Janus Probe st-500 manual

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JANIS Janis Research Company, Inc. 2 Jewel Drive Wilmington, MA 01887 USA Telephone: 1 (978) 657-8750 Fax: 1 (978) 658-0349 Email: [email protected] Web: www.janis.com Page 0 OPERATING INSTRUCTIONS FOR THE JANIS RESEARCH ST-500-1-VMF-HMF MICROMANIPULATED PROBE STATIONS

description

cryogenic probe station operating manual

Transcript of Janus Probe st-500 manual

JANIS

Janis Research Company, Inc. 2 Jewel Drive • Wilmington, MA 01887 USA

Telephone: 1 (978) 657-8750 • Fax: 1 (978) 658-0349 Email: [email protected] • Web: www.janis.com

Page 0

OPERATING INSTRUCTIONS FOR THE JANIS RESEARCH

ST-500-1-VMF-HMF MICROMANIPULATED

PROBE STATIONS

JANIS

Janis Research Company, Inc. 2 Jewel Drive • Wilmington, MA 01887 USA

Telephone: 1 (978) 657-8750 • Fax: 1 (978) 658-0349 Email: [email protected] • Web: www.janis.com

Page 1

TABLE OF CONTENTS SECTION DESCRIPTION PAGE Section 1 INTRODUCTION

1.1 General 2 1.2 Optional Accessories 3

Section 2 UNPACKING AND SET-UP

2.1 Table Top Models 4 Section 3 OPERATION

3.1 Mounting Probes 6 3.1.1 Mounting MW Probes 6 3.1.2 Mounting Low Frequency Probes 7 3.1.3 Mounting and Adjusting Fiber Optic Probes 8

3.2 Replacing Probe Arm Assemblies 9 3.2.1 Removing Probe Arm Assemblies 9 3.2.2 Installing Probe Arm Assemblies 10

3.3 Installing Permanent Ring Magnet 10 3.4 Mounting the Sample 11 3.5 Preparation for Cooldown 18 3.6 Initial Cooldown (with liquid helium) 22 3.7 Initial Cooldown (with liquid nitrogen) 24 3.8 Variable Temperature Operation 25 3.9 Shut Down 26

3.10 Optional Quick Sample and Ring Magnet Changing 26 3.11 Operational Precautions and Maintains 27 3.12 Micromanipulated Probe Stations 28

Section 4 TEMPERATURE CONTROL

4.1 General 28 4.2 Feedback Control 29 4.3 Control Parameters 29

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Janis Research Company, Inc. 2 Jewel Drive • Wilmington, MA 01887 USA

Telephone: 1 (978) 657-8750 • Fax: 1 (978) 658-0349 Email: [email protected] • Web: www.janis.com

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SECTION 1

INTRODUCTION 1.1 GENERAL

CAUTION

The large cylindrical magnets are incredibly powerful and should only be handled by people experienced with the power of the strong magnets. They are killer strong and must be handled accordingly. Please be extremely carefully. Two of these stuck together will take most adults full strength to separate. Monster holding power! MAGNETS ARE VERY FRAGILE AND SHOULD NEVER COME IN CONTACT WITH EACH OTHER, OR CLOSE ENOUGH TO BE ATTRACTED TO EACH OTHER. The Janis Research Model ST-500 continuous flow cryogenic probe stations are dc/low frequency (optional MW and/or Fiber Optics), micromanipulated probe systems (see Figure 1). A system includes an operational temperature range from ~ 3 K to 475 K (450 K maximum for MW probe and 350 K with the ring permanent magnet installed). This is a tabletop model that can accommodate up to four probes. The system is modular in construction so that different probes can be added at future dates, or upgraded to higher frequencies. The system is supplied with a base on which micromanipulated probe stations are mounted. In the center of this base is mounted the ST-500 cryostat, with entry ports on four (six optional) sides for the probes, with a window located on the top of the chamber. The continuous flow ST-500 cryostat includes a thermal radiation shield that surrounds the sample mounting area. This shield is cooled by the escaping helium vapor, and is also used to cool the probe supports through flexible copper cables. The test sample is directly mounted on the sample holder (sometimes referred to as a chuck), and is typically held in good thermal contact with Apiezon N brand (or similar) vacuum grease. The sample mounting stage also holds a temperature sensor and a heater to control the temperature of the sample. Most systems are also supplied with an automatic temperature controller for controlling the temperature of the sample mount. A high efficiency liquid helium/nitrogen transfer line is supplied with the system. The transfer line has a high efficiency flow control valve at the bottom of the storage dewar leg, for precise control of the flow of cryogen for cooling the sample mount. The transfer line is a uniquely designed high efficiency line, which maintains a high degree of flexibility and a small 12” bending radius (with no decrease in efficiency). The system

