navigation
Users Guide
 

Cryostat Manual

last modified 13-09-2006 10:40

Principle

The BM29 cryostat is a Displex cryostat from Oxford Instruments. The cooling power is produced using the Joule-Thomson effect with a coldhead (Edwards) driven by a compressor (Edwards cryodrive 1.5). The sample is mounted independently on a sample rod over the coldfinger, with no contact with vibrating parts of the coldhead or sample chamber in order to avoid signal deterioration due to microphonic effects. Heat exchange between coldfinger and sample is guaranteed by a He gas filling. The cryostat operates from around T=20 K (on the sample) up to 450 K, where the cryostat vessel has to be evacuated for T> 300 K.

The set-up is controlled by an temperature controller (Oxford ITC503) which is programmed either manually or via the control software on the beamline workstation (standard operation mode).

The controller controls the temperature of the sample via the regulation of the He gas flow in the coldhead and a resistive heating. Heating devices are installed both on the cold head and the head of the sample rod, but only one of these can be controlled at a time. It is preferable (one should better say 'highly recommanded') to set the temperature somewhat lower than the temperature one wishes to work at and control only on the coldhead.

The steps to use the cryostat are the following:

  • switch on the cryostat controller and the compressor
  • switch on the primary pump and turbo pump to pump the cryostat isolation vessel
  • pump the sample chamber and flush with He
  • introduce the sample
  • center it with respect to the beam
  • set up the software control

Detailed description on all points can be found below. Reminder: back side of the bench (or simply back side is the side towards the ring inside, front side of the bench means the side towards the outside of the ring, i.e. the side of the door of the experimental hutch. In terms of the coordinate system used for the motor movements, movements towards the are positiv y movements.

  1. Controller
  2. Compressor
  3. Vacuum
  4. Sample holder
  5. Sample change
  6. Control software
  7. Sample alignment
  1. Controller Oxford ITC503
    The controller is sited on the top of the cryostat table, with the operation panel facing towards the front side.

    After switching the controller on, it is in general fully operational in remote mode. All remote programming of the controller is discussed below in the control software section.

    Note that DISPLAY can be toggled between sensors 1, 2 and 3 without affecting the operation of the temperature controller (for example if you want to switch the display between the temperature of the sample and the coldhead, in general S1 and S3, when cooling down).


    Fig. 5.1: The ITC503 temperature controller (on the top of the cryostat table, as looking towards the ring inside). On its top (left to right, seen from their back side) the Leybold Penning gauge controller Pennigvac PM31 (not in use), Thermovac TM20, and Turbotronik TN10.

    In the case that the controller is not in remote mode, follow the following instructions. The full user manuals supplied by the manufacturer are available in the control cabin (beamline staff only).

    Configure ITC503 to remote mode

    1. Switch on the power of the ITC503.

    2. In the ADJUST frame:
      push RAISE and LOWER buttons and hold down while
      in the CONTROL frame:
      push LOC/REM button
      DISPLAY will show: TEST then T 00

    3. In the ADJUST frame:
      push RAISE until DISPLAY reads T 03.
      In the CONTROL frame:
      push LOC/REM; DISPLAY will read G 24
      In the ADJUST frame:
      push LOWER until DISPLAY reads G 00.
      In the CONTROL frame:
      push LOC/REM
      DISPLAY will read TEST then T 00
      and then press LOC/REM again ;
      DISPLAY will read S 01 then PASS then the temperature.

    4. In the HEATER frame:
      push SENSOR until LED is lit corresponding to sample rod being used.
      LED 2 should be lit for sample rod A
      LED 3 should be lit for sample rod B
      This sets the temperature sensor on which the control loop operates.
      Verify that the sample holder cable is connected to the correct port at the rear of the ITC503.

    5. In the HEATER frame:
      push AUTO
      In the GAS FLOW frame:
      push AUTO
      In the PID frame:
      push AUTO

    6. Finally, on the beamline computer in the mono application type: oxsetup

    top

  2. Compressor

    The compressor is placed on the back side of the bench, downstream to the cryostat table (see Fig. 5.2). It need to be switch on for operation of the cryostat. After switching it on, it may take some 5 minutes before the compressor start operation, indicated by the green LED.

