Hall A Scattering Chamber Bremsstrahlung Radiator

• Overview
• Hazards and Safety Issues
• Operations
• Pictures
• Credit
• Responsible Personnel
• Special Instructions

Overview

The Hall A Scattering Chamber Bremsstrahlung radiator built in 2006 is similar in principle, but much modified in design from the previous Hall A radiator system built in 1999 by S Esp and R Gilman and successfully operated in Hall A. The 1999 system was in turn based on the Hall C radiator system built by David Meekins and successfully operated in Hall C.

The main similarities are that both systems use an Oregon Micro Systems VME 44 controller to control an Oregon Micro Systems MH10DX step motor driver, which drives a Slo-Syn M063-LS09 stepper motor, to control the radiator assembly, so that one can position any (or none) of several oxygen-free Cu foils in the beam. Both systems are designed so that the foils can be moved with beam on. The main differences are the following:

• The 1999 system mounts immediately upstream of the scattering chamber, about 72.6 cm from the pivot. The new Scattering Chamber system mounts in the scattering chamber, on the entry plate, with the foils about 36.3 cm from the pivot. The closer distance improves the ability to operate at low energies while maintaining the ability to operate at forward angles.
  
• The 1999 system is water cooled to allow operation at high currents, ~30 µA. The new system is only cooled radiatively; as it has no water cooling the current limit is ~5 µA, which leads to a ~30 K temperature rise in the aluminum target assembly and an additional ~100 K temperature rise in the beam spot on the copper foil.
  
• The 1999 system moves a U shaped copper ladder linearly to move the foils into the beam. To prevent thick metal elements from being struck by the beam, the range of motion is limited by limit switches; hard stops provide a backup. The new Scattering Chamber system rotates a semi-circular aluminum target ladder to move the foils into the beam. Due to the lower current limits, the range of motion is limited only by limit switches; if the limit switches fail, a small section of the target ladder passes through the beam. The beam in this case is tripped by the target ion chambers, or by the radiator temperature exceeding the administrative limits.
  

The radiator targets are used in conjunction with a moderate energy, moderate current beam, leading to a large radiation background in the Hall. No local shielding is added, as calculations indicate that this will not significantly affect dose at the site boundary.

Hazarads and Safety Issues

• The main safety issue concerning the Bremsstrahlung radiator is that of induced radioactivity in the Cu targets. After exposure to beam, the targets will be radioactively hot. The radiator must be surveyed and any work must be coordinated with radcon to minimize radiation exposure to individuals.
• A secondary issue is that the thick aluminum radiator target ladder assembly could be damaged if directly hit by beam for an extended period. In normal operation, the target ladder is never in the beam, and the radiator has to be moved past limit switches to put the target ladder into the beam. The radiator is never moved to the in limit, but it is moved to the out limit. A failure of the OMS VME 44 controller and / or control software could lead to the radiator moving past the limits, so that the thicker target ladder is in the beam. In case of failure, extended exposure of the target ladder to the beam is prevented in two ways. First, high target ion chamber levels should trip the beam through the FSD system. Second, rapid heating of the target ladder will be detected by an RTD installed on the ladder, which leads to a software alarm.

Operations

Special Instructions: The radiator is now being manually controlled in the counting house. The manual operating instructions are here.

The only non-expert operational control consists of moving the radiator target assembly to one of seven designated positions, the six foil positions, one with no foils and five with foils, or the out limit. Software controls of the ladder position are operated by MCC, using EPICS software developed by David Wetherholt of the accelerator division. Calibration constants have been determined to position the foils; any recalibrations need to be coordinated with D Wetherholt. While there is no position feedback except that provided by the OMS VME44 controller, the calibration can be confirmed by monitoring target ion chamber levels vs OMS VME44 position. A standard will be measured during the initial commissioning of the radiator. The radiator may be moved when beam is on target. The motion speed is sufficient to move between foil adjacent positions in several seconds, and across the full range of foil positions in about 30 s. The current speed results from a combination of software settings with a 50:1 gear reducer, on loan from D Meekins for the initial experiments.

There are several notes concerning radiator operations:

• If the Hall A IOCHLA (VME crate in which the OMS VME44-2E controller sits) is rebooted, the remote system ``forgets'' where the radiator is! If the IOC is rebooted, it is important to move the radiator to the out limit to reset the positioning information. This should happen as part of the normal boot procedure, but if the system turns on without the radiator moving to its out limit, the operator should move the radiator to the out limit; in particular the operator should not try to move the radiator to any other position until the out limit has been reached.
• The radiator assembly positioned can be manually confirmed in the Hall by marks drawn on the beam left side on the external rotary feed through. These marks indicate the out limit/no foil position, the 1%, 2%, 3%, 4%, and 5%/in limit positions. The few degrees difference between the two extreme ladder positions and the limits are not distinguishable.
• If there are indications that the calibration has changed, the system can be recalibrated in coordination with Dave Wetherholt. The calibration can be checked without beam using the external marks mentioned above, or with beam by comparing the OMS controller position information to the target ion chamber radiation levels.
  
