A REVIEW OF THE EFFECTS OF CHANGES IN THE VOID FRACTION OF THE SUPERHEATED REGION ON THE TEV OPERATION AND EVAPORATOR STABILITY

A level of superheat is necessary to ensure proper operation of vapor- compression refrigeration systems. An overly superheated vapor prior to entering the compressor can render the system inefficient, while inadequate superheat can result in evaporator instability and compressor damage. The void fraction present in the suction line is directly related to evaporator stability and the hunting phenomenon in the thermostatic expansion valve (TEV). This article aims at reviewing superheated region-related researches in the field of refrigeration as they relate to the operation of the TEV and the stability of the evaporator. In a first part, literature review will explain the causes of the void fraction in the superheated region and the second part describes the effects of the lubricant oil on the void fraction. The third part of this article is a review of the effects of the superheated region on the TEV and on evaporator stability, explaining the causes of the hunting phenomena and evaporator instability

Noise characterization of an atomic magnetometer at sub-millihertz frequencies

Noise measurements have been carried out in the LISA bandwidth (0.1 mHz to 100 mHz) to characterize an all-optical atomic magnetometer based on nonlinear magneto-optical rotation. This was done in order to assess if the technology can be used for space missions with demanding low-frequency requirements like the LISA concept. Magnetometry for low-frequency applications is usually limited by $1/f$ noise and thermal drifts, which become the dominant contributions at sub-millihertz frequencies. Magnetic field measurements with atomic magnetometers are not immune to low-frequency fluctuations and significant excess noise may arise due to external elements, such as temperature fluctuations or intrinsic noise in the electronics. In addition, low-frequency drifts in the applied magnetic field have been identified in order to distinguish their noise contribution from that of the sensor. We have found the technology suitable for LISA in terms of sensitivity, although further work must be done to characterize the low-frequency noise in a miniaturized setup suitable for space missions.Comment: 11 pages, 12 figure

LISA pathfinder micronewton cold gas thrusters: in-flight characterization

The LISA Pathfinder (LPF) mission has demonstrated the ability to limit and measure the fluctuations in acceleration between two free falling test masses down to sub-femto-g levels. One of the key elements to achieve such a level of residual acceleration is the drag free control. In this scheme the spacecraft is used as a shield against any external disturbances by adjusting its relative position to a reference test mass. The actuators used to move the spacecraft are cold gas micropropulsion thrusters. In this paper, we report in-flight characterization of these thrusters in term of noise and artefacts during science operations using all the metrology capabilities of LISA Pathfinder. Using the LISA Pathfinder test masses as an inertial reference frame, an average thruster noise of ~0.17¿¿µN/Hz is observed and decomposed into a common (coherent) and an uncorrelated component. The very low noise and stability of the onboard metrology system associated with the quietness of the space environment allowed the measurement of the thruster noise down to ~20¿¿µHz, more than an order of magnitude below any ground measurement. Spectral lines were observed around ~1.5¿¿mHz and its harmonics and around 55 and 70 mHz. They are associated with the cold gas system itself and possibly to a clock synchronization issue. The thruster noise-floor exhibits an excess of ~70% compared to characterization that have been made on ground on a single unit and without the feeding system. However this small excess has no impact on the LPF mission performance and is compatible with the noise budget for the upcoming LISA gravitational wave observatory. Over the whole mission, nominal, and extension, the thrusters showed remarkable stability for both the science operations and the different maneuvers necessary to maintain LPF on its orbit around L1. It is therefore concluded that a similar cold gas system would be a viable propulsion system for the future LISA mission.Peer ReviewedPostprint (author's final draft

Novel methods to measure the gravitational constant in space

We present two novel methods, tested by LISA Pathfinder, to measure the gravitational constant G for the first time in space. Experiment 1 uses electrostatic suspension forces to measure a change in acceleration of a test mass due to a displaced source mass. Experiment 2 measures a change in relative acceleration between two test masses due to a slowly varying fuel tank mass. Experiment 1 gave a value of G=6.71±0.42(×10-11)¿¿m3¿s-2¿kg-1 and experiment 2 gave 6.15±0.35(×10-11)¿¿m3¿s-2¿kg-1, both consistent with each other to 1s and with the CODATA 2014 recommended value of 6.67408±0.00031(×10-11)¿¿m3¿s-2¿kg-1 to 2s. We outline several ideas to improve the results for a future experiment, and we suggest that a measurement in space would isolate many terrestrial issues that could be responsible for the inconsistencies between recent measurements.Peer ReviewedPostprint (published version

Temperature stability in the sub-milliHertz band with LISA Pathfinder

This article has been accepted for publication in "Monthly notices of the royal astronomical society" published by Oxford University Press.LISA Pathfinder (LPF) was a technology pioneering mission designed to test key technologies required for gravitational wave detection in space. In the low frequency regime (milliHertz and below), where space-based gravitational wave observatories will operate, temperature fluctuations play a crucial role since they can couple into the interferometric measurement and the test masses’ free-fall accuracy in many ways. A dedicated temperature measurement subsystem, with noise levels in 10¿µK¿Hz-1/2 down to 1¿mHz was part of the diagnostics unit onboard LPF. In this paper we report on the temperature measurements throughout mission operations, characterize the thermal environment, estimate transfer functions between different locations, and report temperature stability (and its time evolution) at frequencies as low as 10¿µHz, where typically values around 1¿K¿Hz-1/2 were measured.Peer ReviewedPreprin