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Telephone: 1 (978) 657-8750 • Fax: 1 (978) 658-0349 Email: [email protected] • Web: www.janis.com

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provides excellent temperature stability (typically 50 mK control or better) without requiring an active pumping system to draw the cryogen into the cold head. This comprehensive system design results in a continuous flow cryostat with minimal level of vibrations (nanometer scale) to the sample mount of the probe system. 1.2 OPTIONAL ACCESSORIES Many ST-500 systems are supplied with a video scope, for clear view of the probes and to help in precise manipulation and location of the probe tips on the sample (see Figure 1). The monoscope (stereozoom) is mounted on a horizontal boom with vertical

Figure 1. Typical ST-500-1-VMF-HMF Probe Station

focusing adjustment. The monoscope (stereozoom) is designed for full compatibility with the probe station and is fully tested with the system through out the entire

Vacuum Chamber Evacuation Valve

Line for He Bayonet

StereoZoom Mounting Boom

Camera

Triaxial Probe Arm

Light Source

Triaxial Feedthrough for Cable to Chuck

Helium Vent Port

StereoZoom StereoZoom Focus

Vacuum Chamber Vent Valve

Pressure Relief Valve

Fiber-Optic Light

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temperature operating range of the cryostat. An appropriate ring-light source is also included with the monoscope for illuminating the surface of the sample. Monoscope (stereoscope) imaging systems are also provided with a CCD camera and video monitor. The CCD camera is directly mounted on the video tube of the monoscope (stereoscope), and is supplied with all the appropriate cables connecting it to the video monitor. These systems are designed for full compatibility and are also fully integrated and tested at our facility prior to shipment. Complete systems may also include a turbo-molecular vacuum pumping station for evacuating the sample chamber and providing the isolation vacuum necessary for operating the system. The pumping station is supplied complete on a stand with a turbo-pump, back-up mechanical pump, vacuum gauges and interconnecting stainless steel pumping lines. This pump can be also be used for the occasional evacuation of the vacuum jacket of the flexible cryogen transfer line that is supplied with the system. An optional liquid helium or liquid nitrogen storage dewar may also be supplied with these systems. Such dewars include all the necessary pressure gauges, valves and adapters for the transfer line, and typically require a source of helium or nitrogen gas and pressure regulator to supply the necessary driving force to deliver the cryogen into the cryostat.

SECTION 2

UNPACKING AND SET-UP Upon receipt of the system, please examine the outside of the crate and the inside of the station for any visible shipping damage or evidence of rough handling. Any such damage must be immediately reported to the freight company and to Janis Research Company. Such damage should also be recorded on the receiving documents for future reference and insurance purposes. 2.1 TABLE TOP MODELS The cryostat assembly and all the probe stations are shipped mounted on a circular base, with DC (such as tungsten or beryllium-copper tips) and/or MW probes shipped in a separate box. The transfer line is shipped in a separate crate, as are any optional accessories such as temperature controller, monoscope, light source, camera, video monitor and vacuum pumping station. Please exercise extreme care when removing the various items out of their crates or boxes, since most are quite delicate or fragile. This particularly applies to any eyepieces, objectives, and other optical components, as well as any cameras or video monitors. Remove any protective packing, shipping support material, and plastic cable ties around the probe stations. (see Figure 2).

