5.0   Beamline and Dumps

5.1   Tagger Vacuum Window

This document describes the hazards and corresponding safety measures of the Hall B Tagger Vacuum System.

The tagging magnet hangs from a steel support gantry in the upstream alcove in Hall B. The Magnet is a C magnet with a 2" gap. A large vacuum tank is welded directly to the magnet steel, giving a total vacuum volume of approximately 0.6m3.

The lower surface of the vacuum tank is covered by a thin (less than 20mg/cm2) kevlar-mylar composite window of width 8 inches and length 30 feet. Vacuum is maintained by a 1500 l/s turbo pump at the downstream end of the vacuum tank. The vacuum is continuous with the vacuum in the main hall beam line and with the tagger-dumb beam line, which runs from the downstream end of the vacuum box into the floor of Hall B (the tagger beam dump is located beyond the end of this line, approximately 20 feet beneath the floor). The window faces the interior of the tagger hodoscope box, a mesh-walled box about 3 feet by 4 feet in cross section and 40 feet long.

A shutter system, consisting of 24 interlocking plates of 3/16-inch-thick aluminum which slide on two teflon-covered channels, can be inserted immediately outside the window, at a distance of about 1 inch from the window frame. This shutter system is intended to protect detectors and personnel in case of a sudden window failure. The shutter plates are inserted and removed by one or two persons standing at the bottom end of the hodoscope box, at a distance of more than 6 feet from the closet point of the window and not in direct line of sight of the window.

5.1.1   Hazards

All potential hazards are due to window implosion. They are

  • Being pulled towards the window by rushing air directly after an implosion, and impacting detector structures or the vacuum tank itself.

For example, if an arm were directly across the window, working on a detector, when the window imploded, then that arm could possibly feel a pressure of 15 PSI, or perhaps 150 lb pushing it against the hodoscope.

  • Ear drum damage and/or hearing loss. Ear damage might result from a pressure wave due directly to the imploding membrane, or to loud noise generated by ringing of the vacuum tank, caused by sudden release of the membrane tension.

Experience among members of the CUA and GWU groups exposed to the noise from imploding membranes identical to the current window and a 10-mil mylar window on a 4-foot-long prototype showed that regular safety ear muffs offer good ear protection even within 3 feet of the failure point. During the failure of the mylar window, an observer approximately 20 feet from the failure experienced no temporary hearing loss even though NOT wearing ear protection. Finally, calculations performed at BNL for a window with larger surface area and volume indicated that people at least one meter from the window would be safe wearing ear protection, and that people 6 meters from the window would be safe if unprotected.

5.1.2   Hazard Mitigation

The most important consideration is the very small probability of a window failure. All installed windows are tested with a positive pressure of two atmospheres. If each case the windows are tested three times, for 30 minutes each time. Thus, installed windows are known to be free of manufacturing defects, or weak spots that might cause failure.

Failure due to puncture has been investigated with several prototypes. Even a large puncture, such as from a screwdriver or a knife, will not cause a catastrophic failure (implosion).

Finally, a prototype has been kept under vacuum for over a year. Monitoring results show that the kevlar extension will not reach the manufacturer's predicted break point for many years.

Given that the installed windows are pre-tested for defects, and that windows with no defects are expected to last for many, the chance of an implosion during the time when a person is in close proximity is small. One can minimize this probability by minimizing the time spent near the window when it is under vacuum.

Physical damage due to rushing air is not possible unless a body part is very close to the window and the shutter is not installed. Installing the shutter and wearing eye protection to guard against small debris caught in the stream rushing around the shutter edges will eliminate this hazard.

Noise and pressure wave hazards are mitigated by ear protection and distance. One also expects the shutter to provide substantial buffering of the pressure wave.

Finally, since the window points down, people above it will see only a reflected pressure wave. This decreases the hazard for people working on Level II in the hall or in the tunnel.

5.1.3   Operating Safety Requirements

  1. Ear protection is required for people spending more than a few seconds in the following areas when the window is under vacuum:
    1. In the tagger alcove.

    2. In the upstream beam tunnel.

    3. On the floor of Hall B, in the area limited by the tagger racks on the south, the stairs to Level II on the north, and the downstream end of the tagger racks on the west.

  2. When working inside the hodoscope box when the window is under pressure, workers will wear ear and eye protection.
  3. The shutter will be installed if anyone needs to work inside the box for more than 15 minutes.
  4. The shutter will be installed whenever practical. For example, if beam tests are being performed that will not suffer due to the energy degradation of the electrons passing through the shutter.

5.1.4   Posting Requirements

When the window is under vacuum, signs will be posted in the following areas that say "Caution, tagger window under vacuum, ear protection required for work beyond this point"

  1. On the Hall B floor, at the entrance to the area between the tagger racks and the Level II access stairs.
  2. On the landing of the Level II access stairs, at the entrance to the alcove.
  3. On Level II at the top of the stairs leading to the passage way through the tagger alcove.
  4. In the upstream beam tunnel, facing the door passing the upstream shield wall.

When the window is NOT under vacuum, signs saying "Attention: Tagger window is NOT under vacuum, hearing protection NOT required" will be posted at the same locations.

