Test-mass Charging Research Articles

240 nm UV LEDs for LISA test mass charge control

Test Masses inside the LISA Gravitational Reference Sensor must maintain almost pure geodesic motion for gravitational waves to be successfully detected. LISA requires residual test mass accelerations below 3 fm/s2/√Hz at all frequencies between 0.1 and 3 mHz. One of the well-known noise sources is associated with the charges on the test masses which couple to stray electrical potentials and external electromagnetic fields. LISA Pathfinder will use Hg-discharge lamps emitting mostly around 254 nm to discharge the test masses via photoemission in its 2015/16 flight. A future LISA mission launched around 2030 will likely replace the lamps with newer UV-LEDs. Presented here is a preliminary study of the effectiveness of charge control using latest generation UV-LEDs which produce light at 240 nm with energy above the work function of pure Au. Their lower mass, better power efficiency and small size make them an ideal replacement for Hg lamps.

LISA Pathfinder test-mass charging during galactic cosmic-ray flux short-term variations

Metal free-floating test masses aboard the future interferometers devoted to gravitational wave detection in space are charged by galactic and solar cosmic rays with energies 100 MeV/n. This process represents one of the main sources of noise in the lowest frequency band ( 10−3 Hz) of these experiments. We study here the charging of the LISA Pathfinder (LISA-PF) gold-platinum test masses due to galactic cosmic-ray (GCR) protons and helium nuclei with the Fluka Monte Carlo toolkit. Projections of the energy spectra of GCRs during the LISA-PF operations in 2015 are considered. This work was carried out on the basis of the solar activity level and solar polarity epoch expected for LISA-PF. The effects of GCR short-term variations are evaluated here for the first time. Classical Forbush decreases, GCR variations induced by the Sun rotation, and fluctuations in the LISA-PF frequency bandwidth are discussed.

LISA-PF radiation monitor performance during the evolution of SEP events for the monitoring of test-mass charging

Cosmic rays of solar and galactic origin at energies >100 MeV/n charge and induce spurious forces on free-floating test masses on board interferometers devoted to gravitational wave detection in space. LISA Pathfinder (LISA-PF), the technology testing mission for eLISA/NGO, will carry radiation monitors for on board test-mass charging monitoring. We present here the results of a simulation of radiation monitor performance during the evolution of solar energetic particle (SEP) events of different intensity. This simulation was carried out with the Fluka Monte Carlo package by taking into account for the first time both energy and spatial distributions of solar protons for the SEP events of 23 February 1956, 15 November 1960 and 7 May 1978. Input data for the Monte Carlo simulations was inferred from neutron monitor measurements. Conversely, for the SEP event of 13 December 2006 observed by the PAMELA experiment in space, we used the proton pitch angle distribution (PAD) computed from the Particle Acceleration and Transport in the Heliosphere (PATH) code. We plan to adopt this approach at the time of LISA-PF data analysis in order to optimize the correlation between radiation monitor observations and test-mass charging. The results of this work can be extended to the future space interferometers and other space missions carrying instruments for SEP detection.

IMPLICATIONS OF GALACTIC AND SOLAR PARTICLE MEASUREMENTS ON BOARD INTERFEROMETERS FOR GRAVITATIONAL WAVE DETECTION IN SPACE

Test-mass charging due to galactic cosmic rays (GCRs) and solar energetic particles (SEPs) represents one of the major sources of noise for missions devoted to gravitational wave detection in space. Particle detectors on board future space interferometers will help in monitoring the test-mass charging process. Variations and fluctuations of GCRs and evolution of SEP events of different intensities are discussed here for the correlation of SEP radiation monitor observations and particle fluxes charging the test masses. We consider the performance of the radiation monitors designed for the LISA Pathfinder mission for the results presented in this work. We point out that in addition to the primary use of test-mass charging monitoring, particle detectors on board space interferometers will naturally provide SEP observations at different intervals in heliolongitude and distances from Earth for space weather applications.

Interaction between Stray Electrostatic Fields and a Charged Free-Falling Test Mass

We present an experimental analysis of force noise caused by stray electrostatic fields acting on a charged test mass inside a conducting enclosure, a key problem for precise gravitational experiments. Measurement of the average field that couples to the test mass charge, and its fluctuations, is performed with two independent torsion pendulum techniques, including direct measurement of the forces caused by a change in electrostatic charge. We analyze the problem with an improved electrostatic model that, coupled with the experimental data, also indicates how to correctly measure and null the stray field that interacts with the test mass charge. Our measurements allow a conservative upper limit on acceleration noise, of 2 (fm/s2)/Hz(1/2) for frequencies above 0.1 mHz, for the interaction between stray fields and charge in the LISA gravitational wave mission.