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Figure 2. Remove shipping support material While looking down into the sample space, manipulate the various translation stages to get a feel of the way they move and to the extent they move. This is important as the stages may offer more travel than the required travel of the probes. The accompanying transfer line has a bayonet assembly that will require insertion into a mating section (which is part of the cold head assembly on the vacuum jacket). When inserting the transfer line into the cold head of the cryostat, it is advisable to remove the threaded coupling with the associated metal ring & O-ring (Quick-Connect) and slide them on to the male section of the bayonet. This will aid in smooth insertion of the bayonet without tearing the O-ring. When the bayonet is fully inserted, the O-ring, and metal ring can be pushed into their final position and the threaded coupling tightened to complete the gas tight O-ring compression seal for the cryogen fill. It a good idea to perform this operation and get a feel of how this is done. Install the video scope boom stand to the matching hole located in the baseplate, using the screw provided with the system. Mount the video scope to the boom stand, along with any CCD camera supplied (see Figure 1). Move the video scope up and down to focus on the sample holder and out of the way from the top of the vacuum chamber. Extreme care needs to taken during this operation since the objective can crash into the viewing window and damage the window and/or the objective.

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Telephone: 1 (978) 657-8750 • Fax: 1 (978) 658-0349 Email: [email protected] • Web: www.janis.com

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SECTION 3

OPERATION

3.1 MOUNTING THE PROBES OR PROBE POINTS CAUTION: EXTREME CARE MUST BE TAKEN TO AVOID CONTACT OF THE MW AND LOW FREQUENCY PROBE TIPS WITH ANY OBJECT OR SURFACE TO AVOID DAMAGING THE TIP.

Please note that when the vacuum jacket is evacuated, the force due atmospheric pressure will pull the probes down towards the sample

(chuck) by a fraction of an inch. It is thus very important to raise the probes above the sample BEFORE placing the system under vacuum

(see Figure 3). WARNING: DO NOT EVACUATE THE CHAMBER WITH THE PROBE TIPS TOUCHING THE SAMPLE. 3.1.1 Mounting MW Probes The MW probe holders are mounted on ½” tubes (see Figure 3) that extend out of the vacuum chamber which make up the probe arm. A probe tube O-ring compression seal grips the probe arm and maintains the vacuum (see Figure 4). Using the X translation stage micrometer, slide all stage assemblies radially outwards so that there is a clear space above the sample holder in the chamber. Make sure that the probe is high enough (Z- axis) so that the probe points do not touch the sample holder (placing them at a reasonable distance above the sample holder). If required, use the Z axis stage to move the probe point up. The MW probes are typically shipped in a separate box. These probes are very delicate and very expensive. Do not touch the tips with your fingers or to any other surface. To install (or remove) the MW probe, proceed with the following steps:

- If the vacuum chamber under vacuum, fill it with dry nitrogen or air. - Unscrew and remove the vacuum chamber top flange (see Figure 5). - Remove the four screws holding the radiation shield window flange.

Remove the entire flange, not just the window (see Figure 6). - Unscrew and remove the probe planarizer collar (see Figure 7). - Loosen the MW probe tube compression seal knob (see Figure 8) by ~

one turn. - Rotate the probe arm by ~ 90 degrees (see Figures 9).

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- Loosen the cable compression seal by ~ one turn (see Figure 4) and push the cable into the vacuum chamber (see Figure 9).

- Carefully hold the MW and attach it to the male MW connector. The hexagonal fitting can be tightened using the thumb and index fingers or an 8 mm hexagonal wrench (see Figures 9).

- Rotate the probe holder back by 90 degrees, pool the semi rigid cable out of the vacuum chamber and tighten two M-3 (or 4-40) provided screws on both sides to secure the MW probe into position (see Figure 10)).

