A Conceptual Design of the South Muon Arm Magnet

W. Wayne Kinnison, Walter Sondheim, Melynda Brooks, Joel M. Moss, and P.M. McGaughey

Los Alamos National Laboratory, Los Alamos, NM 87545

Abstract
Physics Goals of the South Muon Arm of PHENIX
The Conceptual Design
Physics Performance of the South Muon Arm Magnet
Summary and Conclusions
APPENDIX A -- Drawings of Magnet

Abstract

A conceptual design of the muon magnet for the South Muon Arm of PHENIX is presented. An overview of the physics and mechanical constraints are described as well as the results of magnetic and physics performance studies as compared to the North Muon Arm magnet.

Physics Goals of the South Muon Arm of PHENIX

A second end-cap for the detection of dimuons in PHENIX, known as the South Muon Arm, was proposed as part of the PHENIX/Spin proposal. More details about the physics goals of PHENIX[1] and PHENIX/Spin[2] can be found elsewhere. For review purposes only, the major physics goals which utilize the detection of muons in the end-caps will be presented here with the emphasis being on how the physics goals drive the muon magnet designs.

The primary driver for the South Muon Arm of PHENIX is the enhancement of the polarized collider experimental program. It significantly increases the dimuon acceptance of the detector. The second end-cap not only essentially doubles the acceptance of low-mass dimuon pairs where both muons go into the same end-cap, but it also increases the mass range available for study and extends the rapidity coverage for dimuons by allowing for one muon to go into each of the end-caps.

A second goal of the South Muon Arm is to enhance the pA, pp, and AA physics programs of PHENIX in non-polarized beam operations at RHIC. Again, the advantages of the second muon arm are significantly enhanced dimuon rates for the measurement of vector mesons, low- and high-mass (Drell-Yan) dimuon pairs. The measurement of the possible suppression of some vector mesons with respect to other nearby vector mesons places a design criteria on the desired dimuon mass resolution obtained by the muon arms. To separate the J/ resonance from the ^' by approximately 6 a mass resolution at the J/ of about 100 MeV/c² is required. To separate the (1s) from the (2s+3s) by approximately 3, the mass resolution at the needs to be 190 MeV/c². This document describes a design of the South Muon Arm Magnet directed towards achieving these goals.

The Conceptual Design

Design Criteria

A conceptual design for the muon magnet for the South Muon Arm has been generated. The design was based on the following criteria:

The mass and momentum resolutions for muons in the South Muon Arm Magnet should be at least as good as the same quantities as measured in the North Muon Arm Magnet.
The South Muon Arm should be as near to identical to the North Muon Arm as is reasonably possible. That design goal helps to reduce EDIA of not only the magnet but also various other detector sub-systems.
The South Muon Arm magnet must be movable and it should be 30 inches above the floor in order for it to use the same style of rail system as is used by the central magnet and the central arm carriages.
The South Muon Arm magnet must be approximately 1.5 meters shorter than the North Muon Arm magnet. This is an integration issue. The South Muon Arm Magnet must be movable and allow for the Central Arm Magnet to be rolled 1.5 meters south of its nominal operating position in order to provide service access on the North Arm and to allow for clearance to roll both magnets out of the detector hall.
No individual piece of the South Arm Muon Magnet should exceed the 40-ton lifting capability of the existing overhead crane in the PHENIX detector hall. This ensures that the South Arm Muon Magnet can be assembled in the detector hall after the other major rigging operations have been completed and possibly after the larger lifting capability is no longer readily available.

The Conceptual Design

With the above criteria in mind, a conceptual design was carried out. Appendix A provides engineering drawings of that design. The design presented here has two coils mounted to the piston. The conceptual design of those two coils is based upon the requirements of the physics program. In designing the coils and the piston, models were produced which kept as much steel as possible under the coils to reduce the saturation. That permits higher current densities (as compared to the North Arm Muon Magnet) resulting in a higher magnetic field. In order to reduce the saturation of the steel under the coils and thus get an

which is nearly the same as for the North Muon Magnet, the piston was designed to be at a 12deg. angle. (The North Muon Arm Magnet has a 10deg. piston.) The design parameters for the two coils are as follows:

