3.0 Magnet Operation
3.1 General Layout of Magnets and Power Supplies
This sections details the high current magnets and power supplies in the Hall B experimental area. Figure 1 shows a schematic of the large magnets and their supplies, power supply specifications are given in Table 1, and magnet specifications are given in Table 2. Several supplies serve dual uses depending on whether the experiment is performing electron scattering or real photon experiments. There are many safety issues for magnets and their power supplies that are addressed in the following sections. The main safety issue is personnel safety, the magnets, supplies and bus lines must be safe enough so that workers can work in the vicinity of the magnets and supplies without any risk of electrical exposure. Equipment protection is also a concern since the magnets and power supply represent a substantial tax payer investment. The equipment is actively protected by hardware interlocks that prevent the power supplies from damaging themselves or the magnets they supply.
Figure 1. Schematic of the large magnets and their power supplies in Hall B. The order shown is the order in which the magnets reside on the beamline, with the beam running left to right. The solid lines between magnets and power supplies represent 535 high current cable, dashed lines represent water cooled bus lines either thick-walled copper or thick walled aluminum pipe.
| magnet | power supply | ||
|---|---|---|---|
3.2 Safety Procedures for Magnets in Hall B
The magnets in the experimental hall are typically energized by remote control to simplify operation when there is beam in the hall. During major down times the magnets are powered down for personal safety reasons as well as to reduce electrical power con sumption. During short interruptions of beam delivery, with hall personnel entering the hall in the controlled access mode, the magnets are typically left energized. The main reason is that the time constants of large size magnets are long (of the order o f hours), and frequent ramping or cycling will lead to inefficient operation. Also, every ramp of a large superconducting magnet involves some risk of permanent damage to the magnet coil.
3.2.1 Identification of Potential Hazards
Personnel working in the proximity of energized magnets are exposed to the following hazards:
- danger of being electrocuted by coming into contact with exposed leads \item danger of metal tools coming into contact with exposed leads, shortening out the leads, depositing a large amount of power in the tool, vaporizing the metal, and creating an arc \item danger of metal objects being attracted by the magnet fringe field, and becoming airborne \item danger of cardiac pacemakers or other electronic medical devices no longer functioning properly in the presence of magnetic fields.
- danger of metallic medical implants (non-electronic) being adversely affected by magnetic fields.
3.2.2 Hazard Mitigation
Two different modes of operation need to be distinguished: (1) routine operation involving work in the vicinity of the magnets, but not in close proximity to the electrical connections, and not involving any work that could result in purposely getting in to contact with the coils or the leads, and (2) non-routine operation involving work on or near the exposed current conductors or connections (typically requiring removal of the shield) or any work that could result in contact, intentional or otherwise, w ith the coils or the leads.
- The current carrying conductors must be protected against accidental contact or mechanical impact by appropriate measures (e.g. run cables in grounded metal conduits or cable trays, use plastic piping to cover water cooled bus).
- All exposed current leads and terminations shall be covered by non-conductive or grounded shields (typically plexiglass or fire-rated lexan) in such a manner as to make it impossible for personnel to accidentally touch exposed leads with either their body or with a tool. Personnel shall be instructed not to reach inside the shields. Warning signs shall to be placed on the shields; the signs shall read:
DANGER
THIS GUARD MAY ONLY
BE REMOVED BY
AUTHORIZED PERSONNEL
UTILIZING JLAB
LOCKOUT - TAGOUT
PROCEDURES
- Whenever a magnet is energized, a flashing light on the magnet or on the magnet support structure must be activated to notify and warn personnel of the associated electrical and magnetic field hazards. The beacons at the magnet are to be activated when th e power supply is turned on, regardless of whether current is actually flowing to the magnet. This serves as notice to any workers in the area that the magnet is presently activated or can be activated at anytime remotely, and any magnetic m aterial should not be placed near the magnet.
- Administrative measures shall be implemented, as appropriate for the situation, to reduce the danger of metal objects being attracted by the magnet fringe field and becoming airborne. (Note that for most magnets strong magnetic fields are only encountered within non-accessible areas inside the magnet.) Areas where these measures are in effect shall be clearly marked.
