Technical Background for mK Setups
effects of resistive wiring
Unlike in 4K-cryostats, heating effects become a major issue when
reaching mK temperatures. Main sources of heating caused by attocube
positioners are: dissipating power from engaging the actuator, ohmic
heating due to finite resistance of the piezoelement, and thermal
connection to RT due to wiring.
Wiring of an attocube Piezo Positioner & Effects of Resistive Wiring
The electrical signal applied to the piezo element consists of a slow
rise and a steep drop (sawtooth signal). Increasing the resistance of
the connected wire causes the sawtooth signal to smear out. A sharp
transition is essential for the movement of the positioner.
In general, attocube suggests to use copper (typ. 0.7 Ω/m @ 0.2 mm
diameter) or brass wiring (typ. 1.5 Ω/m @ 0.25 mm diameter) with a total
resistance (both wires) of not more than 2 Ω (bandwidth 100 kHz). In this
case, attocube fully guarantees the functionality and the specifications of
the positioners. With a resistance of 2 - 5 Ω some specifications might be
altered (e.g. max. load, min. step size, etc.) but the general functionality of
the positioner is kept. If the resistance of the wires is between 5 and 10 Ω it
is recommended to contact attocube.
To balance the thermal and the resistive load, copper wires are
typically used from RT down to the 4 K stage or the 1 K pot. From there,
either superconducting NiTi wires or phosphor-bronze wires lead to
nanopositioners. Phosphor-bronze wires allow for the testing of the
complete setup at RT, where superconducting wires have very high
resistance. attocube's strategic DR suppliers already offer standardized
low-resistance cabling for attocube nanopositioners. Furthermore, in
order to reduce the heat load on the sample stage, the total number of
wires for nanopositioners, as well as for scanners, is reduced by using a
shared ground.
Such wiring specifications are often in conflict with the requirements
for mK setups because of the high specific heat values of the materials
used. Therefore, for mK setups attocube uses a combination of copper
wires and superconducting or phosphor-bronze wires.
Shape of the steep flank of the sawtooth signal as a function of the total wire resistance.