The Münster cluster target for the COSY-11 experiment

NUCLEAR PHYSICS A ELSEVIER

The Miinster

Nuclear Physics A626 (1997) 427c 433c

cluster

target

for the COSY-11

experiment

H. Dombrowski ~ , D. Grzonka b , W. Hamsink ~ , A. Khoukaz ~ , T. Lister ~ , R. Santo" ~Institut f/ir Kernphysik, UniversitSit Mfinster, Wilhelm-Klemm-Str. 9, D-48149 Miinster, Germany* blnstitut ffir Kernphysik, KFA Jfilich, D-52425 3/ilich, Germany

The Mfinster cluster target has been built as a flexible device for internal storage ring experiments. To meet the particular spatial requirements of the COSY-ring at. the KI"A Jfilich special cryopumps have been developed. Cluster beams of all gases except Helium can be produced. An easy adaption of the cluster target to other installations is possible because of its compact modular design. During several beam times the target has successfully been operated within the COSY-11 experiment using hydrogen clusters. Cluster beam densities of up to p ~ 1014 a t o r n s / c m 3 have been reached.

1. I N T R O D U C T I O N W i t h the advent of storage rings using electron or stochastic cooling, high quality p a r t M e beams have become available leading to an increased precision in scattering experiments. To make full use of these advantages thin internal targets are necessary. One group of internal targets are gaseous targets like gas cells, gas-jet, cluster and atomic beam targets. A supersonic gas-jet is generated when gas passes a Laval type nozzle under high pressure. Close to the nozzle high densities of up to 10 lr a t o m s / c m 2 can be achieved in such a gas-jet target [1], but due to the scattering of the gas particles the densitiy distribution perpendicular to the beam is very broad and approaches the density of the residual gas within some cm. Because of its high background of typically 10 -a - l0 -2 mbar this type of target can not be used in a synchrotron ring with UttV conditions. \ molecular beam with densities of typically 101° - 1011 a/;om,s/c~lT~2 can be prepared by culting out the central part of a gas-jet by a special orifice, the skimmer. As a consequence, all of the atoms in this beam nearly move parallel to each other. Such a beam can pass a scattering chamber without influencing the pressure if it is caught in a beam dump. Retaining a similar experimental arrangement, a much higher density is achieved by usi.g a cluster beam instead of a molecular beam. It consists of formations of typically 10"~ - 104 atorl~s [2]. A cluster beam is produced by expanding gas in a nozzle at an appropriate temperature. During the expansion l he t e m p e r a t u r e of the gas is reduced *Supported in part by BMBF and KFA Jiilich 0375-9474/98/$19.00 (c) 1998 Elsevier Science B.M All rights reserved. PII S0375-9474(97)00565-4

428c

H. Dombrowski et al./Nuclear Physics A626 (1997) 427c-433c

below the condensation point and the clustering starts. The typical densities of cluster targets are in the range of 1014atorns/crn 3. Compared to solid state targets gaseous targets have a number of advantages. First of all the target is extremely pure and there are no irradiation damages of the target; material because of the continous refreshing. Moreover losses of beam particles due to Coulomb scattering are negligible and the probability for secondary reactions is low. As a consequence the measurement of extremely low cross sections is possible. Usually cluster target devices consist of two main parts, the cluster source and the cluster beam dump. The cluster target built in M/inster, which is based on earlier studies [3], has drastically reduced geometrical dimensions compared to other existing devices. This was achieved by reducing the pumping capacities to a minimum.

2. E X P E R I M E N T A L A R R A N G E M E N T The cluster target has been designed for measurements with the cooler synchrotron COSY at the KFA Jfilich within the COSY-11 experiment. COSY-11 is an internal experimental arrangement for meson production studies in p-p interactions close to threshold. The target is located in front of a machine dipole acting as a magnetic separator for the ejectiles. Details of the COSY-11 installation can be found in [4].

2.1. V a c u u m s y s t e m The space available for the target is a 30 cm wide gap between a quadrupole and a dipole. Hence, the size of the pumping units had to be reduced as far as possible. A maximum target diameter of lOrnm was planned with a density of 1014atoms/cm 3, keeping the maximum pressure in the scattering chamber in the order of 10-s rnbar. A sketch of the mechanical assembly of the cluster target and typical pressure values in the pumping stages are given in Fig. 1 and Table 1. The first stage, the skimmer stage,

~ - - -

C]tlS[.(2f" slommer stage

i

i

Laval nozzle

/

[)c~ rrl

)

SOUFCC

2rid collimator stage t

COSY be~m ~

durrip

gcattermg chamber

J

i

Figure 1. Construction of the cluster target for the COSY-11 experiment.