- To reduce the danger of magnetic fields to people using pacemakers or other medical implants, warning signs shall be prominently displayed at the entrance to each hall. The sign shall read:
DANGER SAFETY HAZARDS MAY EXIST FROM THE MECHANICAL FORCES EXERTED BY THE MAGNETIC FIELDS UPON MEDICAL IMPLANTS NO PACEMAKERS
The following measures shall be taken during non-routine operation to mitigate the hazards described in Section 3.2.1 :
- All non-routine maintenance shall be performed in strict accordance with the Jefferson Lab EH&S Manual, and in particular, with the chapters on Lockout, Tagout, and Electrical Safety.
- Removal of any protective shield or cover for an electrical conductor shall be performed using administrative lockout procedures. The lockout shall be performed by the cognizant hall engineer (or his designee). The administrative lock shall not be removed until the protective shield or cover has been fully re-installed.
3.3 Sweeper Dipole Magnets
The two blue dipoles downstream of the tagger magnet are used during real photon experiments. These magnets remove secondary charged particles originating at the collimator from the photon beam. Magnetic field stability is not an issue with these magnets. Each dipole achieves maximum field at ~950A with a resulting voltage drop of 38V.
The sweep magnets are powered by an old accelerator arc power supply. The supply is manufactured by Dynafysik and is located on the ground floor of the hall. The supply is light blue in color and is between the tagger supply and the Dyna-A (mini-torus/pai r-spectrometer supply) supply. The two sweep dipoles are connected in series to the power supply. The power supply when connected to the sweep magnets is current limited at 220Amps and this is the operation point.
At present the supply does not have remote control or monitoring, and is run in local mode. The supply should be set to 100% full current from the front panel and current and voltage are read on the front panel.
3.4 Møller Quadrupole Magnets
Møller Quadrupole magnets are an integral part of the Møller polarimeter. Their function is to transport the two electrons to the detectors downstream of the magnets. Since the polarized electron beam can be of different energies the magnets are to be operated at different currents. In fact the first quadrupole must have its polarity changed depending on the beam energy. For this reason the first quadrupole is connected to the Dyna-B supply and Dyna-switch. In order for these supplies to ene rgize the quadrupoles the individual pole coils were reconfigured to be parallel instead of in series. Each pole is still in series with the other poles, thereby each pole still has the same current flowing through it. The bus lines from the power supply or switch are water cooled aluminum bus.
Møller Quadrupole operation checklist
The following checklist must be completed before energizing the Møller Quadrupole magnets to protect personnel and the equipment.
- All flags are to be buffed and cleaned before connecting.
- All protective shields on bus and flags must be in place.
- The warning beacons in the Møller area must be functioning.
- LCW flow verified to power supply, switch, bus lines and magnets.
- The Møller quadrapole area must be clean of magnetic debris.
- The temperature sensor klixons on the magnet and flags must be verified to be connected to the external interlock on the power supply and that the interlock is working.
- The current limit on the supplies must be set to 4000A.
3.5 Mini-Toroid Magnet
The Mini-Toroid magnet consists of six resistive water cooled coils. The magnetic field produced by the Mini-Toroid protects the drift chamber from low energy Møller electrons originating from the target. The magnet was designed for a maximum curre nt slightly more then 8000 Amps, however, the Dyna-A power supply becomes voltage limited at about 7900 Amps. The Mini-Torus is used during electron scattering experiments, and is removed for real photon and polarized target experiments.
3.5.1 Mini-Toroid Installation and Operation
The following items must be performed during installation and operation of the Mini-Torus.
- The circuit breaker for the Dyna-A supply must be locked out by all personnel working on the bus connections.
- The LCW integrity of the coupling at the Mini-Torus must be checked before insertion.
- The electrical isolation of the magnet coils from ground must be verified before connecting them to the power supply.
- All exposed bus, flags and connections must be placed behind protective shields.
- Verify that the temperature sensor klixons on the magnet and flags are connected to the external interlock on the power supply and that the interlock is working.
- All electrical connection surfaces should be void of oxides before connecting.
3.6 Pair Spectrometer Dipole Magnet
The pair spectrometer magnet is located in the downstream tunnel behind the first shield wall. This magnet is only used during real photon experiments and is powered by the Dyna-A power supply. When operating this supply the following items need to be ver ified first:
- All flags are to be buffed and cleaned before connecting.
