1. Self Organized Criticality noise in metals and advanced hinges collective dislocation effects
Riccardo DeSalvo
Sannio University

2. The problem with Dislocations
Collective dislocation activity in metals breakdown the linearity of springs
1/f noise produced
Effects evident at low frequency, present everywhere
This affects the performance of
Seismic attenuators
Seismic sensors
What to do?

3. Dislocations basics Dislocations cannot end inside a crystal
Extend across the entire grain
Mostly move freely across crystal
Internal tensions pull them straight
Bent by stresses
Cannot cross other dislocations
Get pinned at various intervals by
point defects like impurities, atomic vacancies, …
Or on other dislocations
A. Granato, et al., “Theory of mechanical damping due to dislocations”, Jour. Of Appl. Phys., vol 27, n 6, p , 1956

4. Dislocations new understandings
Can turn around a pinning point
Cannot turn around other dislocations
Often the dislocation free length is more limited by entanglement than by pinning

5. Dislocations new understandings
Therefore dislocations will pileup, entangle and disentangle
as a consequence of stress variations

6. Dislocations new problems
IF entanglement and disentanglement are permitted
the ensuing free length changes will cause the following instabilities:
 Young’s modulus
 Equilibrium point
 Loss mechanisms
LIGO-P

7. Self Organized Criticality is a property of dynamical systems
SOC important facts
Self Organized Criticality is a property of dynamical systems
a driving force is needed
Per Bak 1996
How nature works:
The Science of Self-Organized Criticality

8. SOC properties => Does not follow our beloved linear rules ! !
Movement of entangling dislocations is intrinsically Fractal
=> Does not follow our beloved linear rules ! !
=> Avalanches induce random motion
=> 1/f noise at all frequencies

9. Experiment exploring SOC
THE GAS-EMAS filter
A “microscope” for mesoscale effects
see LIGO-G
LIGO-P
Class. Quantum Grav. 26 (2009)

10. Effects of dislocation S O C

11. Hysteresis equal noise ! ! !
Actuator force [mN]
Temperature [oC]
Thermal or position drifts generate hysteresis!
Hysteresis proceeds by avalanches  1/f noise

12. Low frequency instability
65 kg payload can fall indifferently up or down
NOT CREEP,
NOT a GRAVITY DRIVEN EFFECT
It is the Young’s
Modulus that
Suddenly fails!
instability region mm
starting from ~ 0.2 Hz

13. Anomalous Hysteresis at lower frequency
Residual elasticity from crystal
Change of equilibrium point
from disentangled, re-entangled
Network of dislocations

14. Controlling hysteresis
Forced, slowly decaying oscillations successfully smooth out critical slope and eliminate hysteresis
Same tune 0.15 Hz

15. Controlling hysteresis  controlling noise
Bring the system away from the critical slope
means:
to Eliminate the source of avalanching!

16. Frequency (Young's modulus) dependence from. oscillation amplitude
Drops with oscillation amplitude
as dictated by SOC

17. Excess Dissipation at larger oscillation amplitude
increases with oscillation amplitude
as dictated by SOC

18. Big problem Downconversion
thermal drifts + microseismic peak
resonance
downconversion
low microseismic activity
=> low downconversion

19. Summary of Consequences
Hysteresis indicates something is shifting in the material.
Run-offs, changing Young’s modulus, new loss mechanisms
An avalanche dominated 1/f noise is expected
Should extend up to the smallest (fastest) event (MHz)
Should extend down to the largest avalanche (mHz)

20. Ways out
SOC noise is a dynamic effect It needs a power source to generate critical slope No power source, eventually no SOC noise

21. Ways out Keep system quiet, avoid thermal variations
completely block dislocations
Use materials without dislocations
Use mechanics insensitive to dislocations
dilution
Avoid flexures => knife edge hinges
an example

22. A high precision mechanical ground rotation sensor
V. Dergachev, M. Asadoor, A. Bhawal, R. Desalvo, C. Kim, A. Lottarini, Y. Minenkov, C. Murphy, A. O’Toole, G. Pu, A. Rodionov, M. Shaner
DCC: LIGO-G v3

23. Motivation: Advanced LIGO Active isolation requirements
Requirements are more than an order of magnitude more strict than existing tiltmeter performance.
Lantz, B., et al. (2009). “Requirements for a Ground Rotation Sensor for Advanced LIGO." Bulletin of the Seismological Society of America 99(2B):

24. Mechanical tiltmeter Adjustable LVDT position sensors
weights for tuning mechanical properties
LVDT position sensors
Pivot point built as a knife edge on anvil, both of tungsten carbide
feedback actuators

25. Knife-edge hinge
2525

26. Hysteresis measurement
hysteresis virtually eliminated
lower noise performance achieved

27. Exponential decay keeps phase through excitations
Seismic event
Continuing sinusoidal
fit across excitation pulses

28. Better performance achieved !

29. What is still needed to achieve the LIGO requirements ?
lower instrumental noise, limiting performance at high frequencies.
in vacuum operation to improve performance at low frequencies
lower resonant frequency (longer balance)

30. The key point
By designing a geometry not sensitive to dislocation SOC noise
Tiltmeter performance drastically improved
Will the GW interferometer seismic attenuation need the same ?