    Please send the temperature control to 300 K and switch the compressor off after use.

    Possible problems:

    • The compressor stops, red light 'high temperature' is on: There is most probably no water cooling. Check if the valves of the cooling water are open. They are sited on the wall on the ring side, left to the emergency stop button, close to the cryostat.
    • The compressor stops, red LED 'low pressure' is on, or you do not hear the typically noise of the coldhead (happened only once :-) : call support (local contact, probably Bernard Gorges).


    Fig. 5.2: Compressor Edwards cryodrive 1.5 and locations of vacuum pumps used for the cryostat isolation vacuum (as seen from the backside of the bench looking down the beamline).

    3. top

  3. Vacuum

    The cryostat uses a He atmosphere for heat exchange between the coldhead and the sample. It therefore consists of a inner sample chamber surrounded by an isolation vessel. This vessel has to be pumped permanently during operation to guarantee a sufficient good isolation vacuum. The vacuum equipment used for the inner sample chamber is discussed in the correspondent section dealing with sample changes.

    The places of the primary and turbo pump for the isolation vacuum are indicated on Fig. 5.2. You need to check that they are running before going down to low temperatures. There is an isolation valve between the two pumps attached to the primary pump which need to be open for operation. It should be closed when you stop the turbo pump.

    To start the primary pump, use the main switch on the right side of the black box on the top of the pump.

    Start the turbo pump by pressing 'start' on the Turbotronik TN10 controller on the top of the cryostat table (see Fig.5.3). It may take a minute to switch from 'accel' to 'normal' mode.

    Possible problems:

    • the turbo pump stops, the red 'fail' LED is activated on the Turbotronik TN10 controller: The primary pump may not be on, or the valve between the pumps closed. This a a major problem to cause bad isolation vacuum and condensation on the windows or even ice on the outside of the cryostat vessel.
    • In the case of condensation or even formation of ice on the outside of the cryostat isolation vessel, you have to heat up the cryostat immediately to room temperature and check the vacuum pumps.
    • Under normal conditions, there is no condensation or formation of droplets observed on the Kapton windows of the cryostat facing the beam. Nevertheless, during high humidity (somethimes in summer), this may occur, as the air conditioning of the hutch does not dry the air. Therefore we in general blow pressurized air on the windows to avoid this kind of problems. You can see this kind of droplets clearly as dark or grey spots on the flourescence screen downstream the cryostat (fl3).

    top

  4. Sample holder


    Fig. 5.3: Sample holders for the Oxford cryostat. On the left the sampler rod, on the right different sample holder to be mounted on the rods.
    Fig. 5.4: Top view of the sample holders for the Oxford cryostat.

    Fig. 5.5: Sample holders for the Oxford cryostat with dimensions.
    Fig. 5.6: Drawing of positions and dimensions of the threads (2.5mm) on the head of the sample holder rod in order to mount the sample holders in Fig. 5.4 or user sample holders. The dashed line indicates the position of the sample.

    There are different sample holders for standard measurements in the cryostat available. We distinguish here between the main sample holder like shown in Fig. 5.5 and the sample holder rod, Fig. 5.3 and Fig. 5.4.

    At the moment, only sample rod B is in use for standard transmission experiments. The sample holder itself is mounted on the head of the rod via 3 short 2.5 mm srews. A drawing of the position of the correspondent threads can be found in Fig. 5.6. The head is equipped for temperature measurements and heating.

    For pellet samples the 13mm sample holder is used. You may as well use the sample holder with up to 3 samples (Fig. 5.5) either for foil-like samples or even for samples pressed in pellets. In the later case you have to cut the pellet samples in half (or smaller if necessary).

    Your can produce your own sample holders for you experiment, but please limit the dimensions to the one shown in the figures, specially Fig.5.6.