- All protective shields on bus and flags must be in place.
- The warning beacon in the pair spectrometer area must be functioning.
- LCW flow verified to power supply, bus and magnets.
- The pair spectrometer area must be clean of magnetic debris.
- The temperature sensor klixons on the dipole magnets and flags must be verified to be connected to the external interlock of the power supply and that the interlock is working.
3.7 Tagger Magnet and Power Supply
The tagger magnet is a large C magnet which is installed in the upstream alcove in Hall B. Its purpose is to deflect the full-energy electron beam through an angle of 30 degrees into the tagger beam dump, while deflecting lower-energy electrons into the d etectors of the tagging system. The magnet is suspended from a steel support structure called the gantry.
The tagger power supply is located on the floor in Hall B, next to the wall on the north side of the alcove. Power is supplied to the magnet by eight 535 mcm insulated copper cables, four supply and four return. The power cables run in grounded cable tray s along the wall of the hall. The magnet is grounded by a bare 500 mcm copper cable which runs in the same cable trays. The ground cable is connected to the grounding plate in the hall floor, adjacent to the power supply. Additionally, the gantry steel is grounded directly to the magnet steel; ground does not rely on the support connections between the magnet and the gantry. Strain relief is provided for all cables.
The power supply delivers up to 2400 Amps DC, at approximately 70 Volts. At full 2400 Amp excitation, the magnet can deflect a 6.1 GeV electron beam into the tagger beam dump.
The power supply doors are interlocked. Additional interlocks are from a flow meter on the cooling water return for the magnet (not currently connected), and on a series of `Klixon' temperature gauges in contact with the magnet coils. The LCW system is u sed to cool both the power supply and the magnet. The power supply has an internal flow meter for its cooling water that is interlocked as well.
Access to the high-field region is very restricted by the stainless steel vacuum extension to the magnet gap. Furthermore, the field is less than 100 gauss at distances greater than 1 foot from the magnet, and less than 5 gauss at distances greater than 1 0 feet from the magnet.
3.7.1 Safety Hazards and Precautions
The power supply can be run locally using front-panel controls, or remotely under EPICS control. During run conditions, the magnet is controlled via EPICS from MCC.
- Following any modification or maintenance of the magnet or power supply, the following will be verified:
- DC power leads are all in place and torqued down.
- Magnet-to-Gantry and Magnet-to-Ground Plate cables are in place and torqued down.
- LCW water is connected and operating properly.
- Magnet interlocks are connected and verified to trip the power supply.
- DC lead protective covers are in place on the magnet.
-
A sign saying
DANGER
Strong Magnetic Fields - No Pacemakers
No Access Except Authorized Personnelwill be posted at all access points, including the entrance to the alcove from the tunnel.
- The area around the magnet will be roped off.
- The area within 3 feet of the magnet will be searched and cleared of tools and other loose items which may be affected by the field.
- Red rotating or flashing lights on both upper and lower levels (near power connections) will be used at the magnet to indicate `Power On.' These lights will be activated by control power in the near future.
- Immediately prior to energizing the system, a final check will be performed to verify that no one is in contact with the system.
The EPICS control program shall ensure that, whenever beam is present in the Hall B channel, the current in the power supply is within 5 percent of the nominal calculated value. The nominal value is calculated from the beam energy by the EPICS program. Fo r reference, some of the nominal values are listed in Table 3.
3.7.2 Normal Operation
The magnet will be used for its designed purpose during all photon beam runs in Hall B, and may also be used for beam steering and tuneup during electron beam runs. During all beam runs, the power supply is controlled exclusively from MCC via EPICS.
During short controlled accesses to the hall the power supply may remain energized. During longer controlled accesses (longer than 4 hours) or when the Hall is in a state of restricted access, the power supply shall be turned off, or the current set to ze ro, unless an authorized person is present in the magnet area.
3.7.3 Special Procedures
The power supply may be run under local control by authorized personnel for the purpose of making magnetic field measurements, testing the interlock systems, or degaussing the magnet, under local control. The power supply shall be energized only while aut horized personnel are present in the magnet area.