Characteristics and energy dependence of recurrent galactic cosmic-ray flux depressions and of a forbush decrease with LISA Pathfinder

The final publication is available at IOS Press through http://dx.doi.org/10.3847/1538-4357/aaa774Galactic cosmic-ray (GCR) energy spectra observed in the inner heliosphere are modulated by the solar activity, the solar polarity and structures of solar and interplanetary origin. A high counting rate particle detector (PD) aboard LISA Pathfinder, meant for subsystems diagnostics, was devoted to the measurement of GCR and solar energetic particle integral fluxes above 70 MeV n-1 up to 6500 counts s-1. PD data were gathered with a sampling time of 15 s. Characteristics and energy dependence of GCR flux recurrent depressions and of a Forbush decrease dated 2016 August 2 are reported here. The capability of interplanetary missions, carrying PDs for instrument performance purposes, in monitoring the passage of interplanetary coronal mass ejections is also discussed.Peer ReviewedPreprin

Precision charge control for isolated free-falling test masses: LISA pathfinder results

The LISA Pathfinder charge management device was responsible for neutralizing the cosmic-ray-induced electric charge that inevitably accumulated on the free-falling test masses at the heart of the experiment. We present measurements made on ground and in flight that quantify the performance of this contactless discharge system which was based on photoemission under UV illumination. In addition, a two-part simulation is described that was developed alongside the hardware. Modeling of the absorbed UV light within the Pathfinder sensor was carried out with the Geant4 software toolkit and a separate Matlab charge transfer model calculated the net photocurrent between the test masses and surrounding housing in the presence of AC and DC electric fields. We confront the results of these models with observations and draw conclusions for the design of discharge systems for future experiments like LISA that will also employ free-falling test masses.Peer ReviewedPostprint (author's final draft

Analysis of the accuracy of actuation electronics in the laser interferometer space antenna pathfinder

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Armano, Michele, et al. "Analysis of the accuracy of actuation electronics in the laser interferometer space antenna pathfinder." Review of Scientific Instruments 91.4 (2020): 045003. and may be found at https://aip.scitation.org/doi/10.1063/1.5140406The Laser Interferometer Space Antenna Pathfinder (LPF) main observable, labeled ¿g, is the differential force per unit mass acting on the two test masses under free fall conditions after the contribution of all non-gravitational forces has been compensated. At low frequencies, the differential force is compensated by an applied electrostatic actuation force, which then must be subtracted from the measured acceleration to obtain ¿g. Any inaccuracy in the actuation force contaminates the residual acceleration. This study investigates the accuracy of the electrostatic actuation system and its impact on the LPF main observable. It is shown that the inaccuracy is mainly caused by the rounding errors in the waveform processing and also by the random error caused by the analog to digital converter random noise in the control loop. Both errors are one order of magnitude smaller than the resolution of the commanded voltages. We developed a simulator based on the LPF design to compute the close-to-reality actuation voltages and, consequently, the resulting actuation forces. The simulator is applied during post-processing the LPF data.The Spanish contribution has been supported by contracts AYA2010-15709 (MICINN), ESP2013-47637-P, and ESP2015-67234-P (MINECO). M. Nofrarias acknowledges support from Fundacion General CSIC (Programa ComFuturo). F. Rivas acknowledges an FPI contract (MINECO).Peer ReviewedPostprint (author's final draft

Dark Tetrad and workplace deviance: Investigating the moderating role of organizational justice perceptions

This study tested the direct effects of Dark Tetrad traits on organizational and interpersonal counterproductive work behaviors (CWBs). We also examined the moderating effects of the three dimensions of organizational justice – distributive justice, procedural justice, and interactional justice – on the Dark Tetrad-CWBs relationships. Based on the data from 613 employees across different occupations, the results revealed that only psychopathy and sadism had significant effects on CWBs targeted at the organization. The results also supported the direct effect of sadism on interpersonal CWBs. The findings confirmed the moderating role of interactional justice but differentially, depending on the dark trait and the target of workplace deviance. Whereas low and medium levels of interactional justice moderated the relationship between Machiavellianism and CWBs directed to the organization, it did not play any role in narcissism, psychopathy, and sadism. Regarding CWBs aimed at other people, interactional justice emerged as a significant moderator in Machiavellianism and sadism. But, whereas sadistic employees performed more harmful behaviors toward other individuals whatever their level of interactional justice, if people high in Machiavellianism (Machs) perceived a high fair interpersonal treatment, they did not show deviant behaviors directed at other employees. The paper concludes with some suggestions and recommendations about the relevance of organizational justice in the influence of dark personality traits on CWBs