- Email: [email protected]
Development of internal jet targets for high-luminosity experiments
Nuclear Instruments and Methods in Physics Research A 362 (1995) 20-25 _&!9 ELSEVIER
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SectIonA
Development of internal jet targets for high-luminosity experiments J.E. Doskow, F. Sperisen * Indiana
University
Cyclotron
Faciliry
Bloomington,
IN 47408,
USA
Abstract We report on recent developments of the internal supersonic gas jet target at IUCE The emphasis has been on hydrogen jets that can be used as targets of N 1016atoms/cm* thickness in high-luminosity experiments. A method of scanning jets with an ionizing 1 keV electron beam has been employed in order to study jet formation as a function of nozzle geometry and temperature. We summarize our results, obtained by scanning Hz jets from 23 different nozzles, most of them made from glass capillaries. The present setup of the jet target in the Cooler storage ring is described.
1. Introduction Hydrogen gas jets have been the internal targets most in demand since the startup of the electron-cooled light-ion storage ring (“Cooler”) at the Indiana University Cyclotron Facility a few years ago. It is expected that these jets will continue to play an important role in experiments demanding high luminosity, such as pion production near threshold, or as meson production targets in studies of rare decay modes. In this paper we report on recent development efforts to improve the supersonic jet target. ‘Qpically, for the Cooler, with proton beam energies of a few hundred MeV, the time averaged luminosity is a maximum for a Hz target thickness of N 1016atoms/cm* [ 11. This is nearly two orders of magnitude higher than that of HZcluster jet targets, which have commonly been used internally in storage rings [ 21. As the optimum target thickness increases with beam energy, this shortfall of cluster jets becomes even more severe in storage rings at higher energy. It is mainly to overcome this limitation on the luminosity, that we had decided for a supersonic jet instead for the IUCF Cooler [ 31. A supersonic jet is formed by pushing gas through a converging-diverging (de Laval-type) nozzle which is positioned as close as possible (3-7 mm) to the Cooler beam, so that the latter intercepts the jet where it is dense and narrow (a few mm FWHM). The target thickness can easily be controlled via the flow rate (or pushing pressure) and adjusted to the optimum value. The jet is directed into a cone-shaped catcher tube. A large fraction of the jet flow is removed directly through this catcher. The remainder has to be differentially pumped along the Cooler beam and has
* Corresponding author. Fax +I 812 855 6645. e-mail [email protected]. 0168~9OU2/95/$09.50 @I 1995 Elsevier Science B.V. All rights reserved SSDfO168-9002(95)00245-6
in the past represented 20-40% of the target thickness, distributed mainly in the first pumping stage, within f8 cm of the jet. Obviously, the presence of such background gas is generally undesirable. It can complicate particle detection and luminosity determination, among other problems. This is the trade-off for the higher target thickness compared to that of the cluster jet, which represents a well-localized target, surrounded by very little background gas. The goal of the development described here was to improve the localization of our jet target without sacrificing the capability of teaching optimum thicknesses. The main effort for reducing the background gas was directed toward optimizing the jet formation in the nozzle so as to minimize the gas flow rate needed to achieve the optimum jet thickness. As a diagnostic tool, we developed a method of scanning Hz jets with an ionizing electron beam. A similar method had been used previously at our laboratory to study jets of N2, Ar, and H20 [4].
2. Experimental setup Fig. 1 shows a top view of our experimental setup. The nozzle was mounted on the head of a closed-cycle helium expander refrigerator (Leybold RG1040), flanged to a side port of the main vacuum chamber. The jet was directed horizontally towards the center of the chamber into a coneshaped catcher Nbe which was separated from the nozzle by typically 20 mm and pumped through the side port opposite to the cold head and nozzle. The main chamber,as well as the catcher, were pumped by Nrbomolecular pumps (Balzers TPH 2200 and TPU 510, respectively). Becauseof their low compression ratio for light gases like Hz, these turbo pumps were backed by a second turbo (TPH 1500 and TRU 5 10, respectively).
J.E. Doskow. F. Sperisen/Nucl.
Instr. and Meth. in Phys. Res. A 362 (1995) 20-25
21
TO
ELECTRON GUN
PUMP -
JON COLLECTOR
-0 ‘UMP
E-Beam STOPPER GAS & ELECTRlC FEEDTHROUGHS
cm
Fig. I Top view of the setup used to measure gas jet thickness profiles with an ionizing electron beam.