3.8 Toroidal Magnet
This document will give the operator a basic description of the KNUTH computer, and the Oxford Annunciator located in the Hall B Counting House. It will also provide operating and troubleshooting procedures that the operator will need to bring the magnet up to a desired field (current setting). Details of operation can be found in Appendix A.
3.8.1 Equipment Description
The operator will use the KNUTH computer and the Oxford Annunciator to interface with the magnet from the Hall B Counting House.
3.8.2 KNUTH Computer
The KNUTH computer has two monitors for operator interface. The top monitor is configured with the Magnet Power Supply screen and the bottom monitor with the Service Module Mimic screen. The Magnet Power Supply screen is used to adjust magnet current and observe key error indicators. The Service Module Mimic screen shows the status of the cryogen systems valves, pressures and levels.
3.8.3 Oxford Annunciator
The Oxford Annunciator provides a means of alerting the operator to problems that he might not have observed from KNUTH due to operating restraints. The panel will give a audible alarm and display a red light for the following conditions: generator engine fault, pneumatic fault, watchdog timer fault, and power supply setpoint fault (MAG PWR). A yellow light (emergency power) without any audible alarm is given when the generator is providing power. The alarm given when the magnet is not at setpoint is the best indication that a fault has occurred because any fault on the magnet will cause the current to ramp down.
3.8.4 Operations & Troubleshooting Current Control
To ramp the current up or down the operator performs the following steps on the Magnet Power Supply screen:
- Ensure that the Main Power switch is in the ON (or "1") condition.
- Set the Ramp Rate to the proper setting of 1 A/s or 0.5 A/s (IF CURRENT IS ABOVE 2000A USE THE 0.5 A/s SETTING).
- Input the desired current setting (DO NOT EXCEED 3500 A) in the Set Point box.
- Silence the Annunciator MAG PWR alarm and observe the Setpoint Readback change to the desired Set Point setting.
- The Current Readback will show the current ramping up or down to the desired level (it may take a moment to begin ramping).
- Once the Current Readback level has reached the Setpoint Readback setting the Annunciator MGT PWR alarm will reset. If at any time the Current Readback level does not agree with the Setpoint Readback the Annunciator will alarm, alerting the operator that the magnet is not at the desired field.
- At the Magnet Power Screen, ensure that there are not any fast or slow rundown conditions. If a condition exists refer to Fast & Slow Rundown procedure.
- Ensure the Main Power switch is on (displaying a "1").
- Ensure that the Manual Enable switches are on (displaying a "1").
- Depress the green Reset button on the bottom right hand side of the screen.
- After ensuring the current readback is at 0.0 A, cycle the Main Power switch OFF and then ON ("0" and then "1"). Do not cycle this switch multiple times.
- Perform the Current Control procedure.
- If problem persists, contact oncall personnel.
- A rundown will be indicated by the current ramping down, a fault in the Slow or Fast Rundown sections of the Magnet Power Supply screen, and a set ("1") flag in the Status Register.
- Note the cause for on call personnel and depress the Reset button in the right hand corner of the Magnet Power Supply screen.
- Ensure that both Manual Enable switches are ON (`1").
- If the problem persists, cycle the Main Power contactor (OFF and then ON).
- Do not recycle Main Power contactor and contact oncall personnel.
If the screens on either monitor lock-up, perform the following steps:
3.8.5 Reversing Polarity on the Oxford Magnet Power Supply
All personnel must do a walk through of this procedure with a fully trained person before using it. This procedure is meant for reference only. It is assumed that all individuals using this procedure fully understand electrical safety, lock & tag out procedures, and a fundamental knowledge of magnet operations.
To reverse the polarity of the Oxford Magnet Power Supply perform the following steps:
- Ramp the magnet down to a current of zero amps.
- Lock out power from Panel SBESB at the knife switch (the knife switch is located directly in front of the power supply).
- Secure control power to the power supply by unplugging the power cord located on the top of the power supply (the back right hand corner).
- While taking great care to insure that foreign debris doesn't get into the power supply, swap the plates to the desired polarity as shown in the figure on the following page.
- Insure that all bolts are torque to 55 ft/lbs.
- Secure the area and apply power to the magnet.
