System Features

The FCP electromagnets feature continuously adjustable air gaps, allowing rapid air gap changes to suit individual experiments. They have water-cooled coils and precision yokes made of magnetically soft, ultra pure steel, assuring precise pole cap alignment as well as excellent field homogeneity and stability. The H-yoke configurations are oriented at a 45° angle for optimum air gap accessibility. The electromagnets also incorporate compact size with steel eyebolts, permitting convenient bench top mounting to optical tables.

The FCP magnet power supplies provide stable current regulation and true, bipolar power output. The bipolar output allows rapid uniform magnetic field ramping and field reversal, avoiding discontinuities that occur during zero crossover when using unipolar power supplies. The bipolar output also eliminates the need for current reversal contactors or relays. The magnet power supplies are CE compliant and include features such as automatic power output disable upon failure detection, operation as a current or voltage source, and operation in either manual or remote mode.

Unlike other commercial solutions, the Lake Shore FCP system incorporates closed-loop field control (also called feedback control) capability directly into the Model 475 DSP gaussmeter, eliminating the need for a separate computer with redundant software control. The Model 475 includes the electronics and firmware to perform closed-loop, digital PI field control of the electromagnet and magnet power supply.

Control Features

The Model 475 attempts to keep the magnetic field density at exactly the user-entered control setpoint, expressed in gauss, tesla, oersted, or ampere/meter. To do this, the Model 475 uses feedback from the gaussmeter Hall probe to calculate and actively adjust the control (analog) output. In order to maximize control stability, the Model 475 updates the analog output every 33 ms. The result is an internal PI controller providing peak-to-peak field stability of 0.5 G*.

When the setpoint ramp is enabled, the instrument will start ramping from the current field reading, rather than the current setpoint, based on the user settable P and I settings. In addition, the 475 can be configured to ramp the control setpoint from the present field reading to a new value by using a smooth linear transition in field rather than the step response PI control.

Open-loop field control is also possible by using the Model 475 analog output in manual output, meaning feedback is ignored and the analog output stays at the manual user setting. This method makes the magnet power supply operate in constant current mode.

The Model 475 also incorporates user settable control slope limits and analog output voltage limits. These software limits assure that the magnet power supply is not damaged if the field control system is improperly tuned or begins to oscillate.

*Measured at 0 G and 1000 G