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Subsections

Using a ``Displex'' closed-cycle He refrigerator

The closed-cycle He gas refrigerators used at the NCNR are not properly Displex devices, since that term is a trade name. I mention the term ``Displex'' though because the closed-cycle He gas refrigerators at the NCNR are commonly and incorrectly referred to using that name.

Most units can be used to control sample temperatures over the range 10-300 K, but a few units have been modified to allow operation to 450 K or higher. Do not use the standard units above 310 K as this can destroy the cooling head.

While it is possible to manually set the temperature controller attached to the refrigerators, the ICP program can control the temperature of the refrigerator, so a series of measurements can be made at different temperatures automatically. The steps to be followed for automatic usage are outlined below and then will be discussed subsequently in more detail.

Note that the function key references (e.g. F17) refer to the labels on the console computer, not to the the letters on the keys.

Summary of steps

1.: Load a sample, pump down and cool.
2.: Connect the temperature controller to the VAX.
3.: Set the temperature controller type in ICP (TDEV).
4.: Confirm communication (PT).
5.: Define parameters for run in ``Prepare mode.''
6.: Set the data collection time using Automon.
7.: Copy and edit the parameters for additional runs.
8.: Set up a run sequence (RS=...).
9.: Switch to control mode (F17) and check the run sequence timing (HOWLONG/RS).
10.: Check and log the neutron monitor (MRAT) as well as document the sample on: a sample tag, the white board and in the log book.
11.: Start the run sequence (RS).
12.: Remove the refrigerator from the instrument.

Detailed discussion

1.

You should ask for help loading a sample, if you are not comfortable with the task. It is better demonstrated than described. Samples are usually sealed with at least a few percent He gas to promote thermal equilibration and then sealed with a indium gasket, but note that indium melts at temperatures in the operating range of the high-temperature refrigerators.

Attach the sample can to the copper block with screws that are the correct length as screws that are too long can damage the heating element or temperature sensor. Also, make sure these screws are tight, so that the screws do not loosen due to vibration.

The inner heat shield cans should not be screwed on tightly, as they may be impossible to remove after temperature cycling. Back them off by a quarter turn.

Attach the outer vacuum can and pump down with a turbopump. As soon as the pressure is in the range of $10^{-3}$ torr, the cooling can be started. It will typically take about 3 hours to bring the sample to the minimum temperature. I do not recommend attempting to collect room temperature data with the set point at 295 K, while the displex is initially cooled as temperature control may not be good. When I collect data over a range of temperatures, I tend to start at the lowest temperature first, but I cannot defend this choice to be the best.

For runs of a day or so, one can usually operate the refrigerator without continual pumping, but cooling can fail if the pressure rises, so it is usually worth the effort to leave a pump attached while data are collected. Always use a pump with the high-temperature refrigerator, when operating above room temperature in case of outgassing.

2.

Physically connect the BT-1 temperature controller RS-232 connector to the refrigerator. The cable from the VAX is marked ``BT1 Temperature Control'' and is connected either to the rear or the top of the refrigerator temperature controller. The cable can usually be found in the vicinity of the shield wall with the ``white board'' attached. Take care not to use the nearly identical ``BT1 it Magnet Control'' cable.

3.

Instruct ICP (in Control Mode) which temperature controller you will be using with a tdev command (see Figures 5 and 6). Note that the tdev command must be reentered each time ICP is restarted and must be reentered if you change the controller model when a new refrigerator is mounted.

**Figure 5:** Menu of temperature controllers from the `tdev` command.
$\begin{figure} \centerline{ \epsfig {figure=tdev1.ps,height=2.6in} }\end{figure}$

**Figure 6:** Selecting a temperature controller with the `tdev` command.
$\begin{figure} \centerline{ \epsfig {figure=tdev2.ps,height=2.6in} }\end{figure}$

The tdev command sets the T+ flag in ICP, so that the temperature is recorded at each data point.

4.

Check that the temperature controller is reading correctly using the pt command (optional, see Figure 6).

5.

In ``Prepare Mode,'' define a run to be measured in what is called an ``increment buffer'' (see Figure 7). Each ``buffer'' line defines the parameters for a single diffractometer scan.

