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A novel beam-profile monitor for storage rings
Nuclear Instruments and Methods m Physics Research A 333 (1993) 288-293 North-Holland
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
A novel beam-profile monitor for storage rings T. Quinteros, R. Schuch, M. Pajek ', P. Sigray, H. Cederquist, H. Danared, L. Bagge, A. Filevich 2, J . Jeansson, A. Källberg and A. Paâl Manne Siegbahn Institute of Physics, S-104 05 Stockholm, Sweden
Received 29 March 1993 We have developed a novel, nondestructive, beam-profile monitor for ion storage rings. The horizontal profile of the stored ton beam is determined by detecting electrons from the ionization of the residual gas with a two-dimensional position sensitive detector . The strong magnetic field, provided by one of the bending magnets, conserves the information on the initial horizontal positions of ionized electrons during their transport to the detector by means of a vertical electric field. The monitor is tested at the storage ring CRYRING in Stockholm for a 6 MeV/amu D + beam . We show that in this case, where the lifetime of the stored beam is restricted by single scattering on the residual gas, a reduction of the lifetime occurs due to the reduced angular acceptance . 1. Introduction Recently, newly built ion-storage rings have been put into operation in, e.g., Aarhus (ASTRID), Heidelberg (TSR), Darmstadt (ESR), Bloomington (IUCF), Tokyo (Tarn II), Rilich (COSY), Uppsala (Celsius), and Stockholm (CRYRING). The main goals for the construction of such facilities are the storing and phase space cooling of energetic (multi-MeV) ion beams for experiments in atomic, molecular and nuclear physics. Some of these experiments require a high number of beam particles with low momentum spread passing a target per unit time or in some situations, the interactions between stored ions and a target or a laser beam could be observed over substantial periods of time . Some examples of typical ion-storage ring experiments are the measurements of dielectronic and laser-induced recombination at TSR in Heidelberg [1], the measurements of lifetimes of metastable negative ions in Aarhus [2], the studies of bound ß-decay at ESR in Darmstadt [3] and the experiments on dissociative recombination of vibrationally relaxed molecules at CRYRING in Stockholm [4]. It is of course necessary that the information on the beam properties can be retrieved continuously throughout the observation time . Electrostatic pick-ups can be used to give information about the mean position of the beam, while current t Permanent address: Institute of Physics, Pedagogical University, 25-509 Kielce, Poland . z Permanent address: Comisi6n Nacional de Energia At6mica, Laboratorio Tandar, Av . Libertador 8250, 1429 Buenos Aires, Argentina.
transformers give information on the stored beam intensity. The revolution frequency, and thereby the beam velocity, is usually monitored by means of Schottky noise detectors. In quite a few experimental situations it is in addition necessary to gain information about the spatial intensity distributions within the beam envelope . This is sometimes the case in studies of interactions of two stored beams and for some experiments involving laser beams. Beam-profile monitors have now been in operation for a few years at, e.g . TSR [5], KEK-PS at Tokyo [6] or ISR at CERN [7]. The two first monitors detect the recoil ions produced through the ionization of the residual gas that takes place when the stored ions pass by a two-dimensional position-sensitive detector . The monitor at CERN detects the electrons produced in the ionization of atoms from a gas jet crossing the proton beam . Here, we report on experimental tests performed with a new type of beam-profile monitor which is inserted in the strong magnetic field of one of the bending magnets in CRYRING. At present we have been detecting one of the products of the ionization of the residual gas only (e .g . the electrons) to obtain the horizontal beam profile. Within the magnetic field, electrons and recoil ions are transported to opposite channelplate detectors. In the very near future, this monitor will be able to give information about the vertical and the horizontal beam-profiles simultaneously. A projection of the ionized electrons on the position sensitive anode of one of these detectors gives the horizontal beam profile. The vertical beam-profile can be obtained by measuring the time difference between the arrival of recoil ions and ion-
0168-9002/93/$06.00 C 1993 - Elsevier Science Publishers B.V . All rights reserved
T. Quinteros et al. / Beam-profile monitor ized electrons transported electrostatically to two different detectors, mounted above and below the circulating beam . The purpose of the magnetic field, which is parallel with the electric field, is threefold : (i) it assures a high collection efficiency of emitted electrons, (ii) it provides a high resolution by forcing the electrons to spiral closely around the magnetic field lines and (iii) it keeps electrons produced outside the volume probed by the monitor from reaching the detector . The confinement of the electrons to very small cyclotron orbits around the magnetic field lines may provide an opportunity to measure the spatial properties of stored beams with unprecedented resolution . Below, we will show results from test runs with the prototype of our beam-profile monitor . These experiments, were performed at the heavy ions storage ring CRYRING at the Manne Siegbahn Institute of Physics in Stockholm . We have measured the horizontal profile of _ 10 7 D+ ions stored at the energy 6 MeV/amu before and after cooling of the beam . We will, further, show measurements of storage times with and without the beam-profile monitor in operation . First, however,
289
we describe shortly the essential properties of CRYRING and give an account of the detector design .
