1. Magnet type: Superconducting dipole with flux return yoke and open warm bore.
2. Normal operating mode: DC at full specified field. The magnet may be operated at various fields between 0 and full field. Rate of field change is covered under point 16 below. Magnet can be continuously powered or persistent mode.
3. Project scope: Design, fabrication, delivery, and test of complete "turn-key" magnet system including full cryogenic system, charging (power) system, and all instrumentation required for operation.
4. Field orientation: The main field direction in the bore is vertical. Both up and down polarities must be allowed for in the design. Polarity change can be accomplished by external reconnection.
5. Bore orientation: Horizontal. The bore axis is along the initial beam direction, 198 cm above the floor.
6. Bore inside dimensions:
a. Width: 50 cm
b. Height: 15 cm
c. Length: 200 cm (set only by a limit on the overall footprint of the
magnet).
7. Experiment elements located in magnet bore: (supplied and installed by
experimenters)
a. Interaction target: Located on the bore axis at a point no more than 90 cm
from the downstream physical end of the magnet/cryostat system. The location
of the target with respect to the upstream end of the magnet isn't specified.
The final target location will be based on magnetic field considerations (8a).
b. Collimator: Located in the magnet bore downstream of the target. The
collimator weighs about 400 kg. This weight must be supported by the lower
surface of the warm bore.
8. Primary magnetic field component: Vertical orientation. Field strength requirements are
based on a minimum field at the target and a minimum total field integral.
Exceeding either or both of these minimum requirements is desirable.
a. Strength of vertical field component at target: 5.4 T. This
requirement will, in part, dictate the final target location.
b. Integral: 4.6 Tm. Defined as the line integral of the vertical
component of the
magnetic field along any straight line path between the target and the exit
aperture. The exit aperture is taken to be the axial projection of the warm
bore
onto a vertical plane at the downstream physical end of the magnet.
9. Secondary magnetic field components: No specific requirements.
10. Field homogeneity in bore: No specific requirement.
11. Stray field: 50 G at 30 cm from the magnet sides on the horizontal midplane.
12. Operating margin: Must be safe to operate at 105% of specified full field.
13. Coil material: Dictated by performance requirements. No other specific requirement.
14. Yoke material: Magnetic performance at least equal to AISI 1006 steel.
15. Charging (power) system: Fully automatic control of magnet with selectable charging and discharging rates and field (and/or current) target.
16. Field ramp rate: 4 hours between 0 and full field whether charging or discharging.
17. Precision in reaching field setpoint: 1% of setting at any point from 0 to full field (current).
18. Field stability: 0.5% variation per day.
19. Quench protection: Safe from damage due to a quench in any operating condition.
20. Recovery time from a quench: 12 hours to full field.
21. Instrumentation: All important parameters including field, cryogen level, and coil temperature. Must supply alarms with adjustable setpoints for all important parameters and for quench detection. Output accessible through standard IEEE interface.
22. Start-up time: 7 days from beginning insulation space evacuation (if required) to full field.
23. Service interval: 7 days for any required service including LHe but excluding LN2.
24. LHe supply: Periodic supply from standard 500 l storage dewar.
25. LHe consumption: 2 l/hr after initial cooldown.
26. LN2 supply: Periodic or continuous.
27. LN2 consumption: 30 l/hr
28. Dimensional envelope: 2 m along beam, 3 m wide, 698 cm above floor (including stand).
29. Magnet stand and standoffs from 48D48 analyzing magnet (see below): Supplied by BNL
30. Safety: Usual cryogenic and magnetic safety mechanisms including a "crash-off" on the magnetic field. Compatible with BNL safety requirements.
Short description of the E896 experiment:
In the E896 experiment, a high-energy beam of gold nuclei is focused on a fixed gold target. Collisions between the incident and target nuclei result in the production of a wide variety of nuclei and elementary particles. The particles of interest in the experiment are electrically neutral and have a short lifetime. They are identified by their charged decay products as detected in a finely segmented drift chamber tracking system located inside a large conventional electromagnet (a BNL 48D48 analyzing magnet). The drift chamber and analyzing magnet are placed downstream of the target.
Both electrically charged (including some uninteracted gold nuclei) and uncharged particles are emitted from the target. The drift chamber is sensitive to all charged particles entering its active volume and its ability to accurately identify the decay products from the neutral particles is reduced if they are accompanied by a large number of charged particles coming directly from the target. Thus, it is desirable to deflect the charged particles from the target away from the drift chamber active volume. This is accomplished by locating the target inside a powerful dipole magnet (referred to as a sweeping magnet or sweeper) whose field at and immediately downstream of the target is sufficient to deflect most of the charged particles. The important performance parameters of this magnet are given above.
The number of extraneous particles entering the drift chamber is further reduced by locating a shaped collimator inside the sweeping magnet bore, immediately downstream of the target. The weight of this collimator must be borne by the structure of the sweeper.
One of the critical features of this application is that the sweeping magnet is located immediately upstream of the BNL 48D48 electromagnet and there will be a considerable interaction between the two magnets. The major component of the magnetic field in both magnets is vertical. However, during the course of the experiment the two magnets may be operated with any combination of polarities. These factors must be considered in the design of the sweeper. It is understood that a rapid discharge of the 48D48 may cause the sweeper to quench. However, it is required that the sweeper be safe from damage in such a quench and that the recovery time be short enough that the experiment running time will not suffer significantly.