**Figure 7:** Defining a single run in ``Prepare Mode.''
$\begin{figure} \centerline{ \epsfig {figure=setup.ps,height=2.6in} }\end{figure}$

For use with a temperature controller, you will typically need to set the following fields in the buffer: Comment, T0, Wait, Err, Hld, Monit, Prefac. and M-typ, which are used as follows:

Comment: This sets a 1-line file header and the name of the data collection file. Be sure to use letters and numbers (A-Z and 0-9) and no other characters for the first five letters of the Comment as this is used for the file name. If an invalid name is used the file will be named DEFLTxxx.BT1, where xxx is a number in the range 001 to 999.
T0: This specifies the nominal temperature for data collection. This value is sent as the set point to the temperature controller. For the controllers attached to the He refrigerators, this is a temperature in K. If T0 is set to 0, and the T+ flag is set, the temperature will be recorded, but will not be changed and the Wait and Hld0 terms (below) are ignored.
Wait: This specifies a maximum amount of time in minutes that ICP will wait for the sample temperature to be in range (see ERR, below), before starting data collection time. If the desired sample temperature is reached in less time, the remaining time wait is not used. Typical practice is to use a wait that is much longer than the expected time needed to reach the desired temperature, for example 120 to 180 minutes. If you do not want data collection to wait for the temperature to be reached, Wait can be set to 0.
Err: The temperature is considered ``in range'' if the temperature is between T0+ERR and T0-ERR. Note that the value for ERR does not affect the actual stability of the temperature (which is determined by the PID parameters set in the temperature controller) so setting ERR to a small number, can cause data collection to be suspended for long periods when temperature control is flaky. Typical values for ERR are 2 to 5 K for low temperature measurements, but may be 5 to 10 K near room temperature or above.
Hld0: This specifies an amount of time in minutes to wait for temperature to equilibrate after the temperature is reached (or Wait expires) before data collection is started. The desired value for this parameter will depend on the experiment to be performed. A value of 20 minutes is common, but so is 0 as well as longer times.
M-typ: Is either ``NEUT'' or ``TIME''. NEUT is used for most data collection, where the data collection time is adjusted to match the neutron flux on the sample.
Prefac: Each data point is measured ``Prefac'' times and if Prefac is 4 or greater, the measurements are checked for statistical agreement, so that significant noise spikes can be discarded. A rule of thumb is that Prefac should be 4 for runs of 6 hours or less. It may be desirable to increase Prefac by 1 for each additional 6 hours of length, but 4 is a good default value regardless of the data collection time.
Monit: This value, along with Prefac, determines the length of the data collection period. If M-typ=TIME, this specifies a count time in seconds. Most commonly, M-typ=NEUT and Monit is set using the AUTOMON (AMON) feature.

It is very unlikely that you will want change the default values for some fields: A3-beg, A3-end, Inc-3, A4-beg, A4-end, Inc-4 and #pts. The Col field informs ICP of the in-pile collimation (15' or 7'). The default, 15' is usually correct. Note that the A4-*, #pts and Col values are reset every time a buffer is edited. There is one exception to this. If you are setting up runs while the instrument is collecting data and plan to use a different monochromator than the one that is currently in use, you may need to change the A4-beg and A4-end values to match the monochromator you plan to use. Use 3-13 degrees for Cu311 and Si531 and 1.3-11.3 degrees for Ge311. Note that the A- command in control mode turns off the automatic resetting of A4-beg and A4-end.

Note that two fields, Hld and Inc-T, should always be 0. Inc-T causes the temperature to be changed for each data point and Hld creates a delay that is executed at each data point. These processes are almost never of use at BT-1.

6.

Determine the data collection time using the Automon feature. The appropriate monitor value is computed so that the current run will finish at a specified time. Automon is initiated by moving the cursor to the AMON field and pressing Enter. The screen shown in Figure 8 then appears.

**Figure 8:** Using Automon to compute a run length.
$\begin{figure} \centerline{ \epsfig {figure=amon.ps,height=2.6in} }\end{figure}$

The Automon computation can either use or ignore the Wait and Hld0 values. If you answer Y for ``Use Holds,'' the time needed for the Hld0 (and Hld) hold is included in the run length computation. If you answer Y for ``Use TempWait,'' the entire Wait period is included in the run length computation. Since the entire Wait period is usually not used, it is best to say N for ``Use TempWait'' but the answer for ``Use Holds'' is a matter of personal convenience. The number of days and the end time for the run are then entered. Use 1 for delta-days if the run will go past midnight even if the run length is only a few hours. A run starting at 21:00 (9 pm) and ending at 9:00 (9 am) the next morning, would be entered as delta-days=1 and Time=9:00. The computed Monit value is set when Automon completes.

