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Lake Shore Model 625 Superconducting MPS User’s Manual

Operation

4-13

If the persistent mode ramp rate is enabled, then the ramp rate to be used while the magnet is in persistent mode needs to
be entered. Even though this ramp rate is typically much faster than the magnet ramp rate, it should not be set too high as
a fast change in current through the persistent switch can cause the switch to become normal and cause a quench. Note
that the persistent mode ramp rate will not be limited to the maximum ramp rate. Refer to Paragraph 4.11.3 for
information on the maximum ramp rate limit.

To enter a value for the persistent mode ramp rate, continue from the persistent mode ramp rate enable screen or press

PSH Setup

then

Enter

until the following setup screen appears as a prompt for the persistent mode ramp rate.

Persistent Switch Heater Output

Enter a Value For

Persistent Mode Ramp Rate: 5.0000 A/s

Use the data entry keys to enter the persistent mode ramp rate between 0.0001 and 99.999 A/s. Press

Enter

to accept the

new value. Press

Escape

to restart the setting sequence and enter a different value. Press

Escape

again to leave the

setting sequence.

4.15 PSH ON/OFF

The persistent switch heater should be properly set up before attempting to turn it on. Refer to Paragraph 4.14 to setup
the persistent switch heater. The PSH On and PSH Off buttons will not function unless the persistent switch heater is
enabled. When the PSH is off, no current is sourced to the heater and the persistent switch will revert to its
superconducting state. The magnet is said to be in “persistent mode” and any changes to the output current of the supply
are not reflected in the magnet. When the PSH is on, the persistent switch is heated and becomes resistive. The magnet
current can only be changed when the PSH is on.

To turn on the persistent switch heater, press the

PSH On

key. The PSH will begin warming and the PSH On LED will

blink. The output current of the power supply cannot be changed while the PSH is in transition. Once the persistent
switch has stabilized, as determined by the PSH delay time, the PSH will be on and the PSH On LED will be on solid.
Refer to Paragraph 4.14.3 to setup the PSH delay time. Note that the PSH cannot be turned on or off while the output
current is ramping or in compliance.

To turn off the persistent switch heater, press the

PSH Off

key. The PSH will begin cooling and the PSH On LED will

blink. Once the persistent switch has stablilzed, the PSH will be off and the PSH On LED will be off.

When turning on the PSH, it is important that the output current setting of the power supply is equal to the current in the
magnet. If the currents are not equal when the PSH is turned on, there is a possibility that the magnet can quench. The
Model 625 adds an extra layer of protection to keep this from happening. The Model 625 stores the output current
setting of the supply when the PSH was turned off last. If the output current setting does not match this stored setting
when the PSH is being turned on, the following message screen will appear.

Current output setting is not equal to the

setting when the PSH was shut off last.

Current Setpoint: +20.0000 A

Last Setpoint: + 0.0000 A

Turn on PSH anyway?: No

To override the lockout and turn on the PSH anyway, use the

key to select Yes and press

Enter

. This should

only be done if the user is sure that the output current of the power supply matches the current in the magnet. To cancel
this operation, use the

key to select No and press

Enter

or press

Escape

at any time.

NOTE:

If the PSH is turned on when the output current setting does not equal the current in the magnet, the power
supply will ramp the magnet to the output current setting at the compliance voltage limit. It is recommended
that the compliance voltage limit be set to an appropriate value for the magnet before turning on the PSH.