Laser system manual

October 5, 1999

1. Logging in

Login on pcceres7 (user ceres, standard password) and set display by typing "setenv DISPLAY ...". Start the laser control code by typing "laser-tcltk&". Start the mirror control code by typing "manager-tcltk&". Start the diodes monitor code by typing "diodes-tcltk" (without ampersand - it is easier to kill it if something goes wrong). Make sure that the server process diodes_shmwriter is running.
 

2. Operating laser (laser-tcltk)

Press "Pump On" to start cooling. Set voltage to 100, rate between 1 and 10 Hz, Delay to 70, Rate divider to 1. Press "Lamp On" to turn on the flash lamp. Press "Open" to open the shutter. Adjust laser energy by changing the Q-switch delay between 20 and 75.  Lower delays give higher energy and less energy fluctuations. Higher delays give a better beam profile.

 

To turn off the laser, press "Close" to close the shutter, then press "Lamp Off" to switch off the flash lamp, and finally press "Exit" or press the <Escape> key to quit laser-tcltk.

3. Moving mirrors (manager-tcltk)

CERES zone
Mirror 1 is on the small platform on the Jura wall of the zone.  It reflects the beam by 90 degrees to the right, i.e. toward the accelerator beam axis. Mirror 2 is on the TPC axis, 20 cm downstream from the backplate. It reflects the beam by 90 degrees to the right, i.e. toward the target. Rotary stage 1 is on the center of the TPC backplate. The mirror mounted on this stage reflects the beam by 90 degrees outwards. By rotating this stage one changes the phi-coordinate of the laser beam.

The  manager-tcltk window is shown below.  In the following the subwindows will be briefly described.

burst

Select "true SPS" burst if you want to move the mirrors in- and out-of beam axis burst by burst. Select "manual"
if mirrors are allowed to stay in beam all the time.

flippers

The four flippers carry MIR 2, lambda/4, Rotating Mirror, and diode 2.  Selecting a flipper button means inserting the given item into the beam. Selecting "auto" will mean flipping the mirrors in during the burst pause and out during the burst. Flippers should be in when laser beam is being used, and out when the particle beam is coming. This is to avoid background in the TPC; the mirrors themself will not be damaged by the particle beam. The behaviour of the flippers can be watched on the TV monitor, same as the one showing the SPS burst status, but on channel 0 instead of 1.

trigger

If "always" is selected, each laser pulse will send a pulse to the experiment's trigger system. The meaning of the other buttons should be obvious.

beam steering

The MIR 1 and MIR 2 mirrors should only be adjusted by an expert. The Rotating Mirror allows to shoot the laser beam radially towards different TPC sectors. The orientation of the Rotating Mirror can be changed by pressing buttons "< next phi diode" and "next phi diode >".  The mirror can also be moved manually (buttons "<<", "<", ">", and ">>"), and the laser beam can be centered on a diode (button "center").

A typical calibration setting is "manual" burst ,"burst " not selected,  all flippers in, trigger "always".

A typical monitoring setting is "true SPS" burst,  all flippers "auto", trigger 1 per burst,  rot-mir "auto".
 

4. Using diodes to monitor the beam position (diodes-tcltk)

The diodes detect incident laser beam and yield the position to better than 0.1 mm. Each diode has the size of 10x10 mm^2. The diodes are usually located behind the mirrors. The mirrors are partially reflecting. The transmitted beam hits the diode. The particular locations are as follows In diodes-tcltk the position of the laser beam on each diode is represented by a blue spot. The spot size is related to beam energy (not to beam diameter). The spot becomes red if one of the four diode channels has overflow. In this case the position information is unreliable. The window "Diode ..." shows the diode data averaged over all events contained in the buffer. The three numbers are x,y (in mm) and intensity (in ADC channels). "Clear buffer" clears the event buffer. "Save buffer" writes the average positions of all diodes on file diodes-tcltk.dat.
 

Dariusz Miskowiec