GlueX two-dipole tagger specification

Hall D Tagged Photon Spectrometer

Technical Description and Specification

General Description

     The items which make up the complete spectrometer system are as follows:

1. Two identical dipole magnets including coils. 2. Power supply for dipole magnets. 3. Vacuum box. 4. Quadrupole magnet and stand plus power supply. 5. Strongback support structure for the dipoles.

1. Two dipole magnets and coils.

The Hall D Tagger consists of two “C” type identical dipole magnets. They are conventional room temperature magnets with copper conductor coils. Both magnets have rectangular pole faces and all the magnet yokes are made from simple rectangular shapes. These two magnets are placed in series, and the separation between them is around 40 cm. The two dipole magnets are not exactly parallel. The angle between them is 0.892 degrees. Each of the magnets has its own focal plane; and these two focal planes join together with no overlap. The two magnets are designed to operate at 1.5 Tesla but enough margin has been left for operating at 1.8 T if required in the future. When the magnets are running at 1.5 T, a 12 GeV main electron beam will be bent through an angle of 13.4 degrees after traversing the two magnets.

Due to the large electron beam energy, the energy degraded electrons have a small characteristic angle, so the hall D Tagger does not need a large pole gap. At present, a 3 cm pole gap is used in the design.

The pole shoes have a simple rectangular shape. There is a small step around each pole shoe, which provides a sealing surface between the pole shoe and the vacuum chamber. Detailed pole profiles can be found in an AutoCAD drawing of the spectrometer. A three dimensional field calculation has been done by using the same dimensions as are in the AutoCAD drawing. The effective field boundary is found to be approximately 2.5 cm outside the pole stem.

Specifications

1). Main beam momentum 12 GeV/c 2). Momentum range of analyzed electrons 0.6 GeV/c to 9 GeV /c 3). Main beam radius 26.685 m 4). Main beam bending angle 13.4 deg 5). Entrance face angle for the first magnet 5.9 deg 6). Magnetic field 15.0000KGauss 7). Pole gap 30 mm

8). Proposed coil parameters

There are a total of 4 racetrack shaped coils for the two dipole magnets, which are connected in series. These coils are constructed from square section copper conductor (11×11) mm2 with a central water channel 7 mm in diameter. The total number of ampere turns is chosen to provide a maximum field of 18 Kgauss.

Total number of ampere turns for each magnet 61560 Conductor configuration of each coil 7×12 Current density for 61560 amp turns 4.44 amp /mm2 Electrical resistance of each coil 0.124 Ω Current in conductor 366.43 amps Voltage drop across 4 coils connected in series 181.7 volts Total power consumption 67 Kwatt Cooling circuits per coil 7 Total flow rate for 20 deg. Centigrade rise 80 l/min Water supply pressure drop 2.0 atmos

Outside dimension of each coil (92×152) mm2 The coils should be vacuum impregnated.

9). The gap between the pole edge and the inner surface of the coils is 20 mm and should vary by less than ± 1mm. 10). The discrepancy between the calculated and measured effective field boundary (obtained from a field map for the two dipoles) should be less than 1.0 mm along all the pole edges for the analysed electrons. 11). At 1.5 T, the magnetic field homogeneity should be less than 5 parts in 104 along any 100 mm length lying inside an area defined by a line drawn round a pole surface which is two gaps in from the pole stem. Within this area the maximum variation in the magnetic field should be less than 5 parts in 103. 12). The pole faces should have a parallelity such that the variation in the pole gap should be less than 0.01mm along any 100mm length on a pole surface. The variation in gap size should be less than 0.1 mm over the complete area of the poles. 13). The pole surfaces, which form part of the vacuum system, should have a protective covering, e.g. Ni impregnation. 14). The poles for both magnets should have small alignment holes defining the input, output and central ray trajectories: i.e. 3 trajectories per magnet. 15). The variation in the pole gap between zero field and a field of 15 Kgauss should be less than 0.2 mm at any point on the pole surface. 16). There should be tapped holes for attaching jacking supports to the dipole yokes for use in the assembly and disassembly of the dipoles.

2. Power supply for dipole magnet

The coils of the two dipoles will be connected in series. So we need only a single power supply together with a small auxiliary power supply to balance the fields in the two dipoles. The power supply should have the following specifications:

     Power output                                                   80 Kwatt
     Voltage output                                                 200 volts
     Current output                                                 400 amps

The current should be stable to a few parts per million over a period of 48 hours.

The supply should have a zero field option, e.g. it should be capable of maintaining a field of less than 5 ± 0.5 gauss in the magnet by means of feed-back loop controlled by a magnetic field probe.

3. Vacuum box

It is proposed to use the poles as parts of the vacuum system. Details of the vacuum box are shown in the AutoCAD drawing.

The magnet poles have a lip running around their circumference, and the vacuum box is essentially “warped around” each pole with a rubber O-ring seal being made between the top or bottom surface of the vacuum chamber and the lip. Pressure can be applied along the seal by means of tightening brackets which are attached to the yokes.

The vacuum box extends the vacuum system out to the focal plane of the spectrometer. In addition to an entrance port for the incoming beam and an exit port for the main beam, the vacuum box has a large exit window allowing the analyzed electrons to pass through to the focal plane detectors. The exact shape of the vacuum box around the main beam exit port still has to be finalised.

The materials for the vacuum box should be non-magnetic. Stainless steel and aluminium could be the suitable materials. However, since welding can alter the magnetic properties of stainless steel, great attention should be paid to avoid the welds distorting the magnetic field at critical points. For aluminium, since the thermal expansion coefficient of aluminium is significantly different from that of iron, it is probably not acceptable. The box will require external strengthening ribs to prevent it collapsing when the system is under vacuum.

4. Quadrupole magnet , power supply and support. The main specifications of the quadrupole are as follows;

           Aperture diameter                                  30 mm
           Magnetic length                                     400 mm

Max magnetic field gradient 20 T/m

The coil design and the choice of power supply are left to the manufacturer. The magnetic axis and geometrical axis should be co-incident to <0.05 mm along the magnetic length of the quadrupole. A table support is needed for the quadrupole magnet

5. Strong-back support structure.

A single support structure (strongback) is needed for the two dipoles and vacuum chamber, because once they are aligned no relative motion between the two dipole magnets is allowed due to the requirement of precision alignment, vacuum seals, etc, although in addition the entire assembly should be adjustable as a unit. This support structure should also provide the ability to allow accurate adjustment of the magnets. It should be designed to allow independent movement of the magnet by ± 20 mm in the x, y, z direction for each magnet. The final positioning in any direction should be accurate to 0.1 mm. The spectrometer will be mounted horizontally. The beam height will probably be between 1.8 and 2.0 m from the floor. The manufacturer should provide a design for the strongback.