One year of operation at the Heidelberg TSR

Nuclear Instruments and Methods in Physics Research A287 (1990) 268-272 North-Holland

268

ONE YEAR OF OPERATION AT THE HEIDELBERG TSR D. KRÄMER, G. BISOFFI, M. BLUM, A. FRIEDRICH, Ch . GEYER, B. HOLZER, H.W. HEYNG, D. HABS, E. JAESCHKE, M. JUNG, W. OTT, R.E. POLLOCK 1 ), R. REPNOW, F. SCHMITT and M. STECK Max-Planck Institut für Kernphysik Heidelberg, D-6900 Heidelberg, FRG 1) IUCF Bloomington, Indiana 47408, USA

After one year of operation the heavy ion storage ring TSR at the Heidelberg Max-Planck Institut fiir Kernphysik has reached full performance. As designed 1000 turns are accumulated by a combination of multiturn and rf stacking. Due to phase space compression by an electron cooler the momentum spread of the beams is ®p/p =10 -5 -10 -° depending on the heating by intrabeam scattering. The cooled beam lifetime is pushed to the limits set by charge exchange processes as electron capture for bare nuclei and electron stripping for incompletely stripped ion beams. As the vacuum pressure is P _< 10 -1° Torr at present, beam C6+ beam lifetimes range from T = 36 h for 21 MeV protons to 20 s for 7 MeV Be'. Intensities of up to 18 mA (3 x 10 1° particles) have been stacked by applying phase space cooling during injection. For these high intensities the splitting of the longitudinal Schottky noise signal showed irregular behaviour with respect to the expected Af - P/ 2 scaling law.

l. Introduction

The Heidelberg heavy ion storage ring TSR [1] is an experimental facility for accelerator, atomic, plasma and nuclear physics at the MPI MP-tandem-postaccelerator combination [2]. Constructed in the years 1985 to 1988, commissioning of the ring started in May 1988 . With the installation of an electron cooler [3] the ring has become the first heavy ion cooler ring and it has been used for atomic physics experiments starting late 1988 [4]. Fig. 1 shows the floor plan of the 55 .4 m circumference storage ring. The fourfold symmetrical lattice consists of four periods of two dipoles with a focusing c;uadrupole magnet in between and quadrupole doublets at the end;, of the structure. This separated function ' attice, where the main magnetic elements are located near the 90 ° bends, allows for four 5.2 m long straight sections to house the injection, the electron cooler and the rf-system while one complete straight section is available for experimental insertions . The r ng is capable of storing beams of magnetic rigidities up to BPmax. -- 1 .5 T m. Fast energy changes are possible since the magnet yokes are laminated; the ramping rate is B = 0.25 T/s . The electron cooler produces an intensive cold electron beam (transversal electron energies of - 0.1 eV) at densities of up to 10 8 cm - 3, e.g. 1 A at energies of a few

Funded by BMFT under contract No. : 06 HD 852 I . 0168-9002/90/$03 .50 1 Elsevier Science Publishers B.V . (North-Flolland) _1'

keV corresponding to the ion velocity . Under these conditions a decrease of phase space volume by a factor of > 10 4 has been achieved . As the cooling time scales as T - A/Z, 2, electron cooling of heavy ions is a fast and very powerful too? to create high intensity low emittance and small ene-gy spread beams.

Fig. 1 . Floor plan of the Heidelberg heavy ion storage ring TSR. The labels denote the different types of magnets (AM, dipole magnets; QF, QD, quadrupole magnets, SF, SD, sextupole magnets; MS, magnetic septa, El, electrostatic septum).

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D. Krdmer et al. / One year of operation at the Heidelberg TSR

2. Injection schemes at TSR

3. Beam lifetime of stored ions

As the injector tandem has to start with negative ion beams, peak intensities at the ring injection are of the order of some 10 [LA when using a pulsed sputter ion source [5]. A combination of multiturn and rf-stacking injection is applied to fill the horizontal and longitudinal phase space to achieve currents in the mA range . Due to the good beam quality of the injector (momentum spread Ap/p <- 2 x 10 -4 , emittance E = Mir mm mrad) as much as 40 turns are accumulated in the 120 iT mm mrad acceptance by multiturn injection and up to 40 multiturn batches are stacked in longitudinal phase space as the momentum acceptance of the ring is ( + )/ - 3% as was confirmed by measurement. Table 1 lists the results that have been achieved by optimization of multiturn injection, rf-stacking and rfstacking with simultaneous phase space cooling in the case of a 73 MeV C 6+ beam. Especially when adjusting the velocity of the cooling electrons to the velocity of the particles at the stack top the number of stored particles is increased to 18 mA, that is more than 3 x 10 1° particles, well above the design aim. Similar stacking rates have been achieved by successive multiturn injection and setting the cooler to somewhat lower energies. As the phase space of the multiturn batch is cooled the stored particles do not hit the injection septum when the bump magnets fire for the next multiturn. Even without optimizing this "electron stacking method" more than 130 multiturn batches have been accumulated . A more detailed description of particle stacking is to be found elsewhere in these Proceedings [6] .