On the role of radiation monitors on board LISA Pathfinder and future space interferometers

LISA (Laser Interferometer Space Antenna) and its precursor mission LISA Pathfinder (LISA-PF) will carry particle monitors for noise diagnostics. It was proposed to build and place radiation detectors on board the ASTROD missions as well. We present here a study of the solar energetic particle (SEP) events that the LISA-PF radiation monitors are able to detect above the galactic cosmic-ray (GCR) background predicted at the time of the mission data taking in 2015. In order to optimize the correlation between radiation monitor measurements and gravitational sensor test-mass charging, the energy threshold for particles traversing both detectors should be approximately the same. In LISA-PF, the radiation monitor particle energy cut-off was conservatively set at 75 MeV per nucleon (MeV/n) for protons and ion normal incidence, while the minimum energy of the same particles reaching the test masses is 100 MeV/n. We find that SEP events detectable on LISA-PF are characterized by peak fluxes and fluences at energies >75 MeV/n larger than about 45%, on average, with respect to those at energies >100 MeV/n. We conclude that for an accurate correlation between radiation monitor count rates and test-mass charging, it is mandatory to benefit from absolute flux measurements of both galactic and high-energy solar particles provided by experiments carrying magnetic spectrometers in space at the time of LISA-PF (PAMELA, AMS). On the other hand, the role of the radiation detectors on board LISA-PF is crucial allowing for SEP event onset and dynamics monitoring.

Charge mitigation techniques using glow and corona discharges for advanced gravitational wave detectors

Charging of silica test masses in gravitational wave detectors could potentially become a significant low-frequency noise source for advanced detectors. Charging noise has already been observed and confirmed in the GEO600 detector and is thought to have been observed in one of the LIGO detectors. In this paper, two charge mitigation techniques using glow and corona discharges were investigated to create repeatable and robust procedures. The glow discharge procedure was used to mitigate charge under vacuum and would be intended to be used in the instance where an optic has become charged while the detector is in operation. The corona discharge procedure was used to discharge samples at atmospheric pressure and would be intended to be used to discharge the detector optics during the cleaning of the optics. Both techniques were shown to reduce both polarities of surface charge on fused silica to a level that would not limit advanced LIGO. Measurements of the transmission of samples that had undergone the charge mitigation procedures showed no significant variation in transmission, at a sensitivity of ∼ 200 ppm, in TiO2-doped Ta2O5/SiO2 multi-layer coated fused silica.

Test mass charging simulation of ASTROD I due to solar energetic particles at 0.5 AU

Maintaining the geodesic motion of the test mass is vital to ASTROD I space mission. However, the electrostatic charging of the test mass due to cosmic rays and solar energetic particles will result in Coulomb and Lorentz forces and consequently influence the test mass motions. To estimate the size of these effects, a credible simulation of test mass charging processes is critically required. Using the GEANT4 software toolkit, we have modeled the charging processes and predict how the ASTROD I test mass will charge positively at a rate of 217370 e(+)/s, due to solar energetic particles (SEPs) at similar to 0.5 AU caused by the largest SEPs event on 29, September, 1989. In addition to Monte Carlo uncertainty, an error of +/- 30% in the net charging rates was added to account for uncertainties in the spectra, physics and geometry models.

Test-mass module for DECIGO Pathfinder

DECIGO Pathfinder (DPF) is the first precursory satellite mission for DECIGO (DECi-hertz Interferometer Gravitational wave Observatory), which is a future space gravitational wave antenna. The key instrument of DPF is a Fabry-Perot laser interferometer with freely floating test-masses, demonstrating precision laser interferometry, stabilized laser system and drag-free control system in orbit. A test-mass module is one of the sub-components of the DPF instrument, realizing non-contact suspension of the test mass in space, which provides capabilities of local sensing/actuation, test-mass lock mechanism and charge management system. In this article, the test-mass module system is reviewed.