• In case of failure the beam might end up going through the target ladder, leading to higher hall radiation levels and higher target ladder temperatures. In this case the beam should be shut off the the radiator should be moved to its out limit. Normal operations also might lead to a software alarm if the radiator temperature becomes too hot. In this case, after the target cools, beam may be reestablished at lower current, or on a thinner foil.
• The radiator has been tested and moves smoothly. If there is any indication of erratic motion, please contact R Gilman and S Strauch.
  

A backup manual system is in principle available to control the MH10DX step motor driver. The backup system uses a function generator and switch box that is set up in the Hall A in the counting house. The VME 44 controller and the backup system cannot both be hooked up to the control box at the same time. The only people authorized to use the manual control system are R Gilman, D Higinbotham, and S Strauch. Using the backup system requires calibrating it. The function generator is owned and being used by the polarized 3He target, so at present the backup system cannot be easily implemented.

Because the radiator is not water cooled, it is planned for initial commissioning to operate the radiator only at currents up to 5 µA. This leads to an estimated temperature rise of 30 K in the anodized aluminum target assembly, and about 130 K in a 4 (±2) mm square beam spot. The radiator should always be operated with a 4 (±2) mm raster at its maximum current of 5 µA. At lower currents, the raster area may be scaled down proportionally to the current. The current limits depend on radiation levels, monitored by the target ion chambers, and radiator temperature, nomitored by an RTD, and might be changed as a result of the commissioning data.

While the radiator increases the background radiation in the hall, ion chamber trip levels appear to be sufficiently high that this is not a problem for normal operations. Trips levels are generally set to ~25 kRad/hr, while data taking with the 6% foil at about 800 MeV beam energy and 30 µA generates about 1.5 kRad/hr. At the lower beam energies intended for the new scattering chamber radiator, we have no operational experience. It is expected that the radiator thickness will have to be limited to below 6%, because of beam blowup, to ensure the quality of the data. The thickest foil currently installed is 5%.

When the radiator is not being used, the system should be set to the out-limit or no-foil positions. In either position, the radiator is entirely clear of the beam, with all apertures at least ~1.5 cm from the beam.

Power to the control box, and the separate power supply, may be turned off with a front-panel switches if the radiator will not be used for a long time - and should be turned off if work is to be done on the radiator. This deactivates the limit switches, preventing software monitoring of the radiator, but does not affect its position. Both the gear reducer and the torque of the motor should prevent any motion of the ladder. The Hall A technical staff checklist, done as part of preparations for closing the Hall for beam, includes checking the radiator position and the status of the control box.

Debugging

The radiator positioning may be incorrect due to an iochla reboot or to pressing the stop button on the GUI. It is crucial to move the radiator to the out limit to re-establish the positioning information in the VME44 controller. This normally happens as part of the reboot procedure, but if it does not the operator should try to move the radiator to the out limit, and should not try to move the radiator to any other position. If the radiator does not move, there are possible problems either with the state of the software or the hardware. It has been found, for example, that some EDM windows need to be closed and reopened after IOCHLA reboots to function. Rebooting iochla, which takes about 3 minutes, may solve problems. (The readback fields should go "white".)  If the radiator cannot be made to move, possible hardware failures need to be investigated. Check the control power supplies in the hall for about 1.6 Amp holding current on the control box, and 26 V and 0.5 Amps on the power supply. If turning these two boxes off and on does not restore correct values, experts are  needed: D Wetherholt is the software expert, while R Gilman and S Strauch are the hardware experts.

Pictures

Pictures of the new scattering chamber radiator can be found here. The pictures will also be coped to the experiment web site picture section in the near future.

For pictures of the 1999 radiator assembly before installation, the "cross" on the vacumm chamber, and the water cooler, try here.

For pictures of the installed 1999 radiator, the copper foils and target ladder after exposure to beam and the tagged assembly, try here.

Credit

The design for the Brem Radiator for Hall A was done by Susan Esp. The radiator was constructed largely by Rutgers University using the Physics Department machine shop. Assembly was done by J Dumas (RU), G Kumbartzki (RU), Y Rousseau (RU), and S Strauch (SC). Additional installation work was done by Ed Folts (Hall A). The system reuses the controls of the 1999 radiator system, which were partly bought by Rutgers University, and partially contructed by  David Meekins (then with FSU) and Rutgers personnel. EPICS software was developed by David Wetherholt (Accelerator). Jack Segal (Hall A) also contributed to assembly and debugging of the new radiator system.

Responsible Personnel

In case of problems with the radiator, one should contact the people mentioned above for specific details, and / or

• Ron Gilman, x7011
  

Special Instructions

Care must be taken in case of any removal or disassembly of the radiator system is needed. The stepper motor should not be disconnected from the motor driver while power is on; this can lead to damage to the motor and or motor driver.

The Cu targets will certainly be activated in the course of an experiment. Therefore, only remove the Cu target, the target ladder, and / or the whole radiator system after coordinating with  Radcon.