    Mounting a sample on the cryostat sample holder

    After having chosen the kind of sample holder you would like to use for your experiment, you mount the holder on the cryostat rod B (it is the one that your local contact will give you in general. It has 2 similar looking connectors on the top for temperature control and other purposes).

    1. Mount the holder carefully using short 2.5 mm srews. The head of the rod is made out of copper with a nickel surface finish. Copper is soft and you damage the thread if you try to lock up the srew too much. One of the threads is already damaged. It is not necessary to apply force and 2 srews are sufficent as well. Remember that the heat exchange is done via the He gas, therefore the quality of the contact between head and sample holder is not really important ! Watch out for the angle between sample and rod. It is most convenient to have the fixation of the angle indicator orthogonal to the sample.
    2. Place the rod on a table (preferably in the control cabin) somewhat like shown in Fig. 5.3 for the 13mm pellet holder. Put the holder parallel to the table. This is done the easiest (and rather stable) by using a small table (the things to support chemisty equipment, see experimental hutch) or a lead brick supporting the fixation of the alignment tool (See Fig. 5.7 if you do not know what we are talking about) on the top part of the sample rod. The alignment tool should then point upward. In this way the sample holder will in general automatically be parallel to the table (otherwise you did not get the angle right when mounting the holder). Please dont put anything heavy on the rod with the intention to keep it in place ! It may bend which make it useless.
    3. Put you sample(s) in place and fix them with the correspondant frame shown in Fig. 5.6. Your samples are in general more fragile than the frame, so only lock the srews slightly in order to keep the samples in place. You use short (!) 2 mm srews for this.

      In the case you use several samples, make sure that they are all well fixed. Difference in thickness may specially occur with pressed pellets of differing composition. You may use teflon tape or similar as additional spacers.

    Please avoid any loss of sample material inside the cryostat. Not sufficiently fixed samples are good candidates to cause cleaning of the inside of the cryostat, which takes time. For this reasons, fixing the sample with Kapton scotch tape etc. is on your own risk. We did not encounted too many problems, but it may happend that you touch a part of the cryostat inner vessel while introducing the sample and the friction leads to loss of the sample. If you loose a sample, you are obliged to tell your local contact about it. Please note immeadiately in this case composition and other sample caracteristics.

    You can as well have several holders, mount samples in advance, and then change the already mounted holders on the rod. This in general is more delicate, as the sample may more easily damaged during this procedure.

    There are special sample holders for total electron yield (TEY) measurements and work with liquids. In general, you should inquire before asking for beamtime if those are suitable for your experiment. You are free to construct your own sample holders following the dimensions indicated above, but you have to agree with the beamline staff about its use and test it before.

    top

  5. Sample change

    The procedure for the changing sample may therefore vary a bit as a function of what you want to do, temperature, sample holder, etc. The procedure to follow for the case when the cryostat is at low temperare is in principal valid for all cases and is therefore described below. If you want to do anything different, please discuss it beforehand with the staff.

    There are some additional steps that you need to follow, for example in the case that the cryostat is not already running and you have to cool it down. Other point need to be considered if you change the sample rod completely and so on. We start to discus those points and then assume that the experiment is already in operation. We split the second part in a description on how to take out a sample and how to insert a new sample.

    To change the sample, one acts from the back side of the bench. Fig. 5.7 shows the top of the cryostat from that side. It is recommended to use the small ladder (if necessary the big one... the sample insert is unfortunately about 2m from the ground) in order to change the sample. Please consider that safety and your own health have highest priority. Specially during a sample change late at night, do not hurry in order to avoid accidents. By the way, we never had any accidents on the cryostat (not considering the equipment itself).

    Fig. 5.7 shows a photo of the top of the cryostat.


    Fig. 5.7: Top view of the cryostat.

    Filling the sample chamber with He exchange gas

    When you start to cool down the cryostat for the first time, you need to fill the sample chamber with He exchange gas. During an experiment, you may need to add some additional He in the same way as described below, as you loose some He gas each time you change a sample. Changes caused to the compression/expansion due the change of temperature of the exchange gas are in general negligible.

