3. INSTRUMENTATION:

            The SRM dewar system is fitted with one flexible helium level gauge for level when the system is horizontal, and one full length straight level gauge for use when the system is vertical, 4 diode thermometers, 4 resistive heaters and a 3 axis set of field nulling coils.  The level gauges, diodes and heaters are connected through a 15 pin connector on the SQUID end top plate. The cable to the control electronics is fitted with a 9 pin connector and adapter interconnect boxes are supplied to fit between the 15 pin and 9 pin connectors. The box number and function is given in the table below:

                          
3.1 HELIUM LEVEL GAUGES:

            There is one superconducting wire helium level gauge in the helium reservoir on model 755 and 760 systems. This gauge functions as a variable resistor such that the part of the gauge that is below the liquid level is superconducting (zero resistance) and the part above the liquid is resistive. Thus, a measure of the voltage drop across the gauge at a constant current will determine the fraction of the gauge length that is below the liquid, i.e., the liquid depth. The gauge for horizontal operation is a flexible gauge that bends around the reservoir inner diameter. The gauge is connected to pins 3 and 5 of the 15 pin connector on the dewar with pin 5 being the common lead. The resistance of the gauge when the reservoir is FULL is about 50 ohms and about 300 ohms when empty. The helium reservoir is an annular volume (see figures 1-1 and 1-2). The reservoir is constructed of two concentric tubes, the outer one having an inside diameter of 15.25 inches and the inner tube having an outside diameter of 8.25 inches. When operated horizontally the actual helium volume is not a linear function of the level gauge reading. Figures 3-1 and 3-2 give the actual liquid versus level gauge reading for the 755 and 760 magnetometers when operated horizontally.

            When the 755 or 760 is used for vertical operation  a single level sensor is connected to pins 9 and 5. The output voltage is now a linear function of helium level over the volume of the reservoir. To read the vertical level gauge box 3 must be used to connect the pin 9 to pin 3 of the interconnect cable. When the gauge reads empty on all systems approximately 2 liters of liquid is still in the reservoir.

            On systems built prior to Fall 1997 two horizontal helium level gauges were used. Level guage #1 measured one half of the volume and level gauge #2 measured the second half. These gauges were connected to pins 5 and 3 and 5 and 9 respectively.

3.2 THERMOMETERS:

            There is a silicon diode thermometer mounted on the liquid helium reservoir (SQUID temperature) and one on the superconducting shield that is thermally linked to the helium reservoir. Two other diodes are mounted on the vapor cooled shields in the dewar vacuum space. Power to the diodes is supplied from the SQUID control/monitor electronics and its interconnect cable. Diode power is applied whenever the electronics is turned on and the cable is connected. The diode voltage is measured with an external DVM connected to either of two BNC outputs on the rear panel of the control electronics. The diode voltage at critical temperatures is given in the table at the front of this manual for each specific system. Figure 3-3 gives an approximate diode voltage versus temperature for our standard diodes. The voltage change with temperature is approximately 2.7 millivolts per Kelvin from room temperature to about 55K, and is about 0.060 volts per Kelvin from helium temperature to 20 K. We do not calibrate the diodes in the intermediate range of temperatures since this is not important to the SRM operation, and due to the nonlinearity it is difficult to calibrate. When the control/monitor cable is connected directly to the SRM top plate electrical feedthru 15 pin connector, using adapter number 2 the two BNC's monitor the reservoir top (SQUID) temperature and the superconducting shield temperature. The two cryocooler diodes can be read by connecting the control/monitor cable to the 15 pin connector on the SRM using adapter number 3 (see table above for pin connections).

3.3 HEATERS:

There are five heaters built into the system:

1. The superconducting shield heater on pins 4-5 of the electrical feedthru connector is a 500 ohm resistor attached to the superconducting shield. It is used to heat the shield above its transition temperature to change the trapped magnetic field.

2. The SQUID heater (#1) on pins 5 and 6 is a 500 ohm heater attached to the SQUID mounting block. It is used to heat the SQUIDS above their transition temperature to remove any trapped magnetic flux.

3. The strip line heater (#2) on pins 1 and 2 is a 500 ohm heater attached to the superconducting strip lines between the pickup coils and the SQUIDS. It is used to remove large circulating currents from the pickup coil structure without heating the SQUIDS or shield.

4. The fill-safety line heater on pins 5 and 12 is a 500 ohm heater attached to the fill and safety lines at the point where the two lines are thermally grounded to the inner cryocooled shield. This heater is used to heat these two lines up to assist in removing any solid air plug that may form. It is connected using adapter number 3 and powered with the SHIELD heater switch on the control electronics. Note, when this Box #3  is connected the diodes monitored by the electronics are the two cryocooler shield temperatures.

