Model 625 Superconducting Magnet Power Supply

Model 625 features

• 60 A/5 V, bipolar, true 4-quadrant output
• 0.1 mA output setting resolution
• Linear regulation minimizes noise
• Ripple = 0.007% of maximum current (into a 1 Ω load)
• 1 mA per hour stability
• Parallel operation to ±120 A

The Model 625 superconducting magnet power supply is the ideal supply for small- to medium-sized superconducting magnets used in high-sensitivity materials research applications. It is a practical alternative to both the larger, one-size-fits-all superconducting magnet supplies and the endless adaptations of generic power supplies. By limiting output power, Lake Shore was able to concentrate on the performance requirements of the most demanding magnet users. The resulting Model 625 provides high precision, low noise, safety, and convenience.

Precision in magnetic measurements is typically defined as smooth continuous operation with high setting resolution and low drift. Achieving these goals while driving a challenging load, such as a superconducting magnet, requires a unique solution. The Model 625 delivers up to 60 A at a nominal compliance voltage of 5 V, with the supply acting as either a source or a sink in true 4-quadrant operation. Its current source output architecture with analog control enables both smooth operation and low drift. A careful blending of analog and digital circuits provides high setting resolution of 0.1 mA and flexible output programming.

Lake Shore chose linear input and output power stages for the moderate 300 W output of the Model 625. Linear operation eliminates the radiated radio frequency (RF) noise associated with switching power supplies, allowing the Model 625 to reduce the overall noise in its output and the noise radiated into surrounding electronics.

Safety should never be an afterthought when combining stored energy and liquid cryogens in a superconducting magnet system. The Model 625 incorporates a variety of hardware and firmware protection features to ensure the safety of the magnet and supply.

Instrument users have come to rely on Lake Shore for convenience and ease of use. The Model 625 includes the features necessary to conveniently manage a superconducting magnet, such as a persistent switch heater output, calculated field reading, current ramping, and quench detection. Computer interfaces are also integrated for automation of the magnet system. The Model 625 is truly an excellent one-box solution for controlling a superconducting magnet.

Output architecture

True 4-quadrant output capability is ideal for the charge and discharge cycling of superconducting magnets for both positive and negative fields. Tightly integrated analog control of the 4-quadrant output provides smooth current change with very low overshoot on output change. The Model 625 has the ability to charge and discharge magnets up to a 5 V rate.

True 4-quadrant operation eliminates the need for external switching or operator intervention to reverse the current polarity, significantly simplifying system design. The transition through zero current is smooth and continuous, allowing the user to readily control the magnetic field as polarity changes.

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 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 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 assure a smooth ramp rate, the power supply updates the high-resolution DAC 27 times per second. A low-pass filter on the output DAC smooths the transitions at step changes during ramping. Ramping can also be initiated by the trigger input.

The output compliance voltage is settable to a value between 0.1 V and 5 V, with a 100 µV resolution. The voltage setting is an absolute setting, so a 2 V setting will limit the output to greater +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.