An electron gun (Cliftronics type 406) was installed in a little chamber, separated from the main chamber by a differential pumping aperture (9 x 9 mm2), and pumped by an oil diffusion pump (with N 150 l/s). The 1 keV electron beam scanned the jet at a right angle. Its current, typically about 0.7 PA, was measured with a 2.5 cm x2.5 cm tantalum beam stopper, movable with a push-pull feedthrough and biased at +40 V. In order to tune the electron gun and to scan its beam profile, the beam was swept across a 0.3 mm hole at the center of the tantalum plate, horizontally and vertically, while measuring the current on a stopping electrode behind it. The beam profile PWHM was slightly less than 1 mm in both dimensions. The gas jet thickness profile was scanned by sweeping the electron beam vertically across the jet and measuring the positive ion current with a collector electrode at the entrance of the catcher tube. The collector consisted of two concentric rings ( 11 and 25 mm diameter) in a plane perpendicular to the jet axis, held together by three radial wires, designed to minimize interference with the jet. We determined experimentally that the collector had to be biased at -50 V to ensure complete col-
lection of ions from the jet. Jet scanning was automated by a computer (IBM PC) and a programmable electrometer (Keithley model 617) which, for a preset range and step size, put out the voltage to the vertical e-beam deflector and read in the ion current on the collector. Background scans were done by leaking gas elsewhere into the chamber, adjusted to maintain the same background pressure. The jet thickness t = ii / (i,a) , where & and ii are the electron beam and background subtracted ion current, and CT= 2.3 x lo-l7 cm2 is the total ionization cross section for 1 keV electrons on HZ [ 51. As a check on this method, we compared our results with a measurement based on small angle elastic scattering of 25 keV electrons [6], using the same nozzle at room temperature. The thickness profiles agree within the estimated 20% uncertainty of each method.
3. Nozzles All but one of the nozzles used in the present study were made from glass capillaries (the only exception being the
1. INTERNALTARGETS
22
J.E. Doskow, F. Sperisen/
l-
Nucl. Ins~r. and Ueth. in Phys. Res. A 362 (1995) 20-25
I-Zmm Fig. 3. Schematicdrawing of glass nozzle.
copper nozzle used previously for the jet target [3] ). Our development of glass nozzles was initiated mainly by the need for a wide range of nozzle throat diameters and other geometrical parameters. Such nozzles could be made at low cost and at short notice as the investigation progressed. Furthermore, the interior surface finish was much smoother than that of the copper nozzle. The glass used was standard laboratory capillary tubing (Kimax or equivalent) with nominal 6.4 mm outer diameter and l-2 mm inner diameter. Stock was selected for natrow o.d. range to fit in our preparing and mounting fixtures, and also for well centered i.d. with respect to the outside of the tube. The nozzle shape was created using a simple fixture and a natural gas/compressed air torch (see Fig. 2). A small area of the capillary, -35 mm from its end, was heated by the tip of the flame while being held in a vertical position. The tube was slowly rotated as it was being heated to keep it symmetrical. The weight of the glass below the small heated area caused the stretching in that area, and the partial closing of the bore. The timing of the removal of the flame was the critical factor in the resulting throat diameter (see Fig.
Fig. 2. Glass nozzle const~ction fixture with torch.
