The LPL features several energy meters looking at low-energy leakage/diagnostic pick-offs in order to estimate the pulse energy of each stage of each arm of the system. Using an energy meter in the full beam at each location, a comparison can be made to the raw energy measurement of the low-energy leakage/diagnostic pick-off, and a calibration constant can be calculated.
To be sure the calibration constant was calculated correctly, it is usually a good idea to take the average of several shots, watching out for outliers that could belie an instrument malfunction or controls issue.
Once the new calibration constant is calculated, it can be saved into the appropriate field of the laser notepad PVs.
The new calibration constant should also be backed up to the meclas package's GLOBAL class, which protects these values from data loss arising from IOC/host problems.
Most often, even a major re-calibration is still usually only off by a few percent (<10%), but most often the discrepancy is only 1-2%.
If the discrepancy is VERY large, this could point to a different problem. Here are some of the most common:
- are the Gentecs in the pick-off in the diagnostic bay all responding appropriately?
- (usually this is one reason why I shoot each leg multiple times to MAKE SURE that the number updates every time, otherwise it may be a phantom reading from an unresponsive device)
- are the IOCs all configured and working appropriately?
- (if the settings are wrong then it might read out garbage -- for example, if the internal triggering threshold is wrong then it might not catch the full real pulse)
- (if the pulse energy of the pick-off is near the top end of the pre-set energy range of the meter, this can also cause phantom readout values; it's usually better to make sure the expected energy is well below the upper limit of the pre-set energy range of the meter)
- is there any clipping of the pick-off beam headed to the power meter?
- (this can be especially difficult to detect; the fact that the pick-off energies were way too LOW would also be potentially indicative of clipping of the pick-off beams incident on the energy meters)