TECHNICAL SPECIFICATION FOR THE PHOBOS MAGNET SYSTEM
Leslie Rosenberg and Patrick Decowski
Contact: Leslie J. Rosenberg MIT, 24-506 77 Massachusetts Ave. Cambridge MA 02139
617 253-7589 (phone) 617 253-1755 (fax)
ljr@mitlns.mit.edu (email)
Document to be maintained at
URL: http://phobos-srv.mit.edu/magnet/
Revision history:
10Jul97 Version submitted to vendors
5.0 Vendor's Drawing and Design Approval
7.0 Factory and Final Acceptance
The following figures form an integral part of this specification:
Fig. 1.Perspective view of the PHOBOS magnets. This figure shows a perspective view of the two dipole magnets of the PHOBOS experiment, centered around the beam axis. The beam axis is along the z-axis. The Y-axis is vertically up, and the x-axis is defined such that the coordinate system is right handed. The plane z=43cm contains the pole axes.
Fig. 2.The minimum acceptable magnetic field in the mid-plane in the gap of one of the dipole pairs as determined from a 3D TOSCA model of the magnet at power consumption of 342kW. The beam-line is along the z-axis. The nominal interaction point is at x=y=z=0 in the figure. Figures 3a and 3b show the vertical (y) component of the magnetic field along the two indicated trajectories.
Fig. 3a (left panel) The vertical (y) component of the magnetic field (in Gauss) in the mid-plane of the gap along the indicated trajectory (in cm) from Fig. 2; this is the straight-line trajectory leaving the beamline at right angles and continuing through the geometric center of the gap.
Fig. 3b (right panel) The vertical component of the magnetic field in the mid-plane of the gap along the straight-line trajectory starting at the nominal interaction point and continuing through the geometric center of the gap.
Fig. 4. The approximate location of the silicon detectors in the magnet gap relative to the magnet poles (shown in outline). The yoke iron is shown shaded.
The following drawings form an integral part of this specification:
Technical Drawings:
TECHNICAL SPECIFICATION FOR THE PHOBOS MAGNET SYSTEM
This specification defines the minimum requirements for the design and fabrication of a "double dipole" magnet for the PHOBOS Detector for the 10 O'clock collision hall for the RHIC project at Brookhaven National Laboratory (Located on Long Island in New York, United States). RHIC accelerates heavy ions (up to gold) in counter-rotating beams that collide in a collision hall. The PHOBOS magnet straddles the beam-pipe near the collision point (see Fig. 1) and bends charged collision products for momentum analysis. One of the PHOBOS magnet dipoles is located on one side of the beam pipe, the other dipole is on the other side of the beam pipe. The two dipoles are in a series magnetic circuit.
A physics package consisting of planes of silicon detectors occupies the full clear space ("gap") between the pole tips. The package consists of layers of silicon with associated electronics. Some silicon layers are in the high field region between the pole faces, and other layers are in a low field region in the volume between the gap and the collision point (see Fig. 4).
The PHOBOS magnet comprises:
The goal of the PHOBOS magnet is to have as large as possible and as uniform as possible magnetic field in the gap between the poles, and as small as possible magnetic field outside the gap. Fig. 2 represents the minimum acceptable field in the mid-plane of the gap as determined from a 3D TOSCA model of the magnet specified herein with total power consumption of 342kW. Looking at Figure 3a and 3b, the By component of the field should rise from near zero at the axis of the beam pipe (the beam pipe axis satisfies x=y=0 cm--see Fig. 1 for the coordinate system). By 35 cm inwards towards the pole tip axis from the larger circumference of the tapered poles, the By component of the field should rise to > 2 T, and remain relatively constant in By as the x and z coordinates vary over the gap between pole pieces. As large as possible a field in the gap is desirable, consistent with reasonable power consumption (the power consumption is discussed later), reasonable field uniformity, and small field near the "low field" region outside the gap in the direction of the collision point. At 15cm inwards, the field is less than 0.5 T, dropping off to zero as shown in Figures 2 and 3.
The electrical properties of the magnet should be compatible with a power supply providing no more than 378kW (105VDC, 3600 Amperes). Cooling shall be from available RHIC cooling water. RHIC provided cooling water has the following properties: inlet temperature (35±10) °C, operating supply pressure 75 psig (pounds/square inch above atmospheric pressure), operating return pressure 15 psig, maximum conductivity 103 mmho/cm. The magnet shall be designed for continuous operation. Also, we anticipate the magnet will be "ramped up" to full current and "ramped down" to zero current several times per day as part of normal operation and the construction should take this into account.