5.2   Tagger Dump and Faraday Cup

Hall B has two beam dumps, the tagger beam dump and the Faraday cup. When the tagger magnet is on the electron beam is transported to the tagger dump otherwise the beam is to be transported to the Faraday cup. Both dumps are not cooled, and therefore have relatively low power limits. The maximum power that can be deposited into the tagger dump is 1 kilowatt. 1 The maximum power that can be deposited into the Faraday cup is 1 kilowatt. Power near these ratings should not persist for more than one hour. The Faraday cup is located downstream of the last shield wall. This shield wall has a movable lead door that must be in place if the electron beam is being transported to the Faraday cup. This door has a lock on it so that the area is not accessible during a controlled access. The tagger dump which is located in the floor underneath the CLAS detector is not accessible, and the hole in the floor has coverings and railing around it to prevent workers from falling into the hole. The position of the beam at the entrance to the dump can be monitored by a fluorescent screen viewed by a CCTV camera.

5.3   Cryogenic Target

The cryogenic target system was built by Saclay for Hall B electro- and photo-production experiments. It was placed inside CLAS in May of 1997 and operated during commissioning runs during the summer. The target consists of three main separate units:

  1. The control rack containing all automated control and readout devices is installed on level 2 platform in Hall B.
  2. The target chamber with its refrigeration vessel and target cell, attached to the installation cart.
  3. The target gas handling system consisting of bottle supplies and two storage tanks (one for H2 and one for D2) installed permanently in the hydrogen storage area outside of the Hall, and a small storage tank for 3He located in the electronic rack.

The entire system is powered by a single 3-phase cord plug coming from the rear of the control rack, and a UPS. All valves are operated by compressed air available in the Hall. Permanent gas lines and signal cables run from the outside gas shed to the control rack. From there, transfer lines and signal cables run to the target chamber. All relief valves are connected to a common exhaust line.

The target cell is surrounded by a 120 mm thick Kapton foil. The cell has been tested by cycling it 50 times between normal pressure and vacuum. The target vacuum system is connected upstream to the tagger vacuum, and downstream to the alcove beam pipe vacuum. Fast acting valves will isolate the target in case of increased pressure in either upstream or downstream volumes. A detailed description and procedures can be found in Appendix B .

6.0   Operating procedure for the Hall B beam tunnels

There are two tunnels, the upstream tunnel which contains the beam elements that transport the beam to the hall, and the downstream tunnel which contains the elements that transport the beam to the Faraday Cup. At the transition between each tunnel and hall is an alcove. The upstream alcove holds the tagger magnet, and the downstream alcove is the home of the radiative experiment. The tunnels present unique work environments.

6.1   Upstream Tunnel

The upstream tunnel under the scope of this document starts at the green shield wall that separates the accelerator domain from the physics division domain, and continues to the tagger magnet. This region contains both accelerator components and physics division equipment.

6.1.1   Emergency exits

There are two ways out of the upstream tunnel. The most direct route is to pass the tagger magnet onto level 1 of the space frame and exit the hall. The other route is around the green shield wall and follow the beamline upstream to the Beam Switch Yard and exit there. At each exit there are fire extinguishers in case there is a fire in the tunnel.

6.1.2   Magnetic fields

The upstream tunnel holds the final focusing quadrupoles and dipole correctors controlled by the accelerator. Also in this region are the Møller raster magnets, Møller quadrupoles, target rasters, and the Tagger magnet. All physics division magnets can be operated while personnel is present in the tunnel. Red flashing beacons will indicate if the magnets are turned on in the tunnel. If any red beacon is on it is safest to assume that all the magnets are energized. If work is to be done around any magnet in the tunnel the breaker or power supply for the magnet should be locked and tagged. This will prevent any remote excitation of the magnet. All bus work in the tunnel is not accessible by the casual worker, and the energized magnets pose no safety risks. As a matter of convenience the casual worker might want to make sure they are not carrying any magnetic credit cards on their possession when working in the tunnel. Care should be taken not to work with magnetic material near the magnets when they are or can be energized.

6.2   Downstream Tunnel

The downstream tunnel begins after the downstream alcove and continues to the end of the tunnel. Located in this tunnel are several beam diagnostic devices maintained by Hall B. The downstream tunnel also holds the beam dump, which for Hall B is a Faraday Cup. Background from the Faraday cup is blocked by two shield walls, the first shield wall is at the entrance to the tunnel, and the second shield wall is after the beam diagnostic equipment but upstream of the Faraday cup. This second shield wall has a removable lead door which is locked shut during data taking.

6.2.1   Emergency exits

Since the downstream tunnel terminates at the end of the tunnel or at the second shield door if it is locked there is only one way out of the downstream tunnel. It is recommended that all workers in this area follow a two man rule since the area is not visible from outside of the tunnel. All workers in the tunnel should have a working flashlight in case of power failure.

6.2.2   Magnetic Fields

The pair spectrometer magnet can be powered while personnel are in the tunnel. There is no exposed bus and no risk to personnel when this magnet is excited. Care should be taken not to work with magnetic material near the magnet when it is or can be energized.

6.2.3   Confined Space

The region beyond the exhaust tube in the tunnel downstream of the Faraday cup is classified as a Confined space and any entry into this region requires a written Confined Space Work Permit before entering.

 


1. With external cooling, the dump is designed to handle beams up to 10 kW.