3). The smallest throat nozzles were made by allowing the tube to almost appear closed before removing the torch. The length of the converging and the diverging part were affected by how far from the tube’s end the nozzle was created, and how fine a flame tip was used at contact to the tube. Using this method, a particular nozzle geometry could not be achieved in just one attempt. However, a variety of shapes could be obtained rather quickly (N 12/ hour), and nozzles could then be picked out with the desired characteristics. Rough measurements of the throat diameter were made by sliding fine wines of known size through the bores. (This dimension could not be determined by microscopic measutement from the side, due to distortion effects of the complex curvature of the outside of the tube.) Then, nozzles of desired dimensions were cut to the right overall length, using a fine water cooled diamond disc saw. The throat diameter could now be measured with a microscope looking down the nozzle’s bore. Independently, and more accurately, the effective throat diameter was determined from a measurement of gas flow rate versus pushing pressure at known nozzle temperature, and a calculation assuming isentropic expansion through the nozzle. (With the nozzle cooled to 40 K and flow rates up to 1.15 x 102’ molecules/s, maximum pushing pressures ranged from 0.066 to 5.4 bar, depending on the nozzle geometry.) These results agreed with those from the microscope within a few ym, but only when the microscope measurements were obtained after the nozzle was cut at the narrowest point, i.e. the throat diameter could be seen with the microscope at the surface. The nozzles were mounted in the test setup (Fig. 1) , using an indium wire, compressed between it and a beveled seat, for sealing down to cryogenic temperatures (see Fig. 4). The upper copper mounting piece kept the nozzle over the entire length at the desired temperature, which was measured with a platinum resistance thermometer (mod. PT102; Lakeshore, Westerville, OH 43081, USA) and was adjustable between 25 and 300 K with a heater resistor. Thermometer sensor and heater resistor were both sunk into the lower part of the fixture. Also shown in Fig. 4 is the copper nozzle of our previous jet target [ 3 1. In order to study effects on the jet from changes of the
J.E. Doskow, F. Sperisen / Nucl. Instr. and Meth. in Phys. Res. A 362 (1995) 20-25
To (K)
Fig. 4. Cross-sectional and perspective view of the glass nozzle with copper fixture (top),and of the previous copper nozzle (bottom).
divergent part, some nozzles were removed from the test setup and modified repeatedly. For example, the length of the divergent part was gradually shortened by cutting. Other nozzles were re-heated to change the opening angle at the exit. A variety of “blunt” nozzles were also made, by reducing the tube bore just at the end to create nozzles with a very short and wide angled divergence.
00
100
1
200
300
T, (K) Fig. 6. Temperature dependence of the peak thickness (top) and the width (bottom) of an H2 jet profile from same nozzle as for Fig. 5. The distance from the nozzle was 5 mm and the flow rate was 1.04x 10” molecules/s.
4. Results We have scanned jets from 23 different nozzles, most of them transversely at 5 mm from the nozzle exit (typically the distance at which the Cooler beam interacts with the jet) as a function of nozzle temperature below 100 K. In a few cases, we also measured jet profiles as a function of distance from the nozzle, of nozzle flow rate, or of temperature up to 300 K. As an example, Fig. 5 shows a thickness profile of a jet from a nozzle with a throat diameter of 108 pm, diverging over 6 mm to an exit diameter of 1.4 mm. Each of the measuredprofiles was fitted with a bell-shaped
“E 4
6m
10:
++ + +
8I-
z S a (D -0
4r
0
2_-
6_
+ + +
+
++
0
++ i++++ ~~++L~~~~~~~++I++?+ -5 0
+ + + + + ++++ 5
Y (mm) Fig. 5. Thickness profile of an Hz jet from a 108 pm nozzle. measured with a 1 keV electron beam scanning the jet transversely (.v axis) 5 mm from the nozzle tip. The nozzle was cooled to 38 K and the Row rate was 1.04 x 102” molecules/s (corresponding to a pushing pressure of 267 mbar).
function in order to determine the peak thickness and the width. The results can be broadly summarized as follows. First, our original expectation that narrower nozzles (compared to the previous 110 pm throat diameter copper nozzle, see Fig. 4) would produce more forward peaked jet flows and greater thickness, was not born out by our experiments. We used nozzles with throat diameters, d,, ranging from 24 to 220 pm. The optimum dl appears to be in the range of 90-l 20 pm. There, we observed the narrowest jets with the highest peak thickness (for an example see Fig. 5). With regards to other parameters of the nozzle geometry, such as the length and opening angle of the divergent part, we have not observed any pronounced effect on the jet profile. The measured temperature dependence of the peak thickness, tpeb. and the width of a jet from the same nozzle as that of Fig. 5 is shown in Fig. 6. The thickness increases with decreasing temperature until it reaches a plateau at about 40 K, while the jet width narrows until it is minimum at 40 K, then widens again. Obviously, the optimum temperature for this nozzle is 40 K. Generally, the optimum temperature was found to depend somewhat on the nozzle diameter, decreasing with increasing d,. For our widest nozzles, the onset of the tpeakplateau and the minimum width were apparently below the lowest temperature we could reach (-25 K).On the other hand, the thickness of jets from our narrowest nozzles leveled off at higher temperatures (-30% below the highest values). Their width did not have the pronounced minimum shown in Fig. 6. Possibly, viscous effects are more
