The Flow Cryostate

For the investigation of solid state properties it is important to cover a wide range of external conditions. From a thermodynamic point of view there are some quantities which are interesting to vary: the temperature, the magnetic field and to a certain extent the pressure. With our two cryostates we can cover the complete temperature range from twenty Millikelvin up to more than three hundred Kelvin and the field range from zero field up to seven or eight Tesla. We can not yet regulate the pressure for it is correlated to the temperature due to the design of the cryostates.

  

Figure: Sketch of the flow cryostate

In the flow cryostate the sample is cooled with a flow of liquid ⁴He which is heated and evaporated at the sinter below the probe head. A rough regulation is achieved with a needle valve in the supply pipe and a pressure regulation in the exhaust. For the fine adjustement the temperature is regulated by a CONDUCTUS LTC-10 temperature controller. It is measuring the temperature of the ⁴He flow at the sinter and at the probe head with either a Pt-100 resistor for temperatures above 20K or with a semiconductor RuO₂ for temperatures below 20K. With the information of these two thermometers the LTC-10 regulates the heating power given to the sinter heater and the probe head. At the sinter the flow is pre-warmed to a temperature slightly below the desired temperature ensuring that there is not too much power given to the heater at the probe head. We were heating with a maximum power of 2.5W. With this procedure a temperature stability of some fractions of a percent deviation and a range of 15K to 300K can be achieved. The LTC-10 can be controlled from a computer over GPIB, the needle valve not.

We use an 8.5T superconducting magnet with a good field homogeneity over a range of some millimeters. The current in the coil is measured at an external shunt resistor. Before performing a measurement series the field has to be standardized. In our high-T measurements in SrB₆ the center of the ⁶³Cu and ⁶⁵Cu lines gave the ratio

(2.17)

The current in the magnet is controlled by a Hewlett Packard 3497A data acquisition and control unit. It supplies a voltage to the DRUSCH current source in order to stabilize a certain shunt voltage, which is fed back to the HP to digitize it and to send it back to the computer. The DRUSCH achieves a voltage stability of about 2ppm after some days of operation. The HP has a resolution of 12 bits and a voltage range of 10V which results in a resolution of 0.00244V. We divided the output voltage of the HP by ten and therefore increased the resolution to 0.000244V which corresponds to 24.5G. This resolution was not satisfying, but we could only increas the resolution in reducing the range available. And since we wanted to measure at 5 Tesla, a reduction of the field range was not possible. A better proposition would be to insert a constant voltage source. This would bring the system to a freely chooseable offset and we could increase the resolution by another factor of eight or even more. The HP is controlled by the computer using a GPIB interface.

The superconducting magnet is situated in a ⁴He bath which is isolated towards the outside with several vacuums and a surrounding liquid N₂ bath. Nevertheless because the magnet has no persistent mode it always has to be connected to the current supply. This and the isolation, which is not that good as well, makes the cryostate waste a lot of liquid helium. It uses around 40l liquid helium a day, the flow (which is about another ten liters) not included.

For both, the pulse programming and the signal recording, we used a personal computer. It was new in our laboratory not only to send the pulses from a computer board but also to collect and average the signal in a PC and it has the advantage of being much faster than transferring the data from the Le Croy transient recorder. We changed the electronics sketched in [#!patrik!#] only by replacing the fast digital oscilloscope by a eight bit scope board. The oscilloscope is still in use for optimizing a signal for it is quicker to handle than the program.

The software SeveNMR which we used for the computer control is originally designed by Tomaz Apih in Ljubljana. He was so kind to give and explain the source code to us. He also enabled us to change to our purposes. We will go into details about the software in a later chapter .

  

Figure: NMR setup in the flow cryostate

Another point which should quickly be mentioned is the tank circuit. The tank circuit in the flow cryostate is tuneable from outside in a range of about 10MHz width. The quality factor of the tank circuit varies slightly with the position of the dielectric. In some ranges it has to be reduced with a serial resistor of about one to two Ohms. Its working principle is outlined in figure .

  

Figure: The tank circuit in the probe head. The capacitors C₁ and C₂ can be tuned from outside the cryostate by shifting the dielectric or the signal carrier. (conf. figure )