:~

H. Dombrowski et al./Nuclear Physics A626 (1997) 427c 433c

429c

Table 1 Pumping system of the Mfinster cluster target and typical pressure levels at a hydrogen gas flow of about 5.4 mbar l/s. pumping stage

pumps

typical pressure

skimmer

roots pump: 2000 r~3/h (Leybold Ruvac WSL 2001)

2 . 10 -2 mbar

1st collimator

turbo pump: 10001/s (Leybold Turbovac 1000 C)

2.6 • 10 -4 rnbar

2nd collimator

cryopump: 22000 l/.s

6.5 - 10 - r rnbar

scattering chamber

ion pump: 2401/s (Varian Star Cell) t i t a n i u m subl. pump: 1000 l/s

2 • 10 -s mbar

1st beam dump

cryopump: 22000 l/s

9.7 • 10 - s m b a r

2nd beam dump

cryopump: 2 x 2000 I/s (Leybold R P K 1500)

3.7 • 10 -7 mbar

3rd beam dump

cryopump: 39000 I/s turbo pump: 1000l/s (Leybold Turbovac 1000 C)

1.2 • 10 -5 mbar

is equipped with a roots pump of 2000ma/h, sufficient to maintain the pressure below 4 . 1 0 .2 mbar at gas flows of up to 2 0 m b a r l/s. The first collimator stage is evacuated by a turbo molecular pump connected to the skimmer chamber which acts as forepump. In the second collimator stage a self-constructed cryopump is used, which can be linked to the previous chamber for regeneration purposes. The scattering chamber, equipped with an ion pump and with a t i t a n i u m sublimation pump, can be separated from the target chambers by means of two shutters. The following three beam dump stages are p u m p e d by self-constructed and commercial cryopumps. Finally, a turbo molecular pump with a pumping speed of 10001/s is mounted on top of the target, whose forevacuum flange is connected to a roots p u m p followed by a forepump. To obtain a high pumping speed under restricted space conditions special cryopumps have been developed (Fig. 2). On top of a cold head (Leybold RG 510) an array of conical charcoal coated cold sheets is mounted, which can be cooled down below 16 K Thus pumping of all gases, even Helium, is possible. In the center of the cold sheets there is an opening for the passage of the cluster beam. Due to the extremely high absorption capacity, the regeneration of the cryopmnps during a typical experiment run is not neccessary.

430c

H. Dombrowski et al./Nuclear Physics A626 (1997) 427c 433c

2.2. Cluster beam production

ev

d t o" r '

sl pUFE

eotlimator pumpmg stage

II

stai~ stee

i

./

Figure 2. Cluster source of the Mfinster cluster target.

The cluster beam is produced during the expansion of gas in a Laval type nozzle. In the present device nozzles are used, which have a t r u m p e t shaped divergent part and were manufactured at CERN. Inside the Laval nozzle the gas cools considerably down due to adiabatic cooling during the expansion. Up to ,5-10 % of the gas condensates and clusters up to a size of 106 atoms grow [5]. The resulting cluster beam, surrounded by a gas beam with a periodic spatial density structure of nodes, can travel through a vacuum chamber over several meters. To peel off the cluster beam from the gas beam and to prepare a well-defined target a conical skimmer with an opening of 700 # m in diameter is placed behind the nozzle. The diameter of the cluster beam is determined by the diameter of the first collimator. The cluster yield is influenced by the input pressure p0, the gas t e m p e r a t u r e To and the

H. Dombrowski et al./Nuclear Physics A626 (1997) 427c-433c

43 lc

nozzle geometry. The functional dependence is given by empirical scaling laws like d ~L5 L 0.2 N ~ poTo ~~ \ t - 7 ~ o ]

(1)

with the mean number of atoms per cluster N, the length L, the smallest diameter d and the opening angle of the nozzle 0 [5]. According to this formula the cluster yield is higher at low gas temperatures. Therefore, a two stage refrigerator cold head, which generally allows temperatures below 10 K, is used for the precooling. The cooling power is 45 W at 80 K (firsl stage) and 12 W at 20 K (second stage). This would be sufficient to cool hydrogen of a flow of more than 3 0 m b a r l / s down to 20 K. This flow corresponds to the maximum design value of the target gas throughput. Beside the gas cooling further sources of heat transfer to the cold head are present. In general the cooling power Josses are caused by the mechanism of thermal conductance between mechanical components, by the gas flow between zones of different temperature levels and by thermal radiation. To reduce these losses the surfaces of all cooled parts are designed as small as possible. The coldest part, the top of the coldhead (second stage) with the mounted nozzle, is surrounded by the skimmer stage, which is cooled by the first stage of the cold head to act as heat shield. To minimize the heat transport via mechanical connections the parts which have different temperatures are separated by stainless steel bellows with thicknesses of about 0.1 turn. Because the total required power concerning the heat, losses is in the order of 4 W, the coldhead is able to cool down the nozzle to temperatures below 30K at input pressures of up to 18 bar. To prevent a freeze out of the hydrogen and to adjust the nozzle temperature an electric heating system is installed. For the operation of the target ultra high purity hydrogen with impurities of 0.5ppm is used, generated by a special hydrogen purifier (UCAR/Matheson Modell 8373) which contains a palladium membrane. The arrangement of the nozzle, the skimmer and the collimator (Fig. 2) defines the position and the size of the target beam and is therefore the most critical part of tile target. An external geometrical adjustment of all of this components, mounted on a cold head, is feasible. Furthermore, movements of the components due to thermal expansion of the mechanical parts are possible only in cluster beam direction. Therefore, a deadjustment during the cooling process is avoided.