I. INTERNALTARGETS
J. E. Doskow, F. Sperisen / Nucl. Instr. and Ueth. in Phys. Res. A 362 (1995) 20-25
24
.c
BEAM
BEAM
-.c
POSlTlON’
MONl 7 ORS
0
50
cm
Fig. 7. Side view of the jet target setup in the Cooler T section. The differential pumping stagesare labeled by numbers.Stages 2 and 3 each have pomps in common for regions upstream and downstreamof the nozzle. The pumping speedsgiven are nominal values for Hz,
important for narrower nozzles. For comparison with the glass nozzles, we also scanned jets from our previously used 110 ,um copper nozzle [ 3 1. Surprisingly, considering the ragged wall surfaceof this nozzle, the results are quite similar to those shown in Figs. 5 and 6, with the maximum TV being just 7% lower. Finally, we briefly measured another factor affecting the background gas pressure, namely the catcher efficiency, i.e., the fraction of the jet flow removed through the catcher. Measurements done with two catcher tubes (opening diameter: 26 and 29 mm) over a nozzle temperature range of 30 to 90 K indicated little dependence on these two parameters, with a value of 60% for the 26 mm catcher at 40 K. (For heavier gases we observed higher catcher efficiencies: 68% for deuterium, 89% for nitrogen). 5. Jet target in Cooler The jet target in the Cooler [3] requires a considerable amount of differential pumping along the stored beam. The system is of a modular design, making it possible to install
it into rectangular vacuum chambers in the G and T straight sections of the Cooler. Depending on the experimental requirements, mainly regarding the acceptance range of forward going particles, the jet can be installed in three different positions along the beam axis. Recently, the jet target, with the 108 pm glass nozzle, described above for Figs. 5 and 6, has been installed in the Cooler T section for experiments CE38 (pp’ --+ pnrr+ ) and CE49 (pionium production in pd) (see Fig. 7). These experiments use the 6” spectrometer magnet, just downstream of the target. Four differential pumping stages, indicated by numbers in Fig. 7, are separated by 12 pm thick aluminized mylar foils with openings, 18.9 mm wide and 14.2 mm high, for the stored beam to pass through. They are pumped by five turbo and four cryo pumps; two more turbos are used for the catcher and as backing pump on the first stage. In addition, blower pumps are in the fore line of catcher and stage 1, and of stage 2. Based on pressure measurements with a capacitance sensor (mod. Baratron 390H; MKS, Andover, MA 01810, USA) we estimate that the background gas now accounts
25
J. E. Doskow, F. Sperisen / Nucl. Instr. and Meth. in Phys. Rex A 362 (1995) 20-22
for lo-15% of the total target thickness. The jet target with the new glass nozzle performed without problems during several runs, and jet profiles from the luminosity monitor, based on pp and pd elastic scattering [3], will soon be available.
Acknowledgements We would like to thank Richard Ghrist for his help with the development of the experimental method, Tom Rinckel for implementing the PC based scanning controls, and Bill Lozowski for his assistance with the preparation of the glass nozzles.
6. Conclusion We found the optimum nozzle throat diameter to be in the range of 90-120 pm. No dramatic effects from other geometrical parameters of the nozzle were observed. More important seems to be the dependence on the nozzle temperature, the optimum value of which appears to be about 40 K. Any further significant improvement in HZ jet localization would probably have to come from a more radical redesign of the jet target. Background gas could be reduced with one or two jet skimmers between nozzle and the target chamber, and with differential pumping along the jet, similar to the design used for cluster jet targets [ 21. However, this would increase the distance from nozzle to target point considerably, and thus would imply correspondingly higher nozzle flow rates in order to maintain optimum target thickness.
References Ill RX. Pollock et al., Nucl. Instr. and Metb. A 330 (1993) 380. 121C. Ekstrbm, these Proceedings ( 17th World Conf. of the INTDS, Bloomington, IN, USA, 1994) Nucl. Instr. and Meth. A 362 ( 1995) I. [31 F. Sperisen et al., IUCF Scientific and Technical Report, January 1987-
April 1988, p. 194; F. Sperisen et al., IUCF Scientific and Technical Report. May 1989April 1990. p, 117. 141 1. Wellman et al., IUCF Scientific and Technical Report, May 1990April 1991, p. 168; T.W. Bowyer et al., ibid., p. 166. 151 L.J. Kiefer and G.H. Dunn, Rev. Mod. Phys. 38 (1966) 1. 161 F. Sperisen et al., Nucl. Instr. and Meth. A 274 ( 1989) 604.