5. The fill, vent and safety reservoir heater is a 350 ohm heater connected to the three lines just before they enter the helium reservoir. This heater is on pins 5 and 13 of the SRM 15 pin connector and is powered by pins 4&5 of the control monitor with the shield heater switch using adapter number 2.  The heater is used to assist in removal of a solid air blockage at the entrance to the reservoir.

3.4 FIELD NULLING COILS:

            There are three coils mounted outside the dewar vacuum jacket to help establish a low field environment in the SRM measurement region. The calibration constants for these coils are given below and  in the front of this manual. To use the coils to change the field it is first necessary to heat the superconducting shield to above its critical temperature as described in the following section. The coils are referred to as axial (to change the field in the sample access direction) and transverse - two orthogonal sets of saddle coils for changing the field in the directions transverse to the dewar axis. The axial coil is a helmholtz pair of coils wrapped on the dewar vacuum jacket. Each transverse coil is a pair of saddle shaped helmholtz coils 1/2 meter long and 1/2 meter circumferential length.  The approximate field constants for these coils with 15 turns in each coil are given below:

                           transverse  = 70  gamma per milliamp.

                           transverse   = 70  gamma per milliamp.

                           axial = 60  gamma per milliamp.

            To null the SRM field it is advisable to use three independent current supplies for the coils (a simple 3 axis 9 volt battery powered supply is provided with the system) and a three axis flux gate in the access of the SRM. The 2G models 520 and 520A are ideal for this application. Heat the superconducting shield as described in the following section and adjust the coil currents to achieve the desired field. It requires about 45 seconds to heat the shield to its non-superconducting state and it can be maintained at this higher temperature (about 10 kelvin) by turning the heater on for about 10 seconds then off at 4 minute intervals and monitoring the diode voltage. When the desired internal field is achieved with the coils hold the coil currents constant and let the superconducting shield cool to trap the field. Wait until the superconducting shield temperature reaches 5 kelvin or less before turning off the coil currents. This temperature is measured with diode T-1 using box 1 or 2. Please check the actual equilibrium values for the specific SRM. It will require about 20 minutes for the shield to cool after it has been heated to 10 kelvin and another hour before the system lowest noise performance is achieved.

            We have found that magnetometers used in steel shielded rooms without ferromagnetic shields around the magnetometer often have strong gradients, especially in the axial field. These gradients are caused, at least in part, by the fields from the cryocooler cold head. The axial gradient can be removed completely by wrapping about 6 turns of insulated copper wire around the cryocooler end of the magnetometer vacuum jacket. Connect these leads to a DC power supply with at least 0.5 amp adjustable output. Heat the superconducting shield and using this 6 turn coil adjust the current amplitude and polarity to provide gradients below 1 gamma/cm over about a 10 cm axial length centered at the sample measurement region. It normally will require 0.15 to 0.30 amp current. Leave this current on. Next use the 3 axis power supply and built in nulling coils to induce the field at the center of the measurement region to below a few gamma. If the room has other gradients these will also appear in the measurement region. It is best to carefully measure the field in the room at the magnetometer location, and if the field or gradient is too high, re-sweep the room and/or move the magnetometer to another part of the room for the field trapping. Once the field is trapped, the magnetometer can be moved back to its desired position, near a wall etc., and the trapped field will not change.

3.5 SWITCHING THE SUPERCONDUCTING SHIELD:

            The superconducting shield is a crucial part of the SRM. Because of the unique properties of superconducting materials this shield provides very high attenuation of external magnetic field changes and permits the trapping of a specific field environment. Superconducting materials exhibit zero D.C. resistance so any change in magnetic field will produce an in­duced current that flows undiminished so long as the field change is applied. This means that the net magnetic flux linking a superconducting ring will remain constant. For a cylindrical shell structure of the type used on the SRM it can be shown that axial magnetic fields are attenuated by a factor of , where l is the distance in from the open end of the shield measured on axis and r is the shield radius. Transverse fields are attenuated by a factor of . For the shield dimensions used in the 7.6 cm (3.0 in) 760 SRM and the 4.1 cm (1.65 inch) 755 this provides an attenuation of axial fields at the measurement center of  and of transverse fields of .

            The only operational procedure used with the superconducting shield in the SRM is its thermal switching. To trap a given magnetic field environment it is necessary to produce the desired field with coils, magnetic shields, etc. then turn on the shield heater for 1 (one) minute, then turn it off. The field at the SRM access can be measured with a fluxgate magnetometer during this process to determine the value to be trapped. As described in the previous section the field is usually adjusted with the built in nulling coils when the shield is normal. Maintain the desired field as constant as possible for the next 20 minutes. It is best to monitor the superconducting shield temperature (Diode T1, box 1 or 2) and wait until it cools to within 30 millivolts of its equilibrium value, (about 5 K at 2.3 to 2.0 volts for most of the diodes used), before removing the external field source. At this time the shield will be well below its superconducting transition temperature and the field will be trapped by the superconductor. It will take an additional hour or more for the SQUIDS to reach their lowest noise performance.