2.3. Diagnostic tools After the optical alignment of the beam dump to the center of the scattering chamber, the cluster source part is adjusted to the beam dmnp stages by means of two movable rods. These are mounted directly below and above the scattering chamber. When they are moved into the cluster beam one can observe a rise of the scattering chamber pressure, which is proportional to the overlap of the rods with the beam. This method can be used for the measurement of tile cluster beam position and the target width and allows an alignment of the target components by moving and tilting the source part. Additionally, a cluster beam diagnosis can be performed by elastic electron scattering with a 5 k e V electron beam. A movable arm, which contains a complete diagnostic system, allows to scan the target region by moving in a plane perpendicular to the target, beam

432c

H. Dombrowski et al./Nuclear Physics A626 (1997) 427c 433c

cluster \

e> o uo \ .

.

.

.

.

beam

spectrometer /

electron

cup

/

.

///////////////////A

Figure 3. Sketch of the electron scattering monitor system.

direction (Fig. 3). The electron beam is caught in a Faraday cup, which is mounted in the inner part of a spherical electrostatic spectrometer. Electrons, which are elastically scattered at an angle of ~ 21 ° are focussed onto a channeltron behind the Faraday cup for detection. The number of the scattered electrons is proportional to the local areal target density. Thus this system can be used both to measure the spatial areal density distribution of the target beam as well as to monitor the density of the cluster beam during the current experiment. An example of such a density profile is shown in Fig. 4 (see also section 3).

3. E X P E R I M E N T A L R E S U L T S In Fig. 4 a density profile of a hydrogen cluster b e a m is shown. This profile was measured 1.4 m behind the nozzle with the monitor system described above. If the volume density distribution is homogeneous, the areal density is described by a circle function. In contrast, a molecular gas-jet beam has a Gaussian profile. The measured curve fits well to the assumption of a homogeneous cluster beam with a diameter of ~ 19.5 ram. In the scattering chamber a target diameter of ~ 9 m m is expected, because the angular spread amounts to a = 13 mrad. In measurements with the COSY-11 installation the m a x i m u m achieved luminosity amounted to ~ 10a° cm-2s -1. W i t h the known beam current and an estimated beamtarget overlap of 22% a target density of 2 • 1 0 1 4 atorns/crn 2 results. Even at such high densities the pressure in the scattering chamber remains below 1 • 10 - r mbar. I m p o r t a n t parameters in these measurements were: nozzle diameter d = 16 Fro, nozzle t e m p e r a t u r e T = 25 K and hydrogen input pressure p ~ 8 bar. Up to now the target has been in operation for more than 80 days including periods of two weeks of continous operation in the COSY-11 experiment.

4. C O N C L U S I O N S The mechanical assembly of the Miinster cluster target has been presented. It meets the special spatial requirements of the COSY-11 installation and is able to conserve UHV-

H. Dombrowski et al./Nuclear Physics A626 (1997) 427c-433c 6500m

, f , , , , i r , ,i

, r FI~

, , i , , ,i

, ,~'~T,

, F '~T;

'~

433c

,,i,,

'1

nozzle ¢ temperature -pressure

i 6prn 45K 9.7bar

5800

%~5too .~m4400

O o 3700

3000

2300 -24

,,i,,,i,,,i,, -20 -16

~ -12

-fl

-4

0

4

8

, 12

i

, 10

, 20

24

position [mm]

Figure 4. Profile of a cluster beam measured with the monitor system. The fit is based on the assumption of a homogeneous cluster beam with a diameter of 19.5 turn.

conditions in the scattering chamber, while a hydrogen cluster beam with a density of p > 1014a t o m s / c m a passes by. This was reached by a differentially pumped vacuum system in combination with special self-constructed cryopumps. The target density can be checked by an elastic electron scattering monitor system, which also allows to measure the spatial areal density distribution. Due to the special mechanical construction of the cluster source very low gas temperatures in combination with high input pressures are achieved.

ACKNOWLEDGEMENTS The authors would like to thank H.-W. Ortjohann for his support during the design and the installation of the target, Dipl.-Phys. R. Schmidt for technical help during the first test runs and the CERN mechanical workshop for fabricating the nozzles.

REFERENCES 1. E . W . Becker and K. Bier, Z. Naturforsch. 9 A (1954) 975. 2. E. W. Becker, H. D. Falter, O. F. Hagena, W. Hences, R. KlingelhSfer, H. Moser, W. Obert, I. Poth, Proe. Symp. on the Production and Neutralization of Negative Hydrogen Ions and Beams, Brookhaven, 1977, BNL 50 727, p.322. 3. W. Bickel, M. Buschmann, H. Dombrowski, G. Gaul, D. Grzonka, G. Hglker, R. Santo, Nucl. Instr. Meth. A295 (1990) 44. 4. S. Brauksiepe et al., Nucl. Instr. Meth., in press (1996). 5. W. Obert, l l t h Symposium on Rarefied Gas Dynamics, Cannes, 1978, (CEA, Paris, 1978) p.1181.