GG (Galileo Galilei) Test of the Equivalence Principle to 10-17

GG (Galileo Galilei)Test of the Equivalence Principle to 10 -17Results from industrial study and state of the artAnna M Nobili, Dipartimento di Fisica “E. Fermi” Universita’ di Pisa & INFN, Pisa – ItaliaQ2C4, Bremen 21-24 September 2009

GG differential accelerometer (I)NOTE: We do not fly a vacuum chamber (use venting to space instead..)

Test masses material choice in GG (I)Co-rotation makes many disturbing effects DC. Test masses do not need to be manufacturedto very high precision => More freedom in the choice of materials to maximize chance ofEP violation and significance of testEP violation not expected to depend on macroscopic properties of matter (density,chemical, mechanical, electric or magnetic characteristics)Barion number, Lepton number and z component of Isospin (normalized to mass in unit ofthe mass of H atom) have been identified (Fischbach & Talmadge, 1998)B/ µ L/ µ I z/ µ=> choose test masses materials so as to maximize difference in all 3 these properties!

Test masses material choice in GG (III)HDPE (High Density Polyethylene) identified – to be tested in GGG (has interesting sideconsequences.. Not conductive, capacitance read-out possible without capacitance plates inbetween test cylinders … differential by definition .. )

GG lock/unlock mechanisms1. Mechanical stops2. Launch safe lock/unlock (non magnetic actuators)3. Fine lock/unlock (inch-worm actuators)Designed by DTMTechnologies (Ferrari)Lock/unlock of inner test cylinder“bunny ear” lock/unlock ofcoupling arm

Drivers and requirements: some numbers (I)

Drivers and requirements: somenumbers (II)

Drivers and requirements: some numbers (III)

Drivers and requirements: some numbers (IV)

Drivers and requirements: some numbers (V)

Drivers and requirements: some numbers (VI)…passive MLI sufficient

Drivers and requirements: some numbers (VII)Electric charging & patch effects:• passive electric grounding of the test masses• co-rotation of the test masses and the capacitance transdusers (make patch effectsDC or slowly varying if they do slowly vary.. .as they do…)• gold coating• direct measurement of the effects of any patches of charges on test masses, as wehave done in GGG (see later..)

Error budget (I)How it is builtEstablish requirementsImplement Drag Free ControlRun GG SimulatorHeritage from GOCE!Analyze time history of test masses relative displacementsSingle out systematic effects and check their magnitude and signature

Error budget (II)

A simulator in the lab: “GG on the Ground (GGG)”Same number of degrees of freedom; same dynamical properties; position of relativeequilibrium of the test masses in the horizontal plane is NOT stabilized by local gravity (as itshould be as a test of experiment in space…)GGG lab at INFN Pisa-San Piero a Grado

GGG sensitivity: major improvements (I)FFT of relative displacements of GGG test cylinders in the horizontal, not rotating, plane of lab

GGG sensitivity: major improvements (II)PSD of relative displacements of GGG test cylinders in the horizontal, not rotating, plane of lab

GGG current sensitivity to EP violation in the field of the SunGGG has measured 6x10 -9 m at diurnal frequency with coupling period of 13 s =>η sun ~2.3x10 -7Limited by terrain tilts: apparatus not suspended, active tilt control only.Main issue: tilt sensors dependence on temperature

sGGG (suspended GGG) - ASI funds (I)New chamber+ new rotor(undercompletion)New chamber has the right symmetry and has been designed to minimize disturbances on GGGsGGG will be suspended inside chamber by cardanic joint (not rotating) to reduce low frequency terrain tiltspassively, in addition to active tilt control now in use (Note: active tilt control is limited by thermal effects on tiltsensor and requires good thermal stabilization to be effective)An Experiment Simulator will be built by Thales Alenia Space-Italy for the new GGG, similarly to the Simulatorbuilt for the space experiment, to be compared with experimental measurements …

sGGG (suspended GGG) – ASI funds (II)• With the cardanic suspension already manufactured we expect a terrain tilt reduction at low frequencies byabout 5000 (exploit lever effect…)• With active terrain tilt control (+thermal stabilization) plus passive attenuation we expect todetect 1 pm displacements (GG target is 0.5 pm) i.e., with current natural test massesperiod of 13 s: => η sun ~4x10 -11Longer natural period possible (sensitivity would increassequadratically..) but shall we encounter the motor ball bearingsnoise???

Thermal stability in new chamberThermal stability of tiltmeter inside chamber (multi stage thermal control): to a few tenthsof mdeg down at diurnal frequency (requires 20 mW only)

A better tiltmeter to improve active tilt control?Double pendulum (one simple + oneinverted, aligned, coupled by tinycantilever), based on knife edgesuspensions.Capacitance transducer with ad hocelectronic board developed in the labbased on the AD7745 24 bit capacitanceto digital converter capable to measureup to 4 picoFarad to a few tenths offemtoFarad. No additional electronics isneeded outside the vacuum chamber;data are transferred to the computeroutside via USB port.Designed to reach 100 s period(equivalent to a simple pendulum2500 m long!!!) .. Extremelysensitive…On first tests (only rough adjustment ofperiod and alignment) we havemeasured 34.8 s period (equivalent to a300 m simple pendulum..)

Measurement of electric patch effects (I)Apply a force to the external test cylinder with acapacitance plate (both grounded)Since outer and inner test cylinders are coupled, theywill move relative to each otherTheir differential motion is measured by thecapacitance bridges (main sensors) located inbetween the test cylinders

Measurement of electric patch effects (II)FV=o2Q2εSCharge changes sign with applied potential, patchcharge does not!First, apply unipolar potential and measure effect onTMs; then switch to bipolar potential and measureeffect on TMs (square wave with same period…)∆x∆y+ V+ V 0.75µm 0.18µm

Measurement of electric patch effects (III)2QqF± V ≡ Fpatch= qε Sodisplacement q± patch VV= 4 = 4displacement Q V+ V + V + Vis the charge of patch we want to measurepatchby measuring thedisplacements in the twocases we measure V patch …∆x∆y± V± V 0.0275 µ m 0.006 µ mFrom thesemeasurements:Vpatch 0.3V(Plate made of Al liketest cylinders, no goldcoating…. )

Measurement of electric patch effects (III)• No modeling needed, very meat measurement• You measure directly the effect of patch charges on the tests masses• We have done with GGG spinning for 10.6 d, to measure time variation of patch effectamplitude….Note: the effect of the patch, inaddition to being brought to thefrequency of the applied potential isalso amplified by the factor:4 V / VappliedpatchHere it is about 500⇒ the effect of patch charges (nocoating at all) on GGG testmasses, close to diurnalfrequency is a few picometers(GG target requires to measure0.5 picometer)

“GalileoGalilei (GG)”GG undergoing Phase A-2 Study by ASI(Agenzia Spaziale Italiana) Preliminary (April2009) Report available on the Web:http://eotvos.dm.unipi.it/PA2