For heavy ions the beam lifetime is strongly influenced by the vacuum pressure, more particular by the composition of the residual gas . At a pressure of 5 x 10 -1° Torr a typical residual gas spectrum as measured in TSR is shown in fig . 2. Though H2 is the dominant contribution it is the heavy contaminants such as Ar, which give the limit for the beam lifetime of completely stripped ions, as the cross section for electron capture is o =1 .1

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where ß = v/c and y are the usual relativistic parameters, Â is the acceptance, Q the tune and R the radius

Table 1 rf-stacking, ?lumber of stacked turns N in the TSR for different, settings of the accumulation schemes : multiturn injection, currents I; the absolute !Cored in brackets refer to stacking" . The numbers rf-stacking with cooling the stack top as well as "electron C6+ beams the injected intensities are listed. All experiments were performed with Intensities at TSR Stacking scheme

Multiturn injection RFstacking RF-stacking with cooling Electron stacking

Effective turns Injected Multiturn injection peak N current (mAl (stored turns)

I

N

I

(mA)

(stored turns)

(rnA)

0.015

40

(0.6)

0 .009

25

(0.2)

"'00

(6.9)

0.014

20

(0.2)

1.60

(2.~)

0 .002

10

(0.02)

Rf-stacking

Rf-stacking with cooling N 1 (sioreû turns)

(-~")

1300

(18 .0)

Electron stacking N (stored turns)

(mA)

1400

(2.7) V1 .

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ture, electron loss and multiple scattering. The agreement of the numbers is fairly good, as the integral gas composition varies significantly and was assumed to be the mean from measurements at four different locations . For the C6+ at high energies where the losses with respect to capture are small the beam lifetime of the cooled beam is significantly smaller as radiative electron capture in the cooler becomes the dominant loss process. At a vacuum of 5 x 10 -11 Toff in the C6+ beam a cooler we found for the 11.7 MeV/u s production rate of C ' being enhanced by a factor of 560 when the cooling electrons were present . This effect gives a limit of beam lifetime to 1.2 h which agrees with the r =1.14 h measured.

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4. Electron coiling of Hl beams

of the storage ring and P is the equivalent mean nitrogen vacuum pressure in the machine . Thus the beam lifetime under the present conditions is reduced by factors of 2-4 compared to the limits set by electron capture . For incompletely stripped ions the situation at these pressures is completely different. As the cross section for electron loss is typically an order of magnitude larger than for electron capture [9], multiple scattering only has a small influence on the beam lifetime which is predominantly determined by the stripping process . Table 2 gives a comprehensive list of the measured beam lifetimes for different cooled and uncooled beams is comparison to the limits calculated for electron cap-

Phase space compression of beams by electron cooling allows brilliant beams with extreme small energy spread to be generated, independently of the beam intensities. The equilibrium values being determined by the balance of the friction force of the cooling electrons and the heating e.g. intrabeam scattering, scattering with the residual gas, internal target material, etc. Cooled beams at the TSR showed typical equilibrium emittances of E x = E y < 0.5Rr mm mrad at intensities of a few ~LA starting with uncooled beams of E = 120îr mm mrad. Longitudinal cooling is easily extracted from the noise spectrum of coasting beam (Schotiky noise) which is related to the momentum distribution of the beam. For low intensities the noise

Table 2 Experimental beam lifetimes for ion beams in TSR at various energies and theoretical limits for the beam lifetime assuming the rest gas composition from fig. 2 Ion

p Li Be C

O

Charge state (e)

Energy (MeV/u)

Average ring pressure (10-t° mbar)

Beam lifetime Cooled (s)

Uncooled (s)

Electron capture (s)

