The Hall A Bremsstrahlung radiator design is based on the Hall
C radiator system built by David Meekins and successfully operated
in Hall C. It consists of an oxygen-free copper target ladder with six
positions, each containing different thicknesses of oxygen-free Cu foils.
The foils are 6.35 cm wide by 3.175 cm high, with thicknesses of 2% to
6% of a radiation length - the 1% foil position is left as an empty position.
The foils are positioned 3.332 cm apart (center to center). The Bremsstrahlung
radiator is the last element in the Hall A beam line before the scattering
chamber, with the foils at a design distance of about 72.6 cm from the
center of the physics targets. The assembly is mounted oriented vertically,
and is designed so that no part of the system except the target foils can
intersect the beam axis. The ladder itself is U shaped, with a 3.175 cm
wide gap for the target foils. Note that the radiator ladder may be tilted
slightly, leading to a 1/cos(theta) increases in the target thickness.
The target ladder is moved up and down, moving the target foils in and
out of the beam, by use of a stepper motor. To prevent destruction of the
targets by a high current beam, the targets are conductively cooled by
the radiator ladder, which is itself water cooled. Because the radiator
targets are used in conjunction with a high energy, high current (up to
30 microampere) beam, a large amount of radiation background is produced
in the Hall. No local shielding is added for initial tests, as calculations
indicate that this will not significantly affect dose at the site boundary.
The only safety issue concerning
the Bremsstrahlung radiator is that of induced radioactivity in the Cu
targets, or more seriously, in the water used for cooling the targets.
The water cooling system is a closed loop. The heart of the system is a
portable welding torch water cooler, located under the beam line just upstream
of the target. The cooler is kept in a tray which is intended to provide
secondary containment in case of a leak. The cooling system must not be
breached or drained without concurrence from the RCG. Accidental breach
or spill constitutes a radiation contamination hazard. A spill control
kit, capable of containing a system leak or spill, is staged by the door
to the hall. Note that the water cooler FSDs should
be rececked before each experiment.
The only operational control consists of moving
the ladder in and out. Software controls of the ladder position are operated
by MCC, using EPICS software developed by David Wetherholt of the accelerator
division. After the final calibration constants are determined, it is only
necessary to specify which of the foil positions is desired, or to move
the radiator to its out limit - there is no reason to ever move the radiator
to the in limit, although this functionality is provided. The radiator
may be moved when beam is on target.
There are two notes concerning radiator operations:
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 can be set up in either the Hall A equipment
aisle or 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 Ronald Gilman, Xiaodong
Jiang, David Meekins, and Steffen Strauch. The function generator is owned
and being used by the polarized 3He target, so at present the backup system
cannot be easily implemented.
Although the radiator foils are water cooled,
a high current electron beam may deposit enough energy to melt the foils.
At present it is not planned to operate the radiator at currents above
30 microamperes. Assuming a circular beam spot and circular cooled rim,
one may estimate the temperature rise of the beam spot area as T-beam(deg
C) - T-water(deg C) = (P/2 pi k t) ln(r-outer/r-inner), where P is the
power deposited in the foil, k is the thermal conductivity of copper, t
is the foil thickness, r-outer is the radius of the water cooled rim, and
r-inner is the radius of the beam spot. Including a safety factor, we can
limit the temperature rise to 100 degrees C with the following raster radius
as a function of beam current:
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. Trips levels
are generally set to 25 kRad/hr, while data taking with the 6% foil at
about 800 MeV beam energy and 30 microAmps generates
about 1.5 kRad/hr. Note that for lower beam energies
it may be necessary to limit the radiator thickness
to below 6%, because of beam blowup, to ensure the quality of the data.
When the radiator is not being used, the system
should be set to the out-limit position. In the out-limit position, the
radiator will be entirely clear of the beam, with the bottom foil about
1.7 cm above beam height, and the ladder water-cooling tubes about 3.505
cm horizontally away from the beam center. As a safety measure, since the
installed radiator position is being checked, but not adjusted, with surveying,
the lowest ladder position, for the 1% radiator, has been left clear for
the initial series of tests.
Power to the control box may be turned off with
a front-panel switch 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. The separate
power supply should also be turned off. This deactivates the limit switches
and the linear encoder, preventing software monitoring of the radiator,
but does not affect its position. Both the drive screw and the torque of
the motor should prevent any motion of the ladder, However, hard stops
should be installed in this case as an additional safety measure. The Hall
A technical staff checklist, done as part of preparations for closing the
Hall for beam, includes checking the radiator position, the status of the
control box, and the installation of hard stops.
If the radiator water cooler FSDs trip the beam,
and do not reset, it is necessary to go into the hall to check the
status of the cooling system. The cooler, located
upstream of the radiator under the beam line, should be turned
on and showing a pressure of about 55 psi, a
flow rate of about 6 liters/minute, and a trip level of about 2 liters/minute.
Standard problems that have occurred include
evaporation of water from the reservoir, and wearing out of a bearing
that connects the electric motor to the pump
fan. It is almost certain that the Hall A tech on call should be consulted
and called in if there is a problem.
The radiator positioning may be incorrect due
to an iochla reboot or to pressing the stop button on the GUI. In either
case, moving the foil to the out limit should
reset the positioning.
If the radiator does not move, there are possible
problems either with the state of the software or the hardware.
It may be possible to solve software problems
by typing in a new destination number , a few thousandths different
from the current position - as opposed to pressing
a foil button. Rebooting iochla, which takes about 3 minutes, may
also solve problems. (The readback fields should
go "white".) Failing this, 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 the
boxes off and on does not restore correct values,
experts are needed.
Two sets of pictures of the radiator can be found on the Hall A web site.
For pictures of the radiator assembly before installation, the "cross" on the vacumm
chamber, and the water cooler, try
here.
For pictures of the installed radiator, the copper foils and target
ladder after exposure to beam and the tagged assembly, try
here.
The design for the Brem Radiator for Hall A is
based on the Hall C system built by David Meekins. Design work was done
by Susan Esp. The radiator was constructed largely by Rutgers University
using the Physics Department machine shop. Additional hardware and some
modifications were provided by Ed Folts of Hall A. Control circuitry was
built by David Meekins of Florida State. EPICS software is being developed
by David Wetherholt of Accelerator Division.
In case of problems with the radiator, one should
contact the people mentioned above for specific details, and / or
Care must be taken in case 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 in the presence of a
Radcon officer.
The control system uses 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. Read backs include in and
out limit switches, and a linear encoder to determine the actual radiator
position. The VME 44 controller is located in rack 1H75B02 in the Hall
A equipment aisle. The ADC, in slot 5 of the CAMAC crate in the same rack,
is connected through a patch panel to block 30 in rack 1H75B08; channels
15 and 16 are used by the radiator. A custom built control box, containing
the MH10 driver, power supplies, and other control circuitry, is located
in rack 1H75B10. In addition, an external supply used for the
motor is positioned above this box.
Current (uA)
Minimum raster radius (mm)
10
not needed
15
0.2
20
0.7
25
1.3
30
2.1
The Hall A technical staff has documentation for
the radiator, and spares of most major components. Ron Gilman has some
additional spares, including all spare foils.
Please send any comments on this page to Ronald Gilman,
gilman@jlab.org.
Ron Gilman January 15, 2001, minor updates May 2006