2 . Experimental technique CRYRING is a low energy synchrotron and storage ring [8] for ions and molecules over a wide range of q/m ratios . Fig . 1 shows a schematic of the machine and the locations of its main components. Highly charged ions can be produced in an electron-beam ion source (not shown in the figure), while light ions and molecules of moderate charge are produced by a conventional Penning-type ion source (MINIS) . A radiofrequency quadrupole accelerator (RFQ) accelerates either one of these beams to an energy of 300 keV/amu if their charge-to-mass ratios exceed 0 .25 [9] . The ions are then injected into the ring by means of an electrostatic inflector [10] . A nonresonant drift tube [I1] accelerates the stored ions to their final energy, while the electron cooler is used to decrease the longitudinal and transversal momentum spreads of the stored ion beam [12]. The beam-profile monitor is mounted on a mo-
beam-profile monitor Fig . 1 . Layout of CRYRING .
29 0
T Quinteros et al. / Beam-profile monitor
IV41FIIZZIIIIIIIIIIII1111111zllllllllllllll1111 Mci~ ______________
-
end
Fig. 2 . Schematics of the beam-profile monitor. The stored beam ionizes the residual gas atoms. The ionized electrons are driven towards the MCP detector by an electric field in helicoidal trajectories about the magnetic field lines. tion-feedthrough with an ultrahigh vacuum (UHV) lock, which allows the detector to be inserted in and retracted from the ring-vacuum volume without breaking
the vacuum . It is positioned in the dipole magnet following directly after the electron cooler (cf. fig. 1) . The test device in its present form, in which one of the two detectors to be used in the final arrangement was replaced by a potential plate, is shown in fig. 2. It measures the horizontal profile of the stored beam by detecting the electrons generated through ionization of residual gas molecules. The multichannel plate detector assembly (MCP) has 40 mm diameter active area, is equipped with a two-dimensional resistive anode and is designed with a total thickness of 9 mm to minimize the reduction in the aperture of the volume tested by the device . A homogeneous electric field of about 40 V/mm is used to extract the electrons emitted in the ionization of the residual gas. This field, which is directed vertically and extends over a distance of 26 mm, is provided by a repeller plate at a negative voltage and a grounded grid placed at 2.4 mm from the surface of the first channelplate of the position sensitive detector (cf. fig. 2) . The front surface of the detector is set at a small positive voltage of about +60 V. This voltage setting assures that the electrons produced in the extraction field gain enough energy for
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a) Uncooled beam b) Cooled beam Fig. 3 . Horizontal beam profiles taken with 10 7 deuterons stored at 12 MeV. (a) uncooled beam, (b) cooled beam. The upper part shows the projections from where the horizontal beam widths are obtained .