    1. Switch on the primary pump on the front side of the bench. Valve p2 has to be closed, open valve p1 towards the cryostat (Fig 6.1, Ion chamber manual).
    2. Open valve f1 on the filling station (Fig 6.2). On the top of the cryostat, valve h1 is closed, valve h2 in general open, and h3 closed.
    3. If the VAT gate valve is closed, you can assume that the cryostat is still filled with a low pressure of He (this should be the general state), otherwise it is at ambient pressure and you pump the whole. In the first case, you evacuate the four way cross to some 7e-2 mbar by opening valve c2. If someone forgot to close the top of the cryostat with a blank flange, you put of course one before.

      There is a mechanic and a pirani gauge to check the vacuum. The display of the pirani (Thermovac TM20) is placed on the right side of the turbo pump controller Turbotronik TN10 seen in Fig. 5.7.

    4. Close valve c2 and slowly open the gate valve completely.
    5. Now open valve c2 to pump out the sample chamber as well.
    6. Close valve c2. With valve h2 open, you open valve h3 in order to fill the ballon with He. Close valve h3. Be careful, please do not blow up the ballon. You need only very little volume of gas...
    7. Open now valve h1 to fill the desired pressure into the four way cross (and the sample chamber if the gate valve is open..). Close valve h1 again.

      For general operation, some 0.2 bar on the mechanic pressure gauge are optimal. For TEY you may prefer around 1 bar. For flushing, you use 1 bar as well.

      In case of going over 300K, you have to take out all He gas, otherwise you will damage the gaskets on the cryostat (which finishs your experiment).

    8. Repeat the pumping/filling some 3 times to flush the sample chamber if the cryostat was at ambient pressure (air) before.
    9. Close valve f1 on the filling station, switch off the primary pump.

    Remarks:

    • After a sample change, you may as well fill the four way cross with He up to the pressure of the He in the sample chamber. If you work at He pressures as low as 0.2 bar, this is in general not necessary. It is essential in the case you work around 1 bar !
    • For samples decomposing under vacuum conditions, you can you the above described procedure of flushing several times with He in order to avoid to low vacuum. In this case only pumping probably down to 0.1 bar and flush with He some 5 times (or whatever is suitable for your sample and avoid getting air into the sample chamber).

    Adjusting the sample rod

    When you changed the sample rod or use it for the first time, you need probably to adjust the high of the sample rod with respect to the motor that allows you to change the position of the sample in vertical direction.

    1. With the sample rod extracted, move the motor to its negativ limit:
      mvr crysam -40.
      You will get an error message that the negativ limit switch has been touched.
    2. Set this position to '0':
      set crysam 0
      set_dial crysam 0
    3. You now introduce your sample and put it in the lowest position, but not touching the cold head. You can actually feel when you lower the sample on the cold head. Some 3 mm above the cold head is a good distance for the sample.

    Remarks:

    • This procedure has the advantage, that you never risk to drive the sample holder with the motor onto the cold head during the operation. Nevertheless, some positions on the sample holder with 3 positions may not be reachable to satisfaction. The safest way is in this case to move the cryostat rod in or out until you reach all samples within the stroke of the motor EXPERIMENT::crysam. You get a fair idea of the correspondant amount you need to move trough a scan of the motor. The motor has a total stroke of 25 (mm).
    • After a sample change, keeping approxiamtely the same distance between the alignment tool and the angular disk is in general sufficient to be at the same positions (within 1 mm or so).

    Take out a sample from the cryostat

    We consider the general case that the cryostat is working at low temperatures below 300 K. Therefore the pump for the isolation vacuum will be running, controller and compressor working, He atmosphere inside the sample chamber, etc. Of course one can not take the sample out directly by just opening the vacuum clamp on the feedthrough and take out the sample rod, as air would get into the cryostat sample chamber and condense (i.e. at some point you would end up with the sample chamber full of water).

