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Injection system (General activities)

derossi, chiummo, cleva - 16:08 Monday 12 August 2019 (46653)

laser amplifier beam drift investigation

Since we are experiencing a continuous drift of the beam coming out from the NEOVAN laser amplifier (look at the entry 46593 and to the plots 1 to 3, which are the camera images of the beam at the output of the amplifier in April, May and August), this morning we made some tests proposed by F. Cleva to better understand this behaviour.

The hypothesis in fact is that due the pumping diodes ageing, a varying thermal lens is causing the drift in the position of the beam (mainly horizontally). We thus modified the current applied to the diodes, at first by steps od 0.1 A, then by steps of 0.2 A and as a third test by steps of 0.1 A periodically inverted.

The GPSs are given in the 4th plot. The default values for the diodes are 4.7 A for D1, D2 and 5.3 A for D3, D4.

Analysis will follow...

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Comments to this report:

Cleva - 18:18 Tuesday 20 August 2019 (46722)

The idea was to check whether the diode ageing could triggers the beam alignment drift.

# Sensitivity of beam alignement with diode power

plot 1 says that for dP/P = 4% we notice a beam drift by 40 mV on AMP_QD_NF_H

Rem.: the coupling RIN to offset of AMP_QD_NF_H (0.1 V) would lead to only 4 mV, therefore the main contribution is indeed related to beam drift

plot 2 says that since the beg of O3 the neoVAN power has reduced by 1.5 W, that is ~ 1.6 % (upper blue line), in the meantime the AMP_QD_NF_H has drifted by 12.5 / 50 = 0.25 V (*), green line

(*) I used a factor 50 scaling while plotting AMP_QD_DF_H (for display convenience)

Conclusion

The long term trend of the alignment drift is much larger with what would have been expected from the direct calibration made in 46653

Therefore the diode ageing is not the main contributor ot the beam drift

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