Lake Shore 625, Руководство пользователя

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Lake Shore Model 625 Superconducting MPS User’s Manual
1-2 Introduction
At static fields, output current drift is also kept
low by careful attention in the analog control
circuits and layout. The high stability and low
noise of the Model 625 make it possible in many
situations to run experiments without going into
persistent mode. This can help to reduce the time
necessary to gather data.
The Model 625 output architecture relies on low
noise, linear input and output stages. The linear
circuitry of the Model 625 permits operation with
less electrical noise than switch-mode
superconducting magnet power supplies. One key
benefit of this architecture is CE compliance to
the electromagnetic compatibility (EMC)
directive, including the radiated emissions
requirement.
Output Programming
The Model 625 output current is programmed
internally via the keypad or the computer
interface, externally by the analog programming
input, or by the sum of the external and internal
settings. For the more popular internal
programming, the Model 625 incorporates a proprietary digital-to-analog converter (DAC) that is monotonic over the
entire output range and provides a resolution of 0.1 mA.
The Model 625 generates extremely smooth and continuous ramps with virtually no overshoot. The digitally generated
constant current ramp rate is variable between 0.1 mA/s and 99.999 A/s. To ensure a smooth ramp rate, the power supply
updates the high-resolution DAC 28 times per second. A low-pass filter on the DAC output smoothes the transitions at
step changes during ramping. Ramping can also be initiated by the trigger input.
The output compliance voltage of the Model 625 is settable to a value between 0.1 V and 5 V, with a 100 µV resolution.
The voltage is an absolute setting, so a 2 V setting will limit the output to greater than –2.0 V and less than +2.0 V.
Output Readings
The Model 625 provides high-resolution output readings. The output current reading reflects the actual current in the
magnet, and has a resolution of 0.1 mA. The output voltage reading reports the voltage at the output terminals with a
resolution of 100 µV. A remote voltage reading is also available to more accurately represent the magnet voltage by
bypassing voltage drops in the leads connecting the power supply to the magnet. All output readings can be prominently
displayed on the front panel and read over the computer interface.
Protection
Managing the stored energy in superconducting magnets necessitates several different types of protection. The Model
625 continuously monitors the load, line voltage, and internal circuits for signs of trouble. Any change outside of the
expected operating limits triggers the supply to bring the output to zero in a fail-safe mode. When line power is lost, the
output crowbar (SCR) will activate and maintain control of the magnet, discharging at a rate of 1 V until it reaches zero.
Quench detection is necessary to alert the user and to protect the magnet system. The Model 625 uses a basic and reliable
method for detecting a quench. If the current changes at a rate greater than the current step limit set by the operator, a
quench is detected and the output current is safely set to zero.
The remote inhibit input allows an external device to immediately set the output current to zero in case of a failure. This
input is normally tied to an external quench detection circuit, the fault output of a second power supply, or an emergency
shutdown button. The fault output is a relay contact that closes when a fault condition occurs. The contact closure alerts
other system components of the fault.
5 Ampere Charge of an 8 Henry Superconducting Magnet*
* 5 A charge of an 8.6 H AMI magnet with a 95 mA/s ramp rate;
output current monitor measured at 58.88 Hz. rate with a HP
34401
– data multiplied by 10× to obtain output current results.