I. INTERNAL
TARGETS
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SectIonA
Development of internal jet targets for high-luminosity experiments J.E. Doskow, F. Sperisen * Indiana
University
Cyclotron
Faciliry
Bloomington,
IN 47408,
USA
Abstract We report on recent developments of the internal supersonic gas jet target at IUCE The emphasis has been on hydrogen jets that can be used as targets of N 1016atoms/cm* thickness in high-luminosity experiments. A method of scanning jets with an ionizing 1 keV electron beam has been employed in order to study jet formation as a function of nozzle geometry and temperature. We summarize our results, obtained by scanning Hz jets from 23 different nozzles, most of them made from glass capillaries. The present setup of the jet target in the Cooler storage ring is described.
1. Introduction Hydrogen gas jets have been the internal targets most in demand since the startup of the electron-cooled light-ion storage ring (“Cooler”) at the Indiana University Cyclotron Facility a few years ago. It is expected that these jets will continue to play an important role in experiments demanding high luminosity, such as pion production near threshold, or as meson production targets in studies of rare decay modes. In this paper we report on recent development efforts to improve the supersonic jet target. ‘Qpically, for the Cooler, with proton beam energies of a few hundred MeV, the time averaged luminosity is a maximum for a Hz target thickness of N 1016atoms/cm* [ 11. This is nearly two orders of magnitude higher than that of HZcluster jet targets, which have commonly been used internally in storage rings [ 21. As the optimum target thickness increases with beam energy, this shortfall of cluster jets becomes even more severe in storage rings at higher energy. It is mainly to overcome this limitation on the luminosity, that we had decided for a supersonic jet instead for the IUCF Cooler [ 31. A supersonic jet is formed by pushing gas through a converging-diverging (de Laval-type) nozzle which is positioned as close as possible (3-7 mm) to the Cooler beam, so that the latter intercepts the jet where it is dense and narrow (a few mm FWHM). The target thickness can easily be controlled via the flow rate (or pushing pressure) and adjusted to the optimum value. The jet is directed into a cone-shaped catcher tube. A large fraction of the jet flow is removed directly through this catcher. The remainder has to be differentially pumped along the Cooler beam and has
* Corresponding author. Fax +I 812 855 6645. e-mail [email protected]. 0168~9OU2/95/$09.50 @I 1995 Elsevier Science B.V. All rights reserved SSDfO168-9002(95)00245-6
in the past represented 20-40% of the target thickness, distributed mainly in the first pumping stage, within f8 cm of the jet. Obviously, the presence of such background gas is generally undesirable. It can complicate particle detection and luminosity determination, among other problems. This is the trade-off for the higher target thickness compared to that of the cluster jet, which represents a well-localized target, surrounded by very little background gas. The goal of the development described here was to improve the localization of our jet target without sacrificing the capability of teaching optimum thicknesses. The main effort for reducing the background gas was directed toward optimizing the jet formation in the nozzle so as to minimize the gas flow rate needed to achieve the optimum jet thickness. As a diagnostic tool, we developed a method of scanning Hz jets with an ionizing electron beam. A similar method had been used previously at our laboratory to study jets of N2, Ar, and H20 [4].
2. Experimental setup Fig. 1 shows a top view of our experimental setup. The nozzle was mounted on the head of a closed-cycle helium expander refrigerator (Leybold RG1040), flanged to a side port of the main vacuum chamber. The jet was directed horizontally towards the center of the chamber into a coneshaped catcher Nbe which was separated from the nozzle by typically 20 mm and pumped through the side port opposite to the cold head and nozzle. The main chamber,as well as the catcher, were pumped by Nrbomolecular pumps (Balzers TPH 2200 and TPU 510, respectively). Becauseof their low compression ratio for light gases like Hz, these turbo pumps were backed by a second turbo (TPH 1500 and TRU 5 10, respectively).
J.E. Doskow. F. Sperisen/Nucl.
Instr. and Meth. in Phys. Res. A 362 (1995) 20-25
21
TO
ELECTRON GUN
PUMP -
JON COLLECTOR
-0 ‘UMP
E-Beam STOPPER GAS & ELECTRlC FEEDTHROUGHS
cm
Fig. I Top view of the setup used to measure gas jet thickness profiles with an ionizing electron beam.