The magnet shall consist of a steel magnetic yoke, two pole support plates, four support columns, four pole pieces, four potted-epoxy hollow-conductor copper coils and bus work, water manifolds, and other mechanical components. Appropriate insulation shall be provided for turn-to-turn, layer-to-layer, and coil-to-yoke insulation. Provided with this specification is a set of MIT-supplied technical drawings which are an integral part of this specification. The Vendor's design is expected to closely follow that of the MIT-supplied design prints, with deviations clearly spelled-out by the Vendor and approved by MIT.
The magnet shall consist of a rigid assembly of separable yoke, pole support plates, four support columns, and pole pieces which are doweled and pinned together in such a way that their alignment is reproduced upon disassembly and reassembly. Each flux return yoke is made from five pieces of steel; this segmentation into five small pieces allows for easier transportation and assembly of the magnet. The two outer pieces of the yoke run the full length of the magnet and provide the main support against deflection. The magnet gap between the poles (with coils unenergized) is 15.7cm. Under full power (nominally 342kW) and full magnetic field (2T or more maximum gap field) the deflection shall be less than 1mm. The upper and lower yokes are connected by four vertical support columns with welded-on bolt flanges.
The four pole pieces are of cylindrical "double-taper" design, with the addition of one vertical "cut" and one 12 degree "cut" (relative to the vertical) in each. The magnetic field shall be vertically upward-going in one gap, and vertically downward-going in the other gap (the two gaps are in a series magnetic circuit). The pole steel shall be low carbon 1010 steel (or approved equivalent) with care taken during machining to maintain (or restore through annealing) the magnetic properties of the steel. The yoke and support plate steel shall be 1018 (or approved equivalent). The balance of the magnet steel (for support columns, etc.) are not part in a magnetic circuit and therefore need only have mechanical (not magnetic) properties of 1018 or better. The bolts shall be number 8 (or approved equivalent). The two upper poles are mounted on one support plate, the lower two poles are mounted on the other support plate. The plates are in turn mounted to the flux return yoke. In addition to the positioning dowels required from section 3.1, each pole face shall have two survey pins; one pin shall be along the axis of each pole piece to serve as an alignment mark for surveying the magnet into place at RHIC.
In order to provide flexibility in studying new physics signatures and to address concerns over potentially large RHIC backgrounds, the distance between coil pairs shall be adjustable. Adjustment of the distance would be a rare operation, occurring perhaps once per year over the five-year life of the experiment. Two sets of alternate pole bolt hole circles and pole alignment holes shall be provided at +5cm and +10cm offsets for each pole.
All exterior surfaces of the yokes shall be free of rust, primed and painted with blue machinery enamel in such a way not to impair the magnetic properties of the magnet. The pole faces shall not be painted, but instead provided with an anti-rust treatment recommended by the Vendor and approved by MIT. The magnet pieces shall be labeled with part numbers to aid reassembly of the magnet.
The magnet mounting base shall be the responsibility of MIT.
Each of the four coils shall consist of eight (8) double pancakes (in electrical series) with six (6) turns per pancake. The coils shall be vacuum epoxy impregnated using suitable molds. The conductor is square copper 0.650" on a side, with a 0.363" diameter cooling hole through the center. The use of OFHC copper is strongly recommended; alternatives to OFHC copper (for example, deoxidized low phosphorous copper) shall be approved by MIT. In any case, the resistance per pole (with all pancakes in series) shall be less than 0.026 Ohm at 50°C. The conductor insulation is 0.007" fiberglass "half-lap" construction; the insulated thickness of each conductor is then 0.678". The total coil cross section with 0.125" ground insulation is then 11.10" X 4.32". Each double pancake (12 turns) forms a water circuit. The mean turn length is 114", and the number of electrical turns per coil is 2 X 8 X 6 = 96 (in eight parallel water circuits/one circuit per double pancake).
As far as possible, the coil pancakes shall be fabricated using a single length of conductor to avoid brazings inside the coil. Where brazings are unavoidable, the following braze materials are acceptable: "Silfos-15", "Phoson-15", "Castolin 1020'', "Castolin 1802'', and "Sil-72'' or other brazing material approved by MIT. Immediate and thorough cleaning and testing after brazing is required.
Provision should be made (e.g., embedded straps) in each coil to allow for lifing and lowering the coils in place over the poles.
The resistance per coil is approximately 0.75 X 10-6 X 114 X 96 = 0.0264 Ohm (with copper conductor at 50°C). The four coils will be driven in a series-parallel configuration from a power supply providing up to105 VDC at 3600 Amperes. The voltage drop across any one coil is then 0.0264 X 1800 = 47.5 Volts and the total voltage drop across two coils is then 95.0 Volts.