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efficient detection. The magnetic field in the dipole chamber forces the electrons to spiral around the magnetic field lines in small orbits, as they are accelerated electrostatically towards the position-sensitive detector. The radii of such orbits are estimated to be - 60 wm at a magnetic field of 0.6 T and at a mean electron emission energies of 100 eV which is expected to be a maximum value for an ionizing 6 MeV/amu D +-beam [13] . Thus the information on the horizontal position of the ionization event may be preserved with high precision as the electron is transported to the detector . The strong magnetic field assures a collection efficiency of unity for electrons produced inside the volume defined by the electric field and the active area of the resistive anode. It also prevents electrons produced outside the active volume to reach the detector, which has been a very serious problem when detecting the emitted electrons by means of electrostatic extraction without a magnetic field. The cause of this problem is the strong forward emission of electrons in fast ion-atom collisions. 3. Results and discussions The pressure at CRYRING is, at present - 2 X 10 -11 mbar. The measure of the absolute partial pressures of the most abundant residual gas components gives a composition of 90% H2, 5% CH 4, 4% H2O and 0.4% Ar and CO 2. By using the Bethe theory [14] for the ionization of different atomic and molecular
gases [15], we estimate the number of ionization events produced per second in the volume viewed by the beam-profile monitor to be - 32 s -1 , with 10' D + ions stored at 6 MeV/amu . The calculated cross sections have been in agreement with experimental data shown by Rudd et al . [16] . The estimated value should be compared to the measured rate of - 17 s-1 , which thus indicates a detection efficiency of 53% estimated with an error in the order of 50%. This is consistent with the 58% rated open-area ratio of the multichannel plate and the 90% transmission of the grid in front of the detector . The ionization cross section is expected to scale as q2. For stored beams of highly charged ions this means that the count rate will be sufficiently large for convenient monitoring even when the design value of 5 X 10 -12 mbar for the residual-gas pressure in the ring is reached. Fig. 3 shows the horizontal beam-profile for 10' D + ions stored at 6 MeV/amu. The bottom left-hand figure displays a two-dimensional image of the ionization track of this beam before the electron cooler is switched on, while the bottom right-hand figure shows the track produced by the cooled ion beam . The upper figures show the projection of the counts on an axis transverse to the beam direction. The diameter of the beam can be determined from this plot ; it shrinks from - 8 mm to - 1.2 mm (FWHM) as it is cooled by the electrons. No time resolved studies of the cooling process have been made yet. The count rate of the detector, together with the high spatial and temporal resolu-
a
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.
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7 8 9 1 I 0 11 12 13 Time (h) Fig. 4. Lifetime of the stored beam . (a) r = 21 h; (b) r = 10 h.
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tion are well suited, however, for measurements of such quantities as transverse cooling times. The performance of the beam-profile monitor has been compared with the measurements by means of another multichannel plate detector, which is aligned with the electron cooler . This second position-sensitive detector, in the following referred to as the 0°-detector, views particles that have been neutralized in the straight section of the ring containing the electron cooler (see fig. 1) . In this way we could provide two independent measurements of the position and width of the stored ion beam and found them to be in good agreement with one another. The intensity of the stored ion beam was measured as a function of the time after injection with the 0° detector and with the version of the beam-profile monitor consisting of two channelplate detectors, one for electrons and one for recoil ions . For this detector system, the vertical opening is 26 mm . The former measurement was performed with the beam-profile monitor retracted from the beam path and resulted in a lifetime of 21 h, while the measurement with the beam-profile monitor gives a lifetime of 10 h (see fig. 4) . Thus, reducing the vertical aperture in one of the twelve ring dipole magnets from 53 mm to 26 mm leads, in this particular case of a D + beam, to a reduction of the lifetime by a factor of 2. This lifetime reduction depends of