The role of interplanetary electrons at the time of the LISA missions

LISA (Laser Interferometer Space Antenna) is the first space interferometer devoted to the detection of gravitational waves in the frequency range 10−4 to 10−1 Hz. Free-fall gold–platinum test masses constitute the mirrors of the interferometer. Solar and galactic particles charging the test masses induce spurious forces that might mimic genuine gravitational wave signals. Proton and helium nuclei are more than 98% in composition of both galactic and energetic solar particles. The charging due to these ions was carefully studied. However, highly penetrating interplanetary electrons play a role similar to helium nuclei at solar minimum and balance more than half of the net charge induced by galactic protons at solar maximum. In this paper, we report the study of LISA test-mass charging and radiation monitor countrate due to interplanetary electrons under different conditions of solar modulation and global solar magnetic field (GSMF) polarity. The radiation monitors designed for the LISA precursor mission, LISA Pathfinder (LISA-PF), were considered. Solar electrons do not produce any detectable signal in the radiation monitors. No relevant increase in the test-mass charging is generated by solar electrons with respect to protons as well. However, we point out that the detection of electrons of solar origin on-board LISA will allow us to short-forecast incoming, intense solar ion fluxes. An optimized environmental survey would lead us to further improve the test-mass discharging process, reduce the overall noise and, possibly, extend the mission lifetime. Important contributions to solar physics and space-weather investigations will be provided as well.

Simulation of ASTROD I test mass charging due to solar energetic particles and interplanetary electrons

As ASTROD I travels through space, its test mass will accrue charge due to exposure of the spacecraft to high-energy particles. This test mass charge will result in Coulomb forces between the test mass and the surrounding electrodes. In earlier work, we have used the GEANT 4 toolkit to simulate charging of the ASTROD test mass due to cosmic-ray protons of energies between 0.1 and 1000 GeV at solar maximum and at solar minimum. Here we use GEANT 4 to simulate the charging process due to solar energetic particle events and interplanetary electrons. We then estimate the test mass acceleration noise due to these fluxes. The predicted charging rates range from 2247 e +/s to 47,055 e +/s, at peak intensity, for the four largest SEP events in September and October 1989. Although the noise due to charging exceeds the ASTROD I budget for the two larger events, it can be suppressed through continuous discharging. The acceleration noise during the two small events is well below the design target. The charging rate of the ASTROD I test mass due to interplanetary electrons in this simulation is about −11% of the cosmic-ray protons at solar minimum, and over −37% at solar maximum. In addition to the Monte Carlo uncertainty, an error of ±30% in the net charging rates should be added to account for uncertainties in the spectra, physics models and geometry implementations.

Study of test-mass charging process in the LISA missions due to diffuse γ-rays

Gravitational inertial sensors will be placed on board the Laser Interferometer Space Antenna (LISA) and aboard its precursor mission LISA Pathfinder (LISA-PF) in order to detect low frequency gravitational waves in space. Free-floating test-masses (Au7Pt3 cubes) will be housed in inertial sensors for detecting possible laser signal variations induced by gravitational waves. Charging of the LISA test-masses due to exposure of the spacecraft to cosmic radiation and energetic solar particles will affect operation of gravitational inertial sensors. In this paper we report on the role of diffuse γ-rays in charging the LISA and LISA-PF test-masses with respect to protons and helium nuclei. The diffuse γ-ray flux in the Galaxy has been interpolated taking into account the outcomes of recent calculations. A comparison with γ-ray observations gathered by different experiments (COMPTEL and EGRET, Milagro, Whipple, HEGRA, TIBET) has been carried out. Simulations of the test-mass charging process have been performed by means of the FLUKA2006.3b package. Monte Carlo simulations of the interaction of cosmic particles with the LISA spacecraft indicate that the diffuse γ-ray contribution to the average steady-state test-mass charging rate and to the single-sided power spectrum of the charge rate noise is marginal with respect to that due to galactic cosmic-rays.

CHARGE MANAGEMENT FOR LISA AND LISA PATHFINDER

We present an overview of charge management for LISA, including a review of the problems caused by test mass charging and development of the LISA Pathfinder charge management device.

FURTHER TEST MASS CHARGING SIMULATIONS FOR ASTROD I

The electrostatic test mass charging in the ASTROD I (Astrodynamical Space Test of Relativity using Optical Devices I) mission can affect the quality of the science data due to spurious Coulomb and Lorentz forces. To estimate the size of the resultant disturbances, credible predictions of charging rates and the charging noise are required. Using the GEANT4 software toolkit, we present a detailed Monte Carlo simulation of ASTROD I test mass charging due to exposure of the spacecraft to galactic cosmic-ray (GCR) protons and alpha particles (3 He , 4 He ) in the space environment. A positive charging rate of 33.3 e+/s at solar minimum is predicted. The predicted rate reduces by 50% at solar maximum. Based on this charging rate and factoring in the contribution of minor cosmic-ray components, we calculate the acceleration noise and stiffness associated with charging. We conclude that the acceleration noise arising from Coulomb and Lorentz effects are well below the ASTROD I acceleration noise limit at 0.1 mHz both at solar minimum and maximum. The magnitude of coherent Fourier components due to charging are also estimated.