7.

Duplicate the run information for other temperatures that you will wish to run. This is done either by highlighting the buffer to be copied and then pressing the F14 key (actually the F6 key), which will copy the information to all other buffers, or (preferably) the buffer can be copied selectively by pressing the F18 key (actually F10) to enter the ``Buffer Ops'' mode where a buffer can be copied by entering a command such as COPY 2,3 (see Figure 9).

**Figure 9:** Copying a buffer in ``Buffer Ops'' mode.
$\begin{figure} \centerline{ \epsfig {figure=copy.ps,height=2.6in} }\end{figure}$

This copies the parameters in buffer #2 into buffer #3. Exit ``Buffer Ops'' mode by pressing the F20 key (actually F12). Each buffer can then be quickly modified to change the temperature (T0) and possibly the Comment, Hld0, Err and Wait values.

8.

Once a series of runs has been defined, a ``run sequence'' can be defined by pressing the F19 key (actually the F11 key). This brings up the menu shown in Figure 10. If a previous sequence is present, it can be cleared by typing DEL and return.

**Figure 10:** Beginning a ``Run Sequence.''
$\begin{figure} \centerline{ \epsfig {figure=rs1.ps,height=2.6in} }\end{figure}$

Commands are added to the run sequence by typing RI#, where # is the buffer number of the run in the list. Commands may be entered one a time or several commands may be entered at once, separated by semicolons (;). The run sequence in Figure 11 will cause buffer #1 to be collected three times and then buffer #2 to be collected twice. Note that the files will all be named NALICxxx.BT1, so if no other files exist, the data files will be named NALIC001.BT1, NALIC002.BT1 and NALIC003.BT1 for 15 K runs and NALIC004.BT1 and NALIC005.BT1 for the 295 K runs. Exit the RS menu with the F20 key (actually F12).

**Figure 11:** Entering a ``run sequence.''
$\begin{figure} \centerline{ \epsfig {figure=rs2.ps,height=2.6in} }\end{figure}$

9.

Switch to control mode by pressing the F17 (actually F9) key. The length of a run sequence can be estimated using the HOWLONG/RS command (see Figure 12).

**Figure 12:** Using the `HOWLONG/RS` command to determine the expected run time for a ``run sequence.''
$\begin{figure} \centerline{ \epsfig {figure=rs4.ps,height=2.6in} }\end{figure}$

Note that the estimated lengths will be estimated assuming the maximum delay allowed by Wait and the minimum assumes that all Wait times are negligible. In the example shown in Figure 12, there are no actual temperature changes, except between the third run and the fourth, and perhaps before the first run. Assuming that the sample is already at the appropriate temperature and the refrigerator will need approximately one hour to heat from 15 K to 295 K, a good estimate is that the runs will require 32.2 hours. Your mileage may vary.

10.

Before starting the run be sure to:

(a): be sure the shutter is open
(b): measure the monitor using the MRAT command
(c): enter the sample composition and contact info on the white board
(d): put the sample tag in the holder on the white board
(e): enter the sample information in the log book (see Figure 12).

11.

The run sequence is started with a RS command, as shown in Figure 13.

**Figure 13:** Starting a ``run sequence'' with the `RS` command.
$\begin{figure} \centerline{ \epsfig {figure=rs5.ps,height=2.6in} }\end{figure}$

Note that it is possible to modify the run sequence or change the measurement parameters for the runs that have not been started in another (ExtraICP) window while the data collection is in progress.

12.

When removing the refrigerator, close the vacuum valve before turning off the vacuum pump. Let the pump vent completely before removing the vacuum hose - this takes 5 to 10 minutes. The compressor may be turned off at any time. If you plan to unload your sample as soon as possible, you may wish to set the temperature set-point to 295 K, though the cold-head will remain very cold for many hours even when the sample has reached room temperature. When possible, let the refrigerator warm up slowly by letting it sit for a day or so before releasing the vacuum and removing the sample. Remember to leave the sample tag on the refrigerator, so that the identity of the sample is known.

If samples will be changed quickly, it may be necessary to use a heat gun to drive off condensation, but be careful not to heat the cold-head or sample stage to much more than room temperature. The refrigerator can be severely damaged by heating it above about 50 C.

Next: Using the six-position sample Up: A User's Guide to Previous: Running a single sample

Brian Toby
4/22/1999