- Move the probe arm to the original position, level the probe (see Figure 11), and then tighten the compression seal for the probe tube to hold it in place (see Figure 8). The smaller compression seal for the semi-rigid cable should also be tightened to maintain good vacuum for the system.

- Reinstall the probe planarizers (see Figure7). - Using the X translation stage micrometer, slide the stage assembly radially

outwards. - Install all other probes. - Mount the sample (see next section). - Reinstall the radiation shield and vacuum chamber top flanges.

The MW probe arms can be rotated as needed (holding the main collar support) even under vacuum to planarize the MW probes (see Figure 4). 3.1.2 Mounting Low Frequency Probes To install (or remove) the Low Frequency Probe Tips proceed with the following steps:

- If the vacuum chamber under vacuum open the evacuation valve and vent the vacuum (up to one atmosphere) with either dry nitrogen gas or air (if absolutely certain that the system is at room temperature).

- Unscrew and remove the O-ring sealed vacuum chamber top cover (see Figure 5).

- Remove the four screws holding the radiation shield window flange. Remove the entire flange, not just the window (see Figure 6).

- Slide all stage assemblies radially outwards so that there is a clear space above the sample holder in the chamber.

- Slide in (one at a time) the stage assemblies and mount the probes. - The low frequency probe tips are shipped in a separate box. Mount these

points by loosening the set screw on the holder (see Figure 12) and inserting the dull end of the tip from the bottom. It is important to hold the probe holder securely when tightening or loosening the probe tip set screw. This set screw needs only to be snug. Do not over-tighten. Adjust the height of the probe points as required. The probe tips should extend out from the probe holder approximately 0.3” to ensure the largest motion

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of the probes. The tips should extend down (Z direction) from the probe holder by about 0.6”. Any tip length sticking out of the top of the probe should be cut off so it doesn’t scratch the top window. CAUTION: EXTREME CARE MUST BE TAKEN TO AVOID CONTACT OF THE PROBE TIP WITH ANY HARD OBJECT OR SURFACE TO AVOID DAMAGING THE TIP.

After all the probes are properly installed, slide all the probe stations towards the vacuum chamber.

Figure 3. Interior view of sample chamber

3.1.3 Mounting and Adjusting Fiber Optic Probes The probe systems are typically shipped with the fiber optic probe installed (see Figures 13 and 14). If the position of the fiber line capillary should be adjusted then proceed with the following procedure:

- If the vacuum chamber under vacuum open the vent valve and vent the vacuum (up to one atmosphere) with either dry nitrogen gas or air (if absolutely certain that the system is at room temperature).

- Unscrew and remove the vacuum chamber top flange (see Figure 5).

LF Probe Holder

Sample Mount Cable

Sample Mount

Probe Cooling Braid

LF Probe Tip

Re-Entrant Port for Cylindrical

Permanent Magnet

Radiation Shield

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- Remove the four screws holding the radiation shield window flange. Remove the entire flange, not just the window (see Figure 6).

- Loosen the fiber probe ½” diameter tube compression seal knob by ~ one turn (see Figure 8).

- Rotate the probe tube to the position shown on Figure 13. - Loosen the position of the fixing screw on the fiber capillary holder and

adjust the fiber capillary position (see Figure 14). - After the adjustment tighten the fixing screw, return the fiber probe tube

and tighten the compression seal of the probe tube to hold it in place. - Reinstall the radiation shield and vacuum chamber top flanges.

3.2 REPLACING PROBE ARM ASSEMBLIES

CAUTION: EXTREME CARE MUST BE TAKEN TO AVOID CONTACT OF THE PROBE TIP WITH ANY HARD OBJECT OR SURFACE TO AVOID DAMAGING THE TIP.

3.2.1 Removing Probe Arm Assemblies To remove a probe arm proceed with the following steps:

- If the vacuum chamber under vacuum open the vent valve and vent the vacuum (up to one atmosphere) with either dry nitrogen gas or air (if absolutely certain that the system is at room temperature).