course on the acting loss mechanism . In the case of deuterons stored at some MeV/amu in CRYRING, the main losses in the beam intensity are due to single scattering of the ions by the residual gas. If the lifetime of the cooled beam is limited by electron loss or by capture, as expected in the case of heavy ions, it should not be affected at all by reducing the aperture . When the single scattering is the limiting process, the beam lifetime can be calculated by integrating the Rutherford's scattering cross section over deflection angles larger than the maximum angular acceptance of the storage ring : T= I/[pufe,Bmax doo- R th ], where p is the residual gas density and c the ion's velocity . Considering the rectangular geometry of the aperture in the dipole magnets and given that, due to this geometry, the vertical angular acceptance (Bv) in CRYRING is significantly smaller than the horizontal one (00 is r=v 3 e~/-rrp(Z P/A)ZZTr2C 4, where ZP,T are the projectile's and the target's atomic numbers, A the projectile's atomic mass, rP is the classical proton radius and c the speed of light. The maximum amplitude of the betatron oscillations allowed in a storage ring [17] at the dipole mag,6y(d)e, nets in the vertical direction is a y = ; and the angular acceptance ay = E Y (I +ay)/ßy(s) where V the /3-function, ßy(s), depends on the position along the beam trajectory being 6Y(d) its value at the dipole
magnet; e y is the beam emittance and ay = -/3y/2 . Replacing eY and taking the mean value along the circumference of the ring, the vertical angular acceptance is calculated to be, for CRYRING (a y' )(mrad)0.304ay(mm) . The maximum amplitude of the betatron oscillations allowed is half of the smallest aperture in the ring, corrected by closed orbit distortions which are the off-centre position of the stored beam at different locations along its trajectory, due to errors and misalignments in the magnetic fields . At the conditions during the test, the closed orbit was located 8 mm off-center at the dipole aperture and just at the center at the position of the beam-profile monitor. Thus, the maximum angular acceptances with and without the beam-profile monitor were estimated to be 3.9 mrad and 5.77 mrad, respectively . This gives a ratio of the lifetimes T1/'r2 - 0.5 which is in good agreement with the measured value and reproduces the measured lifetimes as well . We have, further, used the beam-profile monitor to measure the change of the stored beam orbit caused by detuning the electron beam energy by 13 eV from its nominal setting at 3 keV for cooling of the 6 MeV/amu beam . This led to a change in the revolution frequency, measured with the Schottky detector, of 8f/f = 1.9 X 10 -3 . Using the calculated value of B = (8f/f )/ (8p/p) = 0.8 and the calculated dispersion of 1.7 m, the 13 eV change yields an expected shift of the beam position of 4.1 mm . We detected, however, a shift of the beam position in the center of the dipole magnet of 7.2 mm . The dispersion function of the ring has been calculated as a function of the position in the ring [10] and it has been measured with uncooled beam by means of electrostatic pick-ups at nine different points. A close agreement between this values was found. Thus, for an explanation of the discrepancy between the beam displacements measured here with a cooled beam and the calculated values, further measurements of the dispersion function as well as the betatron tune and other ring parameters have to be made under these beam conditions . 4. Conclusions We have constructed a new type of beam-profile monitor which allows to measure the horizontal profile of the cooled beam stored in CRYRING. The device, a position sensitive multi-channelplate detector, located in the magnetic field of a dipole magnet above the ion-beam path, detects the electrons produced from ionization of the residual gas under ultrahigh vacuum conditions . For light ions at rather low energy, such as used here, for which the storage time is limited by single scattering, the observed reduction of the lifetime is explained by the reduced angular acceptance . A size
T. Quinteros et al. / Beam-profile monitor of a cooled stored ion beam in the order of 1 mm could be detected within less than a minute counting time .
So the dynamic variation of beam parameters during cooling e.g . the transverse cooling rate can be observed .
[6] [7] [8]
References G. Kilgus, J. Berger, P. Blatt, M. Grieser, D. Habs, B. Hochadel, E. Jaeschke, D. Krämer, R. Neumann, G. Neureither, W. Ott, D. Schwalm, M. Steck, R. Stokstad, E. Szmola, A. Wolf, R. Schuch, A. Müller and M. Wagner, Phys . Rev. Lett . 64 (1990) 737. [2] P. Balling, L.H . Andersen, T. Andersen, H.K . Haugen, P. Hvelplund and K. Taulbjerg, Phys . Rev. Lett. 69 (1992) 1042. M. Jung, F. Bosch, K. Beckett, H. Eickhoff, H. Folger, B. Franzke, A. Gruber, P. Kienle, O. Klepper, W. Koenig,
