Operations

Although the radiator foils are water cooled, a high current electron beam may melt the foils. Beam currents with the radiator will be limited to 30 micro-amperes. Including a safety factor, the raster radii given in Table 1 will limit the temperature rise to 100 $^{\circ}$C.

Table 2.7: Raster radius as a function of beam current.

Current ($\mu$A)	Minimum raster radius (mm)
10	(not needed)
15	0.2
20	0.7
25	1.3
30	2.1

The only operational control consists of moving the ladder in and out. Radiator position is determined by the ratio of the readback voltage from a linear encoder to the voltage applied to it. Table 2 gives the radiator position as a function of this ratio. The foil thicknesses are set so that the thickness, in percent of a radiation length, equals the foil number, except that no foils are mounted in position 1.

Table 2.8: Encoder voltage calibration. See text.

Position	Voltage ratio	$V_{\rm encoder}$ for $V_{\rm supply}$ = 5 V
out limit	0.030	0.15
foil 1	0.102	0.511
foil 2	0.269	1.346
foil 3	0.436	2.179
foil 4	0.603	3.013
foil 5	0.769	3.847
foil 6	0.936	4.681
in limit	0.966	4.831

Software controls of the ladder position are under development; radiator position is changed by calling MCC and requesting that the radiator be set to some foil position, or to the out limit. The position may be changed with beam on. A manual-control backup system also exists.

Both software and manual backup systems control an Oregon Micro Systems MH10DX step motor driver, which drives a Slo-Syn M063-LS09 stepper motor. The MH10 driver, power supplies, and other control circuitry, are in a custom-built box located in the hall in rack 1H75B10. The linear encoder voltage ADC is in slot 5 of the CAMAC crate in rack 1H75B02; radiator inputs use channels 15 and 16, and are connected through a patch panel to block 30 in rack 1H75B08.

When the radiator is not being used, the system should be set to the out-limit position, so that it is clear of the beam. Power to the control box in the hall 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. This deactivates the limit switches and the linear encoder, but does not affect positioning. Additional hard stops should be installed as a 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.