Further computation of the test mass charging and disturbances in ASTROD I

The test mass of ASTROD I (the Astrodynamical Space Test of Relativity using Optical Devices I) will become charged due to exposure of the spacecraft to energetic particles in the space environment. Test mass charging will result in Coulomb and Lorentz forces and hence spurious test mass motions. To estimate the size of these effects, it is important to accurately model the test mass charging process. Earlier work, using GEANT 4 and a simplified ASTROD I geometry, predicted that the acceleration noise associated with test mass charging would be well below the ASTROD I residual acceleration noise target. Here we present results of a more accurate simulation, implementing a more realistic geometry model. We have simulated the charging processes due to cosmic-ray protons and alpha particles using this geometry model, at solar minimum and maximum. The magnitude of acceleration noise and stiffness associated with charging are estimated.

Possibilities for measurement and compensation of stray DC electric fields acting on drag-free test masses

DC electric fields can combine with test mass charging and thermal dielectric voltage noise to create significant force noise acting on the drag-free test masses in the Laser Interferometer Space Antenna (LISA) gravitational wave mission. This paper proposes a simple technique to measure and compensate average stray DC potentials at the mV level, yielding substantial reduction in this source of force noise. We discuss the attainable resolution for both flight and ground based experiments.

Test-mass charging simulations for the LISA Pathfinder mission

We present detailed Monte Carlo simulations of test-mass charging caused by energetic cosmic rays in the LISA Pathfinder mission, using the GEANT4 software toolkit. This effect can lead to a variety of disturbances, threatening the goal of the mission—to achieve a low-frequency acceleration noise within an order of magnitude of that required for LISA. We calculate the test-mass charging rates and charging shot noise for different solar conditions, including solar energetic particle eruptions. Further simulations show that the inclusion of additional masses within the LTP inertial sensor, to balance the gravitational accelerations felt by the test masses, does not significantly alter the overall charging rates. The specifications of a particle monitor for LISA Pathfinder, which can detect variations in the energetic particle flux and enable correlation with test-mass charging, are also described.

LISA test-mass charging process due to cosmic-ray nuclei and electrons

Solar energetic particles and galactic cosmic rays with energies larger than 100 MeV cause progressive charging of the LISA experiment test masses. Consequently, Coulomb forces occur between the test masses and the surrounding conducting surfaces generating spurious signals that might be mistaken for gravitational wave signals. We have parametrized the energy spectra of galactic cosmic-ray nuclei and electrons near the LISA orbit in order to evaluate their role in the test-mass charging relative to the most abundant proton component. This work has been carried out using the FLUKA Monte Carlo program.

Solar physics with LISA

Galactic cosmic rays and solar energetic particles (SEPs) with energies larger than 100 MeV/n are able to penetrate and charge the test masses of the LISA experiment. As this process constitutes one of the major sources of noise for the experiment, a small telescope of silicon detectors will be located on board the LISA PathFinder and, possibly, the three LISA spacecraft. This device will allow us to monitor real-time galactic and solar cosmic-ray incident proton fluxes above 100 MeV. Moreover, spectral information will be provided up to an energy of 500 MeV. We propose to use the above instrument for the evaluation of the test mass charging process and the study of SEPs accelerated by coronal mass ejection (CME) propagation. Because of the peculiar orbit of the LISA spacecraft around the Sun, this experiment offers a unique chance to monitor an evolving CME contemporary at 2° (among spacecraft) and 20° (between LISA and Earth) intervals in longitude at once. These observations are of particular interest for both solar physics and space weather investigations. SEP event occurrence is not predictable and these events are particularly dangerous to astronauts and space equipment.

Evaluation of disturbances due to test mass charging for LISA

This paper concerns the effects of the build-up of electrical charge on the LISA test masses. Charge accumulates on the isolated test masses due to the bombardment of the spacecraft by galactic cosmic rays and solar particles. This will result in forces on the test masses, due to Coulomb and Lorentz interactions, which will disturb their geodesic motion. The three main disturbances associated with this charge are an increase in the test mass acceleration noise, coupling between the test mass and the spacecraft and the appearance of coherent Fourier components in the measurement bandwidth. These disturbances are estimated using the latest charging rate and noise predictions from GEANT4 for both the LISA mission and the technology demonstration mission, LISA Pathfinder, at different times in the solar cycle. The Coulomb disturbances are evaluated based on a detailed 3D, electrostatic, finite element model and submodels of the LTP sensor. These results are compared with those derived using the customary parallel plate approximation to calculate capacitances, and the accuracy of these approximations is assessed for typical parameter settings. The variation of the magnitude of charging disturbances as different parameters are changed, and the management of such disturbances are discussed.