Resistive heating losses within the conductors shall be dissipated by water cooling. The water flow provided by a maximum pressure differential of 60 psig at a maximum inlet temperature of 45°C shall be sufficient to permit continuous operation at a maximum power consumption of 378 kW. Each coil shall be provided with a MIT approved thermal overload device ("Klixons") appropriate for the heat distortion temperature of the epoxy resin being used, and fitted on each double pancake at the water discharge side to protect against loss of cooling. The power per coil is 1800 X 47.5 = 85400 Watts; the power per one double pancake cooling channel is 85410/8 = 10.7 kW or 2560 cal/sec. The length of a cooling channel for each double pancake is 114" X 12 X 0.0254 = 34.7 meters. The inner diameter of the cooling channel is 0.363 X 25.4 = 9.22 mm. From engineering tables relating channel diameter (9.22 mm), length of cooling channel (34.7 meters), differential pressure (4 bars) and water flow rate and temperature rise, we have the flow rate 11 liters/min at a temperature rise of 14°C. The total water flow for the entire magnet (all double pancakes in a parallel cooling circuit) is 11 X 8 X 4 = 352 liters/min.
Coil water connections for each double pancake shall be terminated on a manifold with interconnecting conductors sized to suit the water flow requirements. All manifolding necessary for cooling water distribution shall be provided. Water lines shall withstand 300 psig test pressure, and shall be made from unplated copper, bronze, or stainless steel. All manifold entry and exit points on any one coil shall be located near to one another. Entry and exit connectors, manifolding, and their locations shall be chosen in consultation with MIT.
The Vendor shall notify MIT within fifteen working days prior to conducting any tests. MIT shall have the option to witness any and all tests and the Vendor shall provide all reasonable support to MIT personnel while witnessing tests. All tools, equipment, and accessories necessary to perform the tests shall be supplied by the Vendor. Equipment required for measuring and testing shall be certified by the Vendor to be sufficiently accurate for the type of work to which it is applied and to have been calibrated to traceable to industry standards before and after use. MIT reserves the right to reject test results obtained with improper or substandard equipment. Designated MIT representatives shall upon request be allowed reasonable access to the production and test areas of the Vendor's and/or the Vendor's subcontractor facilities at any time during the progress of the work called for by this specification.
"Factory Acceptance" shall be granted for the magnet upon satisfactory completion and written approval by MIT of all tests specified herein. "Final Acceptance" shall be granted upon satisfactory installation and testing of the magnet by MIT at Brookhaven National Laboratory upon MIT determination that the performance is satisfactory.
The magnet coils shall be tested both prior to and subsequent to epoxy impregnation. Each coil shall be permanently identified with a unique serial number.
Prior to impregnation, the following tests shall be conducted on the coils:
a. The Vendor shall successfully blow a polished steel ball of a diameter not less than 0.6 times the diameter of the cooling channel through each formed coil to check for obstructions to the coolant flow.
b. A test of the turn-to-turn insulation shall be conducted by means of a switched-type ringing tester developing a pulse of approximately 10 Volts per turn. A photograph of the resultant oscilloscope trace showing a slowly decaying waveform shall be submitted to MIT as demonstration of the absence of any shorts. Each photograph shall be clearly marked with the serial number of the coil being tested. A photograph of a shorted turn shall be included for comparison.
c. The formed coil shall be tested for leakage under pressure. There shall be no visible leakage or deformation when the water passages are filled with water, and pressurized to a minimum of 250±10 psig, and held at this pressure for a fifteen-minute period.
d. The coil shall be filled with water and pressurized to a minimum of 250 psig. The coil shall then be isolated from the pressure source. The internal pressure and ambient temperature shall be measured at the beginning and end of a one-hour minimum test interval. The coil shall show no change in pressure due to water leakage after any necessary correction for temperature change during the test interval.
Subsequent to impregnation, the following tests shall be conducted on the coils:
a. Test of coil-to-earth insulation at 2000 VDC shall be done by wrapping the entire coil in aluminum foil and applying the test voltage between the coil terminal and the foil for one minute. The leakage current shall be less than 300 mAmperes and there shall be no evidence of electrical breakdown.
b. A test of the turn-to-turn insulation shall be conducted by means of a switched-type ringing tester developing a pulse of approximately 10 Volts per turn. A photograph of the resultant oscilloscope trace showing a slowly decaying waveform shall be submitted to MIT as demonstration of the absence of any shorts. Each photograph shall be clearly marked with the serial number of the coil being tested. A photograph of a shorted turn shall be included for comparison.
c. The coil resistance shall be measured and recorded to three significant figures at approximately 20°C and a current of 1 Ampere. Coil temperature shall be measured and recorded to within 0.25°C during this test.
d. The water flow through each separate coil passage shall be measured and recorded to the nearest 0.1 gallon/min. Measurement shall be made at a pressure differential of 40, 60, and 80 psig and a water inlet temperature of (25±5)°C. Differential pressures shall be measured at the cooling line connections to the coil.
e. The water temperature rise in each coil at full design current of 1800 Amperes shall be measured at the inlet and outlet water terminations, at a water differential pressure of 60 psig and inlet temperature of approximately 23°C. The observed temperature rise shall be less than 14°C.
