The purpose of the shift was to check the neoVAN seeder beam profile and try to understand why we encounter some power loss (log entry 40855). This shift happens between 3/04/18 11pm LT and 4/04/18 8am LT and should be associated to the logentry https://logbook.virgo-gw.eu/virgo/?r=40976
# upon arrival 3/04/18- 08h43 LT, we found the PMC_TRA_DC = 1,02 V = 62,9 W (AOM is still connected)
# we got also P_LB_M9 = 94,6 W (P at neoVAN output)
# we made a status of the PSL power ( 3/04/18, both calo zeroed. @ 10:21 LT: Step9_1)
we disconnect the AOM: PMC_TRA_DC moves from 1.022 to 1.065 V = 2,65 W (S_PMC_TRA = 61.68 W/V)
# issue_1: we were rather expecting a raise of 3,5 - 4,65 W (see logentry 40434, second plot).
d P_LB_M6 Delta_TE02 R_PMC_V_OR R_PMC_W_OR R_PMC_Reso T_PMC (1-C_R_1) T_PMC_%
m mW mV V W W W
0.55 17.2 280 3.47 88.6 22.6 64.3 0.1744 0.9808 Step9_1
Legend:
R_PMC_V/W_OR: OR for OffReso, V/W for Volt or Watt
1-C_R_1 = R_PMC_Reso/R_PMC_W_OR - Delta_%_TEM02
Delta_%_TEM02 = Delta_TEM02/R_PMC_V_OR
T_PMC_% = (T + R_PMC_Reso)/R_PMC_W_OR
--> the incident power is now 88,6 W instead of 93,7 W in Dec 2017 (see log entry 40855)
--> the PMC transmission is still around 0.98
# we change the current of the neoVAN amplifier to proceed with some calibration
@1206773658: AMP_D2 from 5.3 to 5.2 A (~ 6:54 UTC)
@1206773841: AMP_D2 from 5.2 to 5.3 A
- P_neoVAN decreases by 1,489 %
- P_PMC_TRA increases by 1 %
- P_PMC_REFL decreases by 15 %
--> it moves also the matching wrt PMC: useless for any calibration !?
# we check the seeder beam profile (BP) at the neoVAN input, in transmission of LB_M6 mirror (T~1.084e-3).
we found the BP changed wrt what set on Dec 40260, (logentry 40260, profile "k").
The BP featured a 132 um waist at 0.49 m from LB_L5 and is now
115 um @ 0.50 m from LB_L5 lens.
a change of the telescope focal length from 0.45 m to 0.40 m would explain such a change,
but would also bring the waist closer to the LB_L5 lens by 7 cm, which is not observed.
see plot "k" (old meas.), "l" (4/04/18 new meas.)
-> no clue yet on why the BP_seeder has changed
# we setup a new BP of the seeder (see plot "n")
The BP is now 136/139 um, although not optimal we keep it for the time being (optimal should be 130 um)
the distance(LB_L4 to LB_L5) is now 33 mm (lens to lens) instead of 36 mm.
LB_L4 is set at 98,7 cm from LB_WP5 (used to be 110,7 cm before)
LB_L5 is set at 0.635 m from neoVAN input (=0.32+.21+.105 m)
it means the seeder waist is at 5,5 cm outside of the neoVAN (nominal is 5cm)
we reduce the neoVAN pump current from 4.7/5.3 A to 4.4/5.3 A
This improves the neoVAN modal content and reduces the PMC mismatch.
-> As a result, although the neoVAN output power is less by 4 W the PMC_TRA is improved by 3-4 W
rem.: the seeder together with the pump is responsible for the neoVAN power change
# Issue
the signal PSL_AMP_DC does not see the neoVAN power change: we suspect the beam is too large on the monitoring PD and it may happen that the HOM are clipped
The modal analysis with the PMC shows that 10% of the neoVAN power is spread on HOM of order 4.
Rem.: outer radius of HOM_order4 ~ 3 x waist
# we made a status of the PSL power before leaving ( 4/04/18, @ 06:50 LT: Step9_3)
d P_LB_M6 Delta_TE02 R_PMC_V_OR R_PMC_W_OR R_PMC_Reso T_PMC (1-C_R_1) T_PMC_%
m mW mV V W W W
0.55 17.2 280 3.47 88.6 22.6 64.3 0.1744 0.9808 Step9_1
0.635 xx 0.110 3.48 84.6 15.3 68.2 0.148 0.987 Step9_3
## Conclusion:
- the power seems to have decreased from 93.7 W (December) to 88.6 W (April 2018) at the PMC reflection; not clear whether it is due to the seeder size change
- the neoVAN monitor PD is suspicious
- the neoVAN power measured at neoVAN output with a calorimeter does not seems to have changed
- the PMC throughput is still around 98 %
- the work needs to go on with the seeder optimization together with the neoVAN pump optimization