 Rear Coil:     Current density: 6,428,570 A/m2              
                Cross sectional area: 0.04998 m2             
                Current: 321,300.0 A                         
                Cross-sectional dimension: 8.33 cm thick x   
                60.0 cm-long                                 
                                                             
Front Coil:     Current density: 2,857,140 A/m2              
                Cross sectional area: 0.023334 m2            
                Current: 66,668.5 A                          
                Cross sectional dimension: 3.889 cm thick    
                x 60.0 cm long

Several different shapes for the front of the piston were considered. The drawings included in Appendix A show a piston with a 45-degree chamfer starting at 30-cm in radius. Since, as will be shown below, that configuration presents the "best" field shapes in the region of the nose of the piston, it is considered the baseline design.

The Magnet Performance

Figures 2.1, 2.2, and 2.3 show the results of simulating the magnet using the Vector Fields Opera2d software for this "Chamfer-30" model. As stated above, this model is drawn in Appendix A. Figure 2.1 shows the magnetic field strength in the air of the magnetic volume, Figure 2.2 shows the saturation of the iron, and Figure 2.3 shows the potential lines throughout the magnetic volume. The normal component of the field along lines at 12.5deg. and 15deg. were also calculated and are shown in Figure 2.4.

Figure 2.1: The magnetic field strength in the air region with the proposed piston.

Figure 2.2: The saturation of the iron for the proposed piston model

Figure 2.3: The potential lines through-out the magnetic volume for the proposed piston model.

Figure 2.4: The value of the normal component of the magnetic field along a line at 12.5deg. and a line at 15deg. in the magnet.

The reason for the chamfered nose on the piston tip is shown in Figure 2.5. The figure shows the component of the magnetic field perpendicular to a line at 12.5deg. in the region of the piston tip for four possible configurations of the end of the piston. The "30-chamfer" configuration has the end of the piston chamfered with a 45deg. cut starting at a radius of 30 cm. In the "35-chamfer" configuration, the chamfer starts at 35 cm. The "rounded" configuration has the end of the piston completely rounded off, and the "snubbed" design has it no chamfer at all. As the plot shows, the region of very high field is moved farther from the Station #1 chamber location (from about 1.85 to about 1.95 m) than the other configurations. The absolute size of the dip is also reduced. Therefore, the "30-chamfer" configuration is the preferred design. Figures 2.6 and 2.7 show the magnetic field intensities in the region of the end of the piston for the two extreme cases in order to demonstrate how the chamfer helps to both smooth the field and pull the high-field region away from the chamber location. The conclusion to be drawn is that the magnet should be designed in such a way as to have sharp corners as far as possible from the active regions of the Station #1 and Station #2 chambers.

Figure 2.5: The component of the magnet field perpendicular to a line at 12.5deg. is shown for four possible configurations for the end of the piston as described in the text.

Figure 2.6: The magnetic field potential near the nose of the piston which has been chamfered with a 45-degree cut at a radius of 30 cm (the "30-chamfer" configuration).

Figure 2.7: The magnetic field strength in the region of the piston tip which has been snubbed off.

The line integral has been calculated for this design with all of the above models of the piston. Table 2.1 presents the results of that part of the simulation. The overall performance of this magnet is very similar to the North Muon Arm Magnet.

Table 2.1: The integral of the normal component of the magnetic field along a line ()from a z-position of 1.8 meters, which is the approximate location of the Station #1 chambers, to a z-position of 4.79 meters, which is the approximate location of the Station #3 chambers. The integral is calculated for the various piston models described in the text along five different lines. The integral is also calculated. (The units are kGauss-m and kGauss-m²).