An electron gun (Cliftronics type 406) was installed in a little chamber, separated from the main chamber by a differential pumping aperture (9 x 9 mm2), and pumped by an oil diffusion pump (with N 150 l/s). The 1 keV electron beam scanned the jet at a right angle. Its current, typically about 0.7 PA, was measured with a 2.5 cm x2.5 cm tantalum beam stopper, movable with a push-pull feedthrough and biased at +40 V. In order to tune the electron gun and to scan its beam profile, the beam was swept across a 0.3 mm hole at the center of the tantalum plate, horizontally and vertically, while measuring the current on a stopping electrode behind it. The beam profile PWHM was slightly less than 1 mm in both dimensions. The gas jet thickness profile was scanned by sweeping the electron beam vertically across the jet and measuring the positive ion current with a collector electrode at the entrance of the catcher tube. The collector consisted of two concentric rings ( 11 and 25 mm diameter) in a plane perpendicular to the jet axis, held together by three radial wires, designed to minimize interference with the jet. We determined experimentally that the collector had to be biased at -50 V to ensure complete col-
lection of ions from the jet. Jet scanning was automated by a computer (IBM PC) and a programmable electrometer (Keithley model 617) which, for a preset range and step size, put out the voltage to the vertical e-beam deflector and read in the ion current on the collector. Background scans were done by leaking gas elsewhere into the chamber, adjusted to maintain the same background pressure. The jet thickness t = ii / (i,a) , where & and ii are the electron beam and background subtracted ion current, and CT= 2.3 x lo-l7 cm2 is the total ionization cross section for 1 keV electrons on HZ [ 51. As a check on this method, we compared our results with a measurement based on small angle elastic scattering of 25 keV electrons [6], using the same nozzle at room temperature. The thickness profiles agree within the estimated 20% uncertainty of each method.
3. Nozzles All but one of the nozzles used in the present study were made from glass capillaries (the only exception being the
1. INTERNALTARGETS
22
J.E. Doskow, F. Sperisen/
l-
Nucl. Ins~r. and Ueth. in Phys. Res. A 362 (1995) 20-25
I-Zmm Fig. 3. Schematicdrawing of glass nozzle.
copper nozzle used previously for the jet target [3] ). Our development of glass nozzles was initiated mainly by the need for a wide range of nozzle throat diameters and other geometrical parameters. Such nozzles could be made at low cost and at short notice as the investigation progressed. Furthermore, the interior surface finish was much smoother than that of the copper nozzle. The glass used was standard laboratory capillary tubing (Kimax or equivalent) with nominal 6.4 mm outer diameter and l-2 mm inner diameter. Stock was selected for natrow o.d. range to fit in our preparing and mounting fixtures, and also for well centered i.d. with respect to the outside of the tube. The nozzle shape was created using a simple fixture and a natural gas/compressed air torch (see Fig. 2). A small area of the capillary, -35 mm from its end, was heated by the tip of the flame while being held in a vertical position. The tube was slowly rotated as it was being heated to keep it symmetrical. The weight of the glass below the small heated area caused the stretching in that area, and the partial closing of the bore. The timing of the removal of the flame was the critical factor in the resulting throat diameter (see Fig.
Fig. 2. Glass nozzle const~ction fixture with torch.
