Epics controls

The signals coming from the Unser monitor and from the two cavities are measured every second by three digital multimeters controlled via the Hewlett Packard GPIB system. The bus is used to send commands to the instruments and to receive the measurements.

Via a 50 pins ribbon, the GPIB network is connected to an Industry Pack IEEE488 pluged into an IOC MVME162, located in a regular VME crate in the counting house.

For more information about epics, see the Epics home page.

The operators are able to control the operations via three MEDM screens, all accessible from the main "Hac.adl" display (can easily be opened from any Hac terminal).
The main screen "Hac_bcm.adl" displays a big strip chart representing the current read by the Unser and the 2 BCMs as a function of time. The final value of the current read by the monitor is also displayed, as well as other informations:

• the strip chart range controls
• the last calibration's date
• a "Calibration Required" message window
• buttons for accessing the other screens, and some instructions.

The calibration screen is used to perform the current monitor calibration. The goal is to calibrate the two cavities against an absolute reference, here the Unser monitor. To do so, several operations must be performed:

• measure the offset of the Unser:
  to be able to make this measurement, one must have a step with the beam, measure the two sides of the step and make the difference between the two readings.
  Usually the Faraday Cup 2 of the injector is used to make these steps; the operator just has to insert it in the beam (or remove it).
• calculate the calibration coeffficients:
  after substracting the offset from the Unser reading, divide this current by the output of the cavities to get the two coefficients.
• repeat these operations several times:
  as the offset of the Unser can fluctuate a lot, it's recommended to get some statistics on the previous measurement. Usually, a calibration process is composed of 10 steps.

The calibration process is performed by a State Sequencer Program loaded inside the IOC.

Also it is recommended to do the process with the maximum current available in the machine, in order to get the full accuracy of the Unser.

To perform the calibration, tell the accelerator operators to set it up, then when everybody is ready press the "START" button: status immediately goes from "stopped" to "In Process", and as soon as the first step is done the time elapsed since last calibration resets to zero, and the message stops blinking.
When the desired number of steps is reached, press the "STOP" button: status goes back to "stopped", and the coefficients are updated.

Since the BCMs are drifting over time, the calibration has to be redone every day in order to keep a good accuracy in the measurements.

The multimeters control screen allows the operator to control the two HP3458A and the HP34401A which are used to measure the signals coming respectively from the two cavities and from the Unser.
When the IOC is booted, epics automatically configures the three voltmeters with their default parameters, which can be modify for some particular measurement. However always remember:
Do Not Modify any Parameter during a Run, it might affect the data acquisition.

The default configuration parameters for the HP3458A are:

• FUNC ACV
• END ON
• SETACV SYNC
• ACBAND 990000,1010000
• RES 0.15

The default configuration parameters for the HP34401A are:

• FUNC DCV