C. Kozhuharov, R. Mann, R. Moshammer, F. Nolden, U. Schaaf, G. Soff, P. Spiidtke, M. Steck, Th . Stöhlker and K. Sümmerer, Phys. Rev. Lett . 69 (1992) 2164. [4] M. Larsson, H. Danared, J.R . Mowat, P. Sigray, G. Sundström, L. Broström, A. Filevich, A. Källberg, S. Mannervik, K.G . Rensfelt and S. Datz, Phys . Rev. Lett . 70 (1993) 430. [5] B. Hochadel, Ein Strahlprofilmonitor nach der Methode der Restgasionisation für den Heidelberger Testspeicher-
[9]
[10]
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ring TSR, Max-Plank Institut für Kernphysik, Heidelberg (1990) . T. Kawakubo, T. Ishida, E. Kadokura, Y. Ajima and T. Adachi, Nucl. Instr. and Meth. A302 (1991) 397. B. Vosicki and K. Zankel, IEEE Trans. Nucl . Sci. NS-22 (1975) 1475 . K.-G. Rensfelt, IEEE 91 Part . Accel . Conf., eds . L. Lizama and J. Chew, vol. 5 (IEEE, NY, 1991) p. 2814 . A. Schempp, H. Deitinghoff, J Friedrich, U. Bessler, J. Madlung, G. Rieh], K. Volk, K. Langbein, A. Kippler, H. Klein, A. Källberg, A. Soltan, M. Blörkhage and C.J . Herriander, Proc . 2nd European Particle Accelerator Conf ., Nice, France, 1990, eds. P. Marin and P. Mandrillon (Ed. Frontieres, 1990) p. 1231 . A. Simonsson, Beam Dynamics and Injection in CRY-
RING, Thesis . KTH. Stockholm (1991). [11] K. Abrahamsson, G. Andler and C.B . Bigham, Nucl . Instr. and Meth . B31 (1988) 475. [12] H. Danared, Proc . Workshop on Electron Cooling and New Cooling Techniques, Legnaro, Italy, 1990, eds. R. Calabrese and L. Tecchio (World Scientific 1991) p. 21 . [13] R. Schuch, H. Schöne, P.D Miller, H.F . Krause, P.F . Dittner, S. Datz and R.E. Olson, Phys Rev. Lett 60 (1988) 925. [14] M. Inokuti. Rev. Mod. Phys . 43 (1971) 297. [15] F.F . Rieke and W. Prepejchal, Phys. Rev. A6 (1972) 1507 [16] M.E. Rudd, Y-K. Kim, D.H. Madison and J.W . Gallagher, Rev. Mod. Phys . 57 (1985) 965. [171 P.J . Bryant, CERN Accelerator School, CAS84-04 .
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
A novel beam-profile monitor for storage rings T. Quinteros, R. Schuch, M. Pajek ', P. Sigray, H. Cederquist, H. Danared, L. Bagge, A. Filevich 2, J . Jeansson, A. Källberg and A. Paâl Manne Siegbahn Institute of Physics, S-104 05 Stockholm, Sweden
Received 29 March 1993 We have developed a novel, nondestructive, beam-profile monitor for ion storage rings. The horizontal profile of the stored ton beam is determined by detecting electrons from the ionization of the residual gas with a two-dimensional position sensitive detector . The strong magnetic field, provided by one of the bending magnets, conserves the information on the initial horizontal positions of ionized electrons during their transport to the detector by means of a vertical electric field. The monitor is tested at the storage ring CRYRING in Stockholm for a 6 MeV/amu D + beam . We show that in this case, where the lifetime of the stored beam is restricted by single scattering on the residual gas, a reduction of the lifetime occurs due to the reduced angular acceptance . 1. Introduction Recently, newly built ion-storage rings have been put into operation in, e.g., Aarhus (ASTRID), Heidelberg (TSR), Darmstadt (ESR), Bloomington (IUCF), Tokyo (Tarn II), Rilich (COSY), Uppsala (Celsius), and Stockholm (CRYRING). The main goals for the construction of such facilities are the storing and phase space cooling of energetic (multi-MeV) ion beams for experiments in atomic, molecular and nuclear physics. Some of these experiments require a high number of beam particles with low momentum spread passing a target per unit time or in some situations, the interactions between stored ions and a target or a laser beam could be observed over substantial periods of time . Some examples of typical ion-storage ring experiments are the measurements of dielectronic and laser-induced recombination at TSR in Heidelberg [1], the measurements of lifetimes of metastable negative ions in Aarhus [2], the studies of bound ß-decay at ESR in Darmstadt [3] and the experiments on dissociative recombination of vibrationally relaxed molecules at CRYRING in Stockholm [4]. It is of course necessary that the information on the beam properties can be retrieved continuously throughout the observation time . Electrostatic pick-ups can be used to give information about the mean position of the beam, while current t Permanent address: Institute of Physics, Pedagogical University, 25-509 Kielce, Poland . z Permanent address: Comisi6n Nacional de Energia At6mica, Laboratorio Tandar, Av . Libertador 8250, 1429 Buenos Aires, Argentina.