Field mapping shall be the responsibility of MIT.
5.0 Vendor's Drawing and Design Approval
The Vendor shall submit to MIT within 45 calendar days after award of the contract with copies of drawings and specifications showing:
a. Overall dimensions and total weight of the magnet and separate pieces.
b. Location and details of power connections and water connections.
c. Cross sections of the coils.
d. Details of the insulation materials to be used.
e. Details of alignment.
f. Details of mechanical and electrical test procedures proposed by the Vendor.
g. Any other pertinent information that may be requested by MIT.
MIT will notify the Vendor of design approval and/or submit comments within two weeks after receipt of drawings and specifications listed in this section. Approval by MIT of the Vendor's drawings and specifications shall not be held to relieve the Vendor of any part of the Vendor's obligation to meet all of the requirements of these specifications of the responsibility for the correctness of the Vendor's drawings.
Within one week after shipment of the magnet, the Vendor shall furnish MIT with two sets of manuals or instruction books which includes as-built drawings, parts list, operating instructions, specification sheets of major component parts, and mechanical and electrical test reports.
The Vendor shall include, as part of the proposal, objective evidence of the existence and operation of a quality assurance program during the design, fabrication, and testing of the PHOBOS magnet.
7.0 Factory and Final Acceptance
"Factory acceptance" shall mean satisfactory completion of all tests specified herein at the Vendor's factory. "Final acceptance" shall mean satisfactory installation and testing of the magnet at BNL and determination by MIT that the magnet performance is satisfactory. The Vendor is responsible for assembly and tests of the magnet at the Vendor's factory. MIT is responsible for assembly and Final Acceptance tests of the magnet at BNL.
All elements of the magnet assembly shall be covered by a warranty against material and manufacturing faults for eighteen (18) months from the date of shipment or twelve (12) months from the date of Final Acceptance by MIT, whichever is greater. The Vendor shall either replace or repair, at his cost, any failure under the terms of this warranty.
Following satisfactory completion of inspection and tests at the Vendor's factory, and after receiving written factory acceptance from MIT, the magnet system shall be crated and shipped. Particular care should be exercised to protect machined surfaces from rusting, scratches, and nicks during shipment or packing for shipment. All parts shall be numbered and directions shall be given, referencing these part numbers, in the Vendor supplied manuals or instruction books as to the proper assembly. The components shall be crated in such a manner as to protect the contents from damage while in transit. In particular, the coils shall be prevented from shifting or chafing due to jolts received in transit. The upper two coils and the upper support plate shall be shipped as one complete pinned, bolted and aligned assembly at the minimum pole separation setting. The lower two coils and the lower support plate shall be shipped as another complete pinned and bolted assembly at the minimum pole separation setting. The packing crates shall be placed on pallets suitable for movement using fork lift trucks. All water circuits shall be blown free of any water prior to shipping or storage under freezing conditions. The Vendor shall notify MIT prior to making the shipment. The notice shall include the purchase contract number, origin, date, routing of shipment, approximate date of arrival, and shipping weights.
Figure 1. Perspective view of the PHOBOS magnets. This figure shows a perspective view of the two dipole magnets of the PHOBOS experiment, centered around the beam axis. The beam axis is along the z-axis. The y-axis is vertically up, and the x-axis is defined such that the coordinate system is right handed. The plane z=43cm contains the pole axes.
Figure 2. The minimum acceptable magnetic field in the mid-plane in the gap of one of the dipoles as determined from a 3D Tosca model of the magnet at power consumption of 342kW. The beam-line is along the z-axis. The nominal interaction point is at x=y=z=0 in the figure. Figures 3a and 3b show the vertical (y) component of the magnetic field along the two indicated trajectories.
Figure 3a (left) shows the vertical (y) component of the magnetic field (in Gauss) in the mid-plane of the gap along the indicated trajectory (in cm) from Fig. 2; this is the straight-line trajectory leaving the beamline at right angles and continuing through the geometric center of the gap. Figure 3b (right) shows the vertical component of the magnetic field in the mid-plane of the gap along the straight-line trajectory starting at the nominal interaction point and continuing through the geometric center of the gap.
Figure 4. The approximate location of the silicon detectors in the magnet gap relative to the magnet poles (shown in outline). The yoke iron is shown shaded.