    1. Take carefully off plug of the cable to the temperature controller. Some of the pins are already a bit worn out, please...
    2. Undo a bit the four srews on the feedtrough to keep the rod in place.
    3. Move the sample rod carefully upwards through the feedtrough.
      You may touch a bellow inside the sample chamber. In this case you should carefully turn the sample rod a bit while continue the movement. Do not use force !
      Move the rod up until it is completely out (approximately 1 m).
    4. Close the VAT gate valve.
      Be careful again, if you did not take out the sample completely in the last step, you will now touch of course the sample rod or sample. In this case, you have to stop and move the sample rod further upwards !
      At the point where you think the valve is closed, it is likely to be not... push now hard with one hand on the lever of the valve on the left side, the other one supporting from the right side, until you hear a clear click (your local contact will demonstrate this in general to you).
    5. Once the gate valve is completely closed, you can take off the vacuum clamp.
      Now take of the rod. As there is in general some under-pressure in the sample chamber, you can use the flat side of a suitable srewdriver as a lever arm between the flange of the feedtrough and the DN40KF flange of the cryostat top cross to let some air in.

      Watch out, the sample is in general cold...

    Insert a sample into the cryostat

    1. Switch on the primary pump on the front side of the bench. Valve p2 has to be closed, open valve p1 towards the cryostat (Fig 6.1, Fig 6.1, Ion chamber manual).
    2. With the sample rod fully extracted (feedtrough in the closest position to the sample holder, which is the position after taken out the sample), attach the sample rod on the four way cross on the top of the cryostat. The clamp is a DN40KF fast close/open Edwards clamp, your local contact will show you how to open and close it, please ask if you don't know.
    3. Fix the sample rod in its upmost position by fasten slightly the four srew on the feedtrough. Otherwise you risk that the sample moves rapidly down and is only stop abruptly by the gate valve, which may destroy your sample. You do not need to apply force when fixing the srews, specially you do not need a srew driver !
    4. Open valve c2 (Fig 5.7) to pump out the air that is now in the four way cross. Normally, you pump down to somewhat below 1e-1 mbar (lets say some 7e-2 mbar).

      For sample that can not support vacuum conditions, you may just pump sligthly on the sample and flush with He several times. See description above of how to fill the sample chamber with He.
      Close valve c2.

    5. Open slowly the VAT gate valve. If you work with He exchange gas at pressures of 0.2 bar or so, there is only a small difference in pressure and there is no problem. If for some reasons you wish to work at different pressures (anyway not higher than ambient pressure !), the pressure difference between the cross and the sample chamber is too big and you may damage the seals on the gate valve ! In this case fill up the cross first to the same pressure as in the sample chamber using valves h1, h2, h3 as described below.
    6. Now undo the four srews on the feedtrough and introduce the sample rod into the sample chamber.
      Be as careful as described above when taking out the sample. In case of touching a bellow, turn slightly the rod before going on. You stop in general in approximately the same position as used before. The distance between the alignment tool and the angular disk give you a good idea. Through the window on the cryostat vessel you can see the position of the sample. Check that it is orthogonal to the beam - or whatever angle you want. If you checked before that the alignment tool and the sample are approximately orthogonal, you may use the scale off the angular disk to afdjust the angle, for example to 45 degrees with respect to the beam for fluorescence measurements.
    7. Connect the cable to the temperature controller to the sample rod. For help, there are marks on both parts. In the right position, the marks are facing each other. Please do not use force in order not to destroy the pins.
    8. Switch off the pump on the front side. Residual vibration transfer trought the pipes onto the sample may introduce a significant loss in statistic !
      Close valve p1 towards the cryostat (Fig 6.1).
    9. Wait until the correspondant temperature is reach on the sample before starting the next scan. You can use this time to center the sample, preparing a macro, do a test scan, etc.

    top

  6. control software

    .. not finished ....

    top

  7. Sample alignment

    .. not finished ....

    top

 
European Synchrotron Radiation Facility