Operator's Manual
Issue 14
/
Mar 2016 / UMC0071
Mercury
iPS
©2016 Oxford Instruments NanoScience. All rights reserved.
Page
41
usually used to energise the magnet to a target current (switch heater ON) then to run down the
power supply, with the switch heater OFF, to leave the magnet energised at the target current.
The basic configuration is shown in Figure 8 (protection circuits not shown). It is important to
remember that the current in the switch equals the difference between the magnet current and
the power supply current
=
−
thus when running down the leads (power supply) after energising a magnet the switch will be
approaching its maximum as the power supply current approaches zero. It is important not to
ramp the leads too fast as this is the rate at which current is building in the switch. Switches
can quench (break open) because they over heat if they are ramped too fast.
Figure 8. Diagram showing a simple solenoid magnet fitted with a persistence switch. In this example the persistence
switch heater is OFF, the switch is “closed” (analogy with relay contacts), so current flows through the switch so two
current loops exist. The current in the switch is the difference between the psu current and the magnet current.
At the instant when the switch opens (or closes) there will be a perturbation in the power supply
output as the relatively large inductance load of the magnet is connected (or disconnected) from
the psu and the control loop time constant changes.
4.1 Quench
In normal magnet operation this is a very unusual event. This is when the superconductor of
the magnet coil(s) reverts to its normal resistive state. For low-temperature superconductors
such as NbTi and Nb
3
Sn the propagation of the normal state is very fast and the electrical
resistance rise is large. The results of this transition is a rapidly rising voltage transient across
