Flood, B. G. Harvey, F. G. Resmini and E. A. McClatchie, Nucl. Instr. and Meth. 72 (1969) 61;

io) il) 12) i3)

W. C. Parkinson and J. Bardwick, Nucl. Instr. and Meth. 78 (1970) 245; E. Huenges, H. ROsler and H. Vonach, Phys. kett. 46B (1973) 36l; J. D. Goss, C. P. Browne, A. A. Rollefson and P. L. Jolivette, Phys. Rev. C 11 (1975) 710. F. Hinterberger, P. v. Rossen, R. Jahn and B. Schfiller, Nucl. Instr. and Meth. 130 (1975) 347. F. Hinterberger, P. v. Rossen, H. G. Ehrlich, B. Schfiller, R. Jahn, J. Bisping and G. Welp, Nucl. Phys. (in press). R. Rieger, private communication. F. Blumenberg, private communication.