transformers give information on the stored beam intensity. The revolution frequency, and thereby the beam velocity, is usually monitored by means of Schottky noise detectors. In quite a few experimental situations it is in addition necessary to gain information about the spatial intensity distributions within the beam envelope . This is sometimes the case in studies of interactions of two stored beams and for some experiments involving laser beams. Beam-profile monitors have now been in operation for a few years at, e.g . TSR [5], KEK-PS at Tokyo [6] or ISR at CERN [7]. The two first monitors detect the recoil ions produced through the ionization of the residual gas that takes place when the stored ions pass by a two-dimensional position-sensitive detector . The monitor at CERN detects the electrons produced in the ionization of atoms from a gas jet crossing the proton beam . Here, we report on experimental tests performed with a new type of beam-profile monitor which is inserted in the strong magnetic field of one of the bending magnets in CRYRING. At present we have been detecting one of the products of the ionization of the residual gas only (e .g . the electrons) to obtain the horizontal beam profile. Within the magnetic field, electrons and recoil ions are transported to opposite channelplate detectors. In the very near future, this monitor will be able to give information about the vertical and the horizontal beam-profiles simultaneously. A projection of the ionized electrons on the position sensitive anode of one of these detectors gives the horizontal beam profile. The vertical beam-profile can be obtained by measuring the time difference between the arrival of recoil ions and ion-
0168-9002/93/$06.00 C 1993 - Elsevier Science Publishers B.V . All rights reserved
T. Quinteros et al. / Beam-profile monitor ized electrons transported electrostatically to two different detectors, mounted above and below the circulating beam . The purpose of the magnetic field, which is parallel with the electric field, is threefold : (i) it assures a high collection efficiency of emitted electrons, (ii) it provides a high resolution by forcing the electrons to spiral closely around the magnetic field lines and (iii) it keeps electrons produced outside the volume probed by the monitor from reaching the detector . The confinement of the electrons to very small cyclotron orbits around the magnetic field lines may provide an opportunity to measure the spatial properties of stored beams with unprecedented resolution . Below, we will show results from test runs with the prototype of our beam-profile monitor . These experiments, were performed at the heavy ions storage ring CRYRING at the Manne Siegbahn Institute of Physics in Stockholm . We have measured the horizontal profile of _ 10 7 D+ ions stored at the energy 6 MeV/amu before and after cooling of the beam . We will, further, show measurements of storage times with and without the beam-profile monitor in operation . First, however,
289
we describe shortly the essential properties of CRYRING and give an account of the detector design .
2 . Experimental technique CRYRING is a low energy synchrotron and storage ring [8] for ions and molecules over a wide range of q/m ratios . Fig . 1 shows a schematic of the machine and the locations of its main components. Highly charged ions can be produced in an electron-beam ion source (not shown in the figure), while light ions and molecules of moderate charge are produced by a conventional Penning-type ion source (MINIS) . A radiofrequency quadrupole accelerator (RFQ) accelerates either one of these beams to an energy of 300 keV/amu if their charge-to-mass ratios exceed 0 .25 [9] . The ions are then injected into the ring by means of an electrostatic inflector [10] . A nonresonant drift tube [I1] accelerates the stored ions to their final energy, while the electron cooler is used to decrease the longitudinal and transversal momentum spreads of the stored ion beam [12]. The beam-profile monitor is mounted on a mo-
beam-profile monitor Fig . 1 . Layout of CRYRING .
29 0
T Quinteros et al. / Beam-profile monitor
IV41FIIZZIIIIIIIIIIII1111111zllllllllllllll1111 Mci~ ______________
-
end
Fig. 2 . Schematics of the beam-profile monitor. The stored beam ionizes the residual gas atoms. The ionized electrons are driven towards the MCP detector by an electric field in helicoidal trajectories about the magnetic field lines. tion-feedthrough with an ultrahigh vacuum (UHV) lock, which allows the detector to be inserted in and retracted from the ring-vacuum volume without breaking
the vacuum . It is positioned in the dipole magnet following directly after the electron cooler (cf. fig. 1) . The test device in its present form, in which one of the two detectors to be used in the final arrangement was replaced by a potential plate, is shown in fig. 2. It measures the horizontal profile of the stored beam by detecting the electrons generated through ionization of residual gas molecules. The multichannel plate detector assembly (MCP) has 40 mm diameter active area, is equipped with a two-dimensional resistive anode and is designed with a total thickness of 9 mm to minimize the reduction in the aperture of the volume tested by the device . A homogeneous electric field of about 40 V/mm is used to extract the electrons emitted in the ionization of the residual gas. This field, which is directed vertically and extends over a distance of 26 mm, is provided by a repeller plate at a negative voltage and a grounded grid placed at 2.4 mm from the surface of the first channelplate of the position sensitive detector (cf. fig. 2) . The front surface of the detector is set at a small positive voltage of about +60 V. This voltage setting assures that the electrons produced in the extraction field gain enough energy for
1 2 mm
8 mm
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, .,
E E
T
40
40
30
30
20
20
10
10
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40
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a) Uncooled beam b) Cooled beam Fig. 3 . Horizontal beam profiles taken with 10 7 deuterons stored at 12 MeV. (a) uncooled beam, (b) cooled beam. The upper part shows the projections from where the horizontal beam widths are obtained .