3). The smallest throat nozzles were made by allowing the tube to almost appear closed before removing the torch. The length of the converging and the diverging part were affected by how far from the tube’s end the nozzle was created, and how fine a flame tip was used at contact to the tube. Using this method, a particular nozzle geometry could not be achieved in just one attempt. However, a variety of shapes could be obtained rather quickly (N 12/ hour), and nozzles could then be picked out with the desired characteristics. Rough measurements of the throat diameter were made by sliding fine wines of known size through the bores. (This dimension could not be determined by microscopic measutement from the side, due to distortion effects of the complex curvature of the outside of the tube.) Then, nozzles of desired dimensions were cut to the right overall length, using a fine water cooled diamond disc saw. The throat diameter could now be measured with a microscope looking down the nozzle’s bore. Independently, and more accurately, the effective throat diameter was determined from a measurement of gas flow rate versus pushing pressure at known nozzle temperature, and a calculation assuming isentropic expansion through the nozzle. (With the nozzle cooled to 40 K and flow rates up to 1.15 x 102’ molecules/s, maximum pushing pressures ranged from 0.066 to 5.4 bar, depending on the nozzle geometry.) These results agreed with those from the microscope within a few ym, but only when the microscope measurements were obtained after the nozzle was cut at the narrowest point, i.e. the throat diameter could be seen with the microscope at the surface. The nozzles were mounted in the test setup (Fig. 1) , using an indium wire, compressed between it and a beveled seat, for sealing down to cryogenic temperatures (see Fig. 4). The upper copper mounting piece kept the nozzle over the entire length at the desired temperature, which was measured with a platinum resistance thermometer (mod. PT102; Lakeshore, Westerville, OH 43081, USA) and was adjustable between 25 and 300 K with a heater resistor. Thermometer sensor and heater resistor were both sunk into the lower part of the fixture. Also shown in Fig. 4 is the copper nozzle of our previous jet target [ 3 1. In order to study effects on the jet from changes of the
J.E. Doskow, F. Sperisen / Nucl. Instr. and Meth. in Phys. Res. A 362 (1995) 20-25
To (K)
Fig. 4. Cross-sectional and perspective view of the glass nozzle with copper fixture (top),and of the previous copper nozzle (bottom).
divergent part, some nozzles were removed from the test setup and modified repeatedly. For example, the length of the divergent part was gradually shortened by cutting. Other nozzles were re-heated to change the opening angle at the exit. A variety of “blunt” nozzles were also made, by reducing the tube bore just at the end to create nozzles with a very short and wide angled divergence.
00
100
1
200
300
T, (K) Fig. 6. Temperature dependence of the peak thickness (top) and the width (bottom) of an H2 jet profile from same nozzle as for Fig. 5. The distance from the nozzle was 5 mm and the flow rate was 1.04x 10” molecules/s.
4. Results We have scanned jets from 23 different nozzles, most of them transversely at 5 mm from the nozzle exit (typically the distance at which the Cooler beam interacts with the jet) as a function of nozzle temperature below 100 K. In a few cases, we also measured jet profiles as a function of distance from the nozzle, of nozzle flow rate, or of temperature up to 300 K. As an example, Fig. 5 shows a thickness profile of a jet from a nozzle with a throat diameter of 108 pm, diverging over 6 mm to an exit diameter of 1.4 mm. Each of the measuredprofiles was fitted with a bell-shaped
“E 4
6m
10:
++ + +
8I-
z S a (D -0
4r
0
2_-
6_
+ + +
+
++
0
++ i++++ ~~++L~~~~~~~++I++?+ -5 0
+ + + + + ++++ 5
Y (mm) Fig. 5. Thickness profile of an Hz jet from a 108 pm nozzle. measured with a 1 keV electron beam scanning the jet transversely (.v axis) 5 mm from the nozzle tip. The nozzle was cooled to 38 K and the Row rate was 1.04 x 102” molecules/s (corresponding to a pushing pressure of 267 mbar).
function in order to determine the peak thickness and the width. The results can be broadly summarized as follows. First, our original expectation that narrower nozzles (compared to the previous 110 pm throat diameter copper nozzle, see Fig. 4) would produce more forward peaked jet flows and greater thickness, was not born out by our experiments. We used nozzles with throat diameters, d,, ranging from 24 to 220 pm. The optimum dl appears to be in the range of 90-l 20 pm. There, we observed the narrowest jets with the highest peak thickness (for an example see Fig. 5). With regards to other parameters of the nozzle geometry, such as the length and opening angle of the divergent part, we have not observed any pronounced effect on the jet profile. The measured temperature dependence of the peak thickness, tpeb. and the width of a jet from the same nozzle as that of Fig. 5 is shown in Fig. 6. The thickness increases with decreasing temperature until it reaches a plateau at about 40 K, while the jet width narrows until it is minimum at 40 K, then widens again. Obviously, the optimum temperature for this nozzle is 40 K. Generally, the optimum temperature was found to depend somewhat on the nozzle diameter, decreasing with increasing d,. For our widest nozzles, the onset of the tpeakplateau and the minimum width were apparently below the lowest temperature we could reach (-25 K).On the other hand, the thickness of jets from our narrowest nozzles leveled off at higher temperatures (-30% below the highest values). Their width did not have the pronounced minimum shown in Fig. 6. Possibly, viscous effects are more