   Angle  12.5deg. 15.0deg. 20.0deg. 25.0deg. 30deg.    
                       "Snubbed" Model                                         
   -10.002  -7.738    -4.937    -3.442    -2.549    
   -29.578  -22.923   -14.048   -9.480    -6.821    
                      "35-chamfer" Model                                       
   -9.996  -7.695    -4.940    -3.449    -2.558    
  -29.957  -22.843   -14.084   -9.503    -6.847    
                      "30-chamfer" Model                                       
   -9.889  -7.680    -4.938    -3.457    -2.564    
  -30.371  -22.918   -14.108   -9.536    -6.871

Physics Performance of the South Muon Arm Magnet

A study was carried out to compare the physics performance of the proposed design of the South Muon Arm Magnet in comparison to the North Muon Arm Magnet. That work is reported in another PHENIX Technical Note and will only be summarized here.[3] It should be noted that in the study, the North Arm copper absorber was 28-cm thick and the South Arm copper absorber was 12-cm thick. The final configuration of copper absorbers is yet to be determined, but it is more likely to be symmetric. In the study the chambers in the North Arm were located at z positions of 180, 348, and 600 cm from the nominal PHENIX interaction vertex. The South Arm had chambers located at 180, 288, and 450 cm from the vertex.

Figure 3.1 shows a plot of the mass resolutions obtained at the , J/, and in both the North Arm and the South Arm with 100-um chamber resolutions. As can be seen from the figure, the South Arm performance in the regions of the and J/ is at least as good as the North Arm. The fact that the South Arm appears to be a little better than the North Arm is due to the reduced copper absorber thickness. A study has shown that the momentum resolution for the and the J/ muons is the same in both arms. Consequently one can safely assume that if both arms have the same thickness of material through which the muons pass, both arms will give the same resolutions up to the J/.

On the other hand, the mass resolution in the region of the is somewhat degraded and studies have indicated that it is significant. In the North Arm the resolution of the is 220 MeV/c² and in the South Arm it is 245 MeV/c². The difference is due to the fact that in the South Arm the momentum resolution in the tracking region is a factor of 1.4 worse than in the North Arm. Consequently, in the South Arm the mass resolution has a stronger dependence upon the chamber resolution than is the case for the North Arm. That dependence upon chamber resolution is shown in Figure 3.2. In order to try to overcome the loss in mass resolution at the , the above mentioned study looked at what could be achieved if the magnetic field in the South Arm Magnet could be increased by 20%. In that case, the mass resolution in the South Arm would then improve to 225 MeV/c², which is almost identical to the performance for the North Arm.

Figure 3.1: The mass resolution at the , J/, and from simulations in the two arms with 100-um chamber resolution is shown.

Figure 3.2: The invariant mass resolution of reconstructed muons from upsilons in the South Muon Arm Magnet as a function of chamber resolution.

Summary and Conclusions

The conceptual design of the South Muon Arm Magnet which is presented here would result in a magnet which would fulfill the design criteria and mechanical constraints placed upon it and whose physics performance would be comparable to the North Muon Arm. In order to accomplish that, however, the piston angle was increased from 10 degrees to 12 degrees which will result in the South Muon Arm having even less acceptance for

s than the North Arm. It will, however, greatly increase the total acceptance for J/

s, and

s. It should be remembered in going to the final design that any degradation in the performance of the magnet from what is presented here will quickly degrade the spectrometer's momentum resolution and thus the physics capabilities.

As pointed out above, in the final design care should be taken to consider methods of avoiding sharp edges on the piston near the locations where the Station #1 and Station #2 chambers will be mounted so that there will not be large changes in the magnetic field in the chambers themselves. It is suggested, also, that the final tip of the piston, which was studied here, be a bolt-on piece. That will allow it to be manufactured separately from the rest of the magnet and decouples the detailed design of the piston tip from the rest of the magnet.

APPENDIX A -- Drawings of Magnet

This Appendix contains a set of concept drawings of the South Muon Magnet which has been used in the Magnet Performance and Physics Performance Studies presented in this document. The drawings contained in this Appendix were originally done by Walter Sondheim in collaboration with Orin Fackler, Bob Yamamoto, and Joel Bowers. They are concept drawings only and should not be used for construction.

This Appendix contains the followings drawings:

Figure A-1: Full-cut assembly side view of the magnet
Figure A-2: Head-on assembly view of the magnet
Figure A-3: Detail of one of the two side pieces of the magnet rear plate
Figure A-4: Detail of the center section of the magnet rear plate
Figure A-5: Detail of the big donut of the magnet
Figure A-6: Detail of a lampshade panel
Figure A-7: Detail of the magnet piston. "30-chamfer" piston shown.
Figure A-8: Detail of the small donut of the magnet

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