291
T. Quinteros et al. / Beam-profile monitor
efficient detection. The magnetic field in the dipole chamber forces the electrons to spiral around the magnetic field lines in small orbits, as they are accelerated electrostatically towards the position-sensitive detector. The radii of such orbits are estimated to be - 60 wm at a magnetic field of 0.6 T and at a mean electron emission energies of 100 eV which is expected to be a maximum value for an ionizing 6 MeV/amu D +-beam [13] . Thus the information on the horizontal position of the ionization event may be preserved with high precision as the electron is transported to the detector . The strong magnetic field assures a collection efficiency of unity for electrons produced inside the volume defined by the electric field and the active area of the resistive anode. It also prevents electrons produced outside the active volume to reach the detector, which has been a very serious problem when detecting the emitted electrons by means of electrostatic extraction without a magnetic field. The cause of this problem is the strong forward emission of electrons in fast ion-atom collisions. 3. Results and discussions The pressure at CRYRING is, at present - 2 X 10 -11 mbar. The measure of the absolute partial pressures of the most abundant residual gas components gives a composition of 90% H2, 5% CH 4, 4% H2O and 0.4% Ar and CO 2. By using the Bethe theory [14] for the ionization of different atomic and molecular
gases [15], we estimate the number of ionization events produced per second in the volume viewed by the beam-profile monitor to be - 32 s -1 , with 10' D + ions stored at 6 MeV/amu . The calculated cross sections have been in agreement with experimental data shown by Rudd et al . [16] . The estimated value should be compared to the measured rate of - 17 s-1 , which thus indicates a detection efficiency of 53% estimated with an error in the order of 50%. This is consistent with the 58% rated open-area ratio of the multichannel plate and the 90% transmission of the grid in front of the detector . The ionization cross section is expected to scale as q2. For stored beams of highly charged ions this means that the count rate will be sufficiently large for convenient monitoring even when the design value of 5 X 10 -12 mbar for the residual-gas pressure in the ring is reached. Fig. 3 shows the horizontal beam-profile for 10' D + ions stored at 6 MeV/amu. The bottom left-hand figure displays a two-dimensional image of the ionization track of this beam before the electron cooler is switched on, while the bottom right-hand figure shows the track produced by the cooled ion beam . The upper figures show the projection of the counts on an axis transverse to the beam direction. The diameter of the beam can be determined from this plot ; it shrinks from - 8 mm to - 1.2 mm (FWHM) as it is cooled by the electrons. No time resolved studies of the cooling process have been made yet. The count rate of the detector, together with the high spatial and temporal resolu-
a
Jy
1 a.
0'- detecLor
.
daLa
ioaiization deLecLor data 6
7 8 9 1 I 0 11 12 13 Time (h) Fig. 4. Lifetime of the stored beam . (a) r = 21 h; (b) r = 10 h.
14
15
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T.