I. INTERNALTARGETS
J. E. Doskow, F. Sperisen / Nucl. Instr. and Ueth. in Phys. Res. A 362 (1995) 20-25
24
.c
BEAM
BEAM
-.c
POSlTlON’
MONl 7 ORS
0
50
cm
Fig. 7. Side view of the jet target setup in the Cooler T section. The differential pumping stagesare labeled by numbers.Stages 2 and 3 each have pomps in common for regions upstream and downstreamof the nozzle. The pumping speedsgiven are nominal values for Hz,
important for narrower nozzles. For comparison with the glass nozzles, we also scanned jets from our previously used 110 ,um copper nozzle [ 3 1. Surprisingly, considering the ragged wall surfaceof this nozzle, the results are quite similar to those shown in Figs. 5 and 6, with the maximum TV being just 7% lower. Finally, we briefly measured another factor affecting the background gas pressure, namely the catcher efficiency, i.e., the fraction of the jet flow removed through the catcher. Measurements done with two catcher tubes (opening diameter: 26 and 29 mm) over a nozzle temperature range of 30 to 90 K indicated little dependence on these two parameters, with a value of 60% for the 26 mm catcher at 40 K. (For heavier gases we observed higher catcher efficiencies: 68% for deuterium, 89% for nitrogen). 5. Jet target in Cooler The jet target in the Cooler [3] requires a considerable amount of differential pumping along the stored beam. The system is of a modular design, making it possible to install
it into rectangular vacuum chambers in the G and T straight sections of the Cooler. Depending on the experimental requirements, mainly regarding the acceptance range of forward going particles, the jet can be installed in three different positions along the beam axis. Recently, the jet target, with the 108 pm glass nozzle, described above for Figs. 5 and 6, has been installed in the Cooler T section for experiments CE38 (pp’ --+ pnrr+ ) and CE49 (pionium production in pd) (see Fig. 7). These experiments use the 6” spectrometer magnet, just downstream of the target. Four differential pumping stages, indicated by numbers in Fig. 7, are separated by 12 pm thick aluminized mylar foils with openings, 18.9 mm wide and 14.2 mm high, for the stored beam to pass through. They are pumped by five turbo and four cryo pumps; two more turbos are used for the catcher and as backing pump on the first stage. In addition, blower pumps are in the fore line of catcher and stage 1, and of stage 2. Based on pressure measurements with a capacitance sensor (mod. Baratron 390H; MKS, Andover, MA 01810, USA) we estimate that the background gas now accounts
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for lo-15% of the total target thickness. The jet target with the new glass nozzle performed without problems during several runs, and jet profiles from the luminosity monitor, based on pp and pd elastic scattering [3], will soon be available.
Acknowledgements We would like to thank Richard Ghrist for his help with the development of the experimental method, Tom Rinckel for implementing the PC based scanning controls, and Bill Lozowski for his assistance with the preparation of the glass nozzles.
6. Conclusion We found the optimum nozzle throat diameter to be in the range of 90-120 pm. No dramatic effects from other geometrical parameters of the nozzle were observed. More important seems to be the dependence on the nozzle temperature, the optimum value of which appears to be about 40 K. Any further significant improvement in HZ jet localization would probably have to come from a more radical redesign of the jet target. Background gas could be reduced with one or two jet skimmers between nozzle and the target chamber, and with differential pumping along the jet, similar to the design used for cluster jet targets [ 21. However, this would increase the distance from nozzle to target point considerably, and thus would imply correspondingly higher nozzle flow rates in order to maintain optimum target thickness.
References Ill RX. Pollock et al., Nucl. Instr. and Metb. A 330 (1993) 380. 121C. Ekstrbm, these Proceedings ( 17th World Conf. of the INTDS, Bloomington, IN, USA, 1994) Nucl. Instr. and Meth. A 362 ( 1995) I. [31 F. Sperisen et al., IUCF Scientific and Technical Report, January 1987-
April 1988, p. 194; F. Sperisen et al., IUCF Scientific and Technical Report. May 1989April 1990. p, 117. 141 1. Wellman et al., IUCF Scientific and Technical Report, May 1990April 1991, p. 168; T.W. Bowyer et al., ibid., p. 166. 151 L.J. Kiefer and G.H. Dunn, Rev. Mod. Phys. 38 (1966) 1. 161 F. Sperisen et al., Nucl. Instr. and Meth. A 274 ( 1989) 604.
I. INTERNAL
TARGETS