LISA Pathfinder Performance Confirmed in an Open-Loop Configuration: Results from the Free-Fall Actuation Mode

We report on the results of the LISA Pathfinder (LPF) free-fall mode experiment, in which the control force needed to compensate the quasistatic differential force acting on two test masses is applied intermittently as a series of “impulse” forces lasting a few seconds and separated by roughly 350 s periods of true free fall. This represents an alternative to the normal LPF mode of operation in which this balancing force is applied continuously, with the advantage that the acceleration noise during free fall is measured in the absence of the actuation force, thus eliminating associated noise and force calibration errors. The differential acceleration noise measurement presented here with the free-fall mode agrees with noise measured with the continuous actuation scheme, representing an important and independent confirmation of the LPF result. An additional measurement with larger actuation forces also shows that the technique can be used to eliminate actuation noise when this is a dominant factor.

Figure 1

LPF capacitive actuation along $x$ and housing coordinate systems. $g_1$ and $g_2$ indicate the stray acceleration experienced by TM1 and TM2, respectively.

Figure 2

Time series of TM2 force (top), relative displacement, $\mathrm{\Delta }x$ (middle), and $\mathrm{\Delta }g$ (bottom) measured in June 2016. The thin lines refer to a noise measurement with continuous control on TM2 in URLA authority, while the thick lines indicate a free-fall mode measurement in nominal authority. The discontinuity in the top panel stems from the use of different telemetry packets. The transient phase between the two runs is discarded in the bottom panel.

Figure 3

Fit results to free-fall data, measured in December 2016, to remove the bias from the spectrum (see the text for details). The peaks are excluded from the analysis and hence not fitted.

Figure 4

Acceleration noise estimate with free-fall mode in URLA $\varphi$ authority compared with that in continuous actuation mode in URLA and the LISA requirements [2]. The free-fall ASD (dots) results from 20 periodograms, while the continuous noise run (asterisks) is $\sim 18$ days long (78 periodograms), both calculated at $1\sigma$ confidence interval and according to the method presented in [10]. Shadowed area: estimate of the “native” free-fall spectrum. Dashed lines: actuation noise predictions.

Figure 5

Comparison of $\mathrm{\Delta }g$ ASD between the free-fall mode experiment executed in June 2016 in nominal $\varphi$ authority (thick solid line, 2 periodograms) and the continuous control mode run carried out in May 2016 in the same authority (thin solid line, 3 periodograms). The dashed lines are the noise predictions as explained in the text.