Quinteros et al. / Beam-profile monitor
tion are well suited, however, for measurements of such quantities as transverse cooling times. The performance of the beam-profile monitor has been compared with the measurements by means of another multichannel plate detector, which is aligned with the electron cooler . This second position-sensitive detector, in the following referred to as the 0°-detector, views particles that have been neutralized in the straight section of the ring containing the electron cooler (see fig. 1) . In this way we could provide two independent measurements of the position and width of the stored ion beam and found them to be in good agreement with one another. The intensity of the stored ion beam was measured as a function of the time after injection with the 0° detector and with the version of the beam-profile monitor consisting of two channelplate detectors, one for electrons and one for recoil ions . For this detector system, the vertical opening is 26 mm . The former measurement was performed with the beam-profile monitor retracted from the beam path and resulted in a lifetime of 21 h, while the measurement with the beam-profile monitor gives a lifetime of 10 h (see fig. 4) . Thus, reducing the vertical aperture in one of the twelve ring dipole magnets from 53 mm to 26 mm leads, in this particular case of a D + beam, to a reduction of the lifetime by a factor of 2. This lifetime reduction depends of course on the acting loss mechanism . In the case of deuterons stored at some MeV/amu in CRYRING, the main losses in the beam intensity are due to single scattering of the ions by the residual gas. If the lifetime of the cooled beam is limited by electron loss or by capture, as expected in the case of heavy ions, it should not be affected at all by reducing the aperture . When the single scattering is the limiting process, the beam lifetime can be calculated by integrating the Rutherford's scattering cross section over deflection angles larger than the maximum angular acceptance of the storage ring : T= I/[pufe,Bmax doo- R th ], where p is the residual gas density and c the ion's velocity . Considering the rectangular geometry of the aperture in the dipole magnets and given that, due to this geometry, the vertical angular acceptance (Bv) in CRYRING is significantly smaller than the horizontal one (00 is r=v 3 e~/-rrp(Z P/A)ZZTr2C 4, where ZP,T are the projectile's and the target's atomic numbers, A the projectile's atomic mass, rP is the classical proton radius and c the speed of light. The maximum amplitude of the betatron oscillations allowed in a storage ring [17] at the dipole mag,6y(d)e, nets in the vertical direction is a y = ; and the angular acceptance ay = E Y (I +ay)/ßy(s) where V the /3-function, ßy(s), depends on the position along the beam trajectory being 6Y(d) its value at the dipole
magnet; e y is the beam emittance and ay = -/3y/2 . Replacing eY and taking the mean value along the circumference of the ring, the vertical angular acceptance is calculated to be, for CRYRING (a y' )(mrad)0.304ay(mm) . The maximum amplitude of the betatron oscillations allowed is half of the smallest aperture in the ring, corrected by closed orbit distortions which are the off-centre position of the stored beam at different locations along its trajectory, due to errors and misalignments in the magnetic fields . At the conditions during the test, the closed orbit was located 8 mm off-center at the dipole aperture and just at the center at the position of the beam-profile monitor. Thus, the maximum angular acceptances with and without the beam-profile monitor were estimated to be 3.9 mrad and 5.77 mrad, respectively . This gives a ratio of the lifetimes T1/'r2 - 0.5 which is in good agreement with the measured value and reproduces the measured lifetimes as well . We have, further, used the beam-profile monitor to measure the change of the stored beam orbit caused by detuning the electron beam energy by 13 eV from its nominal setting at 3 keV for cooling of the 6 MeV/amu beam . This led to a change in the revolution frequency, measured with the Schottky detector, of 8f/f = 1.9 X 10 -3 . Using the calculated value of B = (8f/f )/ (8p/p) = 0.8 and the calculated dispersion of 1.7 m, the 13 eV change yields an expected shift of the beam position of 4.1 mm . We detected, however, a shift of the beam position in the center of the dipole magnet of 7.2 mm . The dispersion function of the ring has been calculated as a function of the position in the ring [10] and it has been measured with uncooled beam by means of electrostatic pick-ups at nine different points. A close agreement between this values was found. Thus, for an explanation of the discrepancy between the beam displacements measured here with a cooled beam and the calculated values, further measurements of the dispersion function as well as the betatron tune and other ring parameters have to be made under these beam conditions . 4. Conclusions We have constructed a new type of beam-profile monitor which allows to measure the horizontal profile of the cooled beam stored in CRYRING. The device, a position sensitive multi-channelplate detector, located in the magnetic field of a dipole magnet above the ion-beam path, detects the electrons produced from ionization of the residual gas under ultrahigh vacuum conditions . For light ions at rather low energy, such as used here, for which the storage time is limited by single scattering, the observed reduction of the lifetime is explained by the reduced angular acceptance . A size
T. Quinteros et al. / Beam-profile monitor of a cooled stored ion beam in the order of 1 mm could be detected within less than a minute counting time .
So the dynamic variation of beam parameters during cooling e.g . the transverse cooling rate can be observed .
[6] [7] [8]
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