1+ 1+ 1+ 3+ 4+ 5+ 6+ 6+ 3+ 3+ 5+ 6+ 7+ 8+ 8+

21 1 .3 0.8 2.1 3.7 4.3 6.1 11 .7 1 .2 4.6 2.4 3.4 8.9 6.1 11 .7

0.8 1.0 0.6 6.0 8.0 9.0 15 .0 0.7 10.0 10.0 8.0 2.0 0.7 10 .0 3.0

130000 26

11000 20 12 3 10

2 x 10 9 27000 9500 650 13000

IV

70V

ic

720 4100

16 400 258

Theoretical beam lifetime

155 17 000 0.6 1.6 4.5 14 196 3600

980 230000 58 5800 160 270 198000 17000 20000

Electron loss (s)

Multiple scattering (s)

22 12 3 10

12000 6000 7100 640 580

0.9 2.1 5.0 19 490 -

300 16000 200 2000 340 410 9100 380 3600

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where the rl-parameter is 0.9 for TSR and fn is the frequency of the nth harmonic of the revolution frequency and AI is the frequency spread of the signal at FWHM . At particle numbers larger than a critical value [101 N>

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the noise spectrum to a double peak structure, where the peak splitting is proportional to the square root of the beam intensity whereas the spectrum form contains the momentum spread. Fig. 3 shows Schottky noise spectra of a 2.6 mA C6+ beam; the broad distribution is that of an uncooled beam, the narrow distribution is measured when the cooler is working. As the setting for the spectrum analyzer was unchanged, the strong signal suppression for cooled beams is obvious and the integrated spectral power is no longer a measurement of the beam intensity . Increasing the intensity the frequency splitting of the spectrum enlarges. Fig . 4 shows a Schottky noise spectrum of a cooled 15 mA C6+ beam (3.1 x 10 10 particles) . The double peak structure is very pronounced and the splitting is as big as 0.18% of the nth harmonic f . Measurement of the frequency splitting at different currents taken at the 28th harmonic show a nonlinear scaling near currents of about 4 mA, fig. 5. The splitting follows the expected scaling up to particle numbers of 7 x 10" [111 If = QnIo ( Zu/n)1

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with values for the impedance Z11 =1 .5 k S2 whereas at higher currents the slope of the curve is significantly changed. This might be caused by an increase in ernittance which enters into Z,I . A hint that the beam properties are drastically changed when crossing this threshold is the observation that below threshold the beam can perform large transversal oscillation which often leads to a decrease in beam lifetime, whereas above threshold the beam was "quiet" with no transverse oscillations detectable. Acknowledgements

Start 12 .2819 MHz

Fig. 4. Schottky spectrum of -3 15 mA cooled C6 + beam .

We would like to acknowledge the advice and help of many collegues from CERN, DESY, GSI, INS and V 1. BOOSTERS

272

D. Krämer et al. / Oneyear of operation at the Heidelberg TSR

IUCF whom we cannot name all individually. We also would like to thank the technicians of MPI for their enthusiastic and skillfull work which made these experiments possible at the TSR.

References (1]

G . Bisoffi et al., in : Proc . 1989 Part. Acc. Conf, March 20-23, 1989, Chicago and Proc . of the Europ. Part . Acc . Conf ., June 7-11, 1988, Rome, to be published . (2] B . Huck, H . Ingwersen, E. Jaeschke, B . Kolb, R . Repnow and Th . Walcher, IEEE Trans . Nucl . Sci . NS-28 (1981) 3516 . (3] M . Steck, G . Bisoffi, M . Blum, A . Friedrich, C . Geyer, M . Grieser, B. Holzer, E . Jaeschke, M . Jung, D . Krämer, K . Mad, W . Ott and R . Repnow, these Proceedings (5th Int .

[4] (51 (6]

[7] [8] [9] [10] [11]

Conf . on Electrostatic Accelerators and Associated Boosters, Strasbourg-Heidelberg, 1989) Nucl . Instr . and Meth. A287 (1990) 324. D. Habs et al ., MPI H - 1989 - V18 and submitted to Nucl. Instr . and Meth . B . MPI Ann . Report Heidelberg (1987) p . 7 . G. Bisoffi et al ., these Proceedings (5th Int . Conf. on Electrostatic Accelerators and Associated Boosters, Strasbourg-Heidelberg, 1989) Nucl. Instr. and Meth . A287 (1990) 320. A .S. Schlachter, J .W . Sterns, W .G . Graham, K .H . Berkner, R.V . Pyle and J.A. Tanis, Phys. Rev . A27 (1983) 3372. W . Hardt, CERN ISR-300/GS/68-11 (1968). V .S. Nikolaev, Sov . Phys. Usp. 8 (1965) 269 . S . Chattopadhyay, CERN 84-11 (1984). V .V . Parkhormchuk and D.V . Pestrikov, Sov. Phys . Tech . Phys . 25 (1980 816 .