- Unscrew and remove the vacuum chamber top flange (see Figure 5). - Remove the four screws holding the radiation shield window flange.

Remove the entire flange, not just the window (see Figure 6). - For LF Probe Arm remove the probe tip (see Figure 12 and Section

3.1.2). For MW Probe Arm remove the MW probe (see Section 3.1.1). For Fiber Optic Arm (see Figures 13, 14 and Section 3.1.3) cut and remove the Teflon thread (closest to the capillary), pull the fiber line out of the capillary, loosen the capillary position fixing set screw and remove the capillary. Gently attach the fiber to the capillary holder to avoid damaging it during the probe army removing (see below).

- Loosen the ½” diameter tube compression seal knob by ~ one turn (see Figure 8).

- Move the probe arm tube into the vacuum chamber and disconnect the cooling braid terminal by unscrewing it (see Figure 15). (There are no cooling braids for Fiber Optic Arm)

- Rotate the probe arm tube by 180 degree and disconnect the second cooling braid terminal (see Figure 15).

- Pool out the probe arm tube (see Figure 8).

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3.2.2 Installing Probe Arm Assemblies To install a probe arm proceed with the following steps:

- For MW Probe Arm unscrew and remove the MW probe holder from the arm assembly (see Figures 16 and 17)

- Gently insert arm tube into the probe station vacuum chamber through the compression tube (see Figure 8).

- Move the probe arm tube into the vacuum chamber and connect the cooling braid terminal by screwing it to the probe holder (see Figure 15).

(There are no cooling braids for Fiber Optic Arm). - Rotate the probe arm tube by 180 degree and connect the second cooling

braid terminal (see Figure 15). - For LF Probe Arm install the probe tip (see Figure 12 and Section 3.1.2).

For MW Probe Arm reinstall the MW probe holder and MW probe (see Section 3.1.1). For Fiber Optic Arm (see Figures 13 and 14 and Section 3.1.3) insert capillary into the capillary holder slot and gently tighten the capillary position by fixing the set screw. Insert the fiber line into the capillary and fix the fiber position by gently attaching the fiber line to the capillary holder with a Teflon thread (Dental Floss). Make sure that the end of the fiber line is out of the capillary (see Figure 13).

- Rotate the arm tube into the proper position and tighten the tube compression seal knob (see Figures 4).

3.3 INSTALLING PERMANENT RING MAGNETS

CAUTION: These ring magnets are very powerful and must be handled accordingly. Please be

extremely careful. Do not heat the sample mount above 350 K when a ring permanent magnet is

installed.

1. If the cryostat is cold, it must be fully warmed up to room temperature prior to opening. Note that even if the temperature of the sample mount is at room temperature, the radiation shield and other internal components may still be cold.

2. Open the evacuation valve and vent the vacuum (up to one atmosphere) with either dry nitrogen gas or air (if absolutely certain that the system is at room temperature).

3. Remove the O-ring sealed sample chamber top cover (see Figure 5). 4 Remove the four screws holding the radiation shield window flange. Remove the

entire flange, not just the window. (see Figure 6).

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5 Move all the probes out to clear the space above the sample holder. 6 Install the ring magnet spacer (except for the strongest 10 mm thick ring magnet)

(see Figure 18). 7 Install the corresponding ring magnet (see Figure 19). 8 When taking out a ring magnet and a spacer remove the electrical isolation ring

before removing the spacer (see Figure 20). 3.4 MOUNTING THE SAMPLE Standard ST-500-2 probe stations include a sample holder that is directly attached to the cold head (sample mount) of the cryostat (see Figure 3). These systems are designed to allow sample mounting through the top of the vacuum chamber. The following steps must be taken to attach the sample to and remove it from the sample holder: 1. If the cryostat is cold, it must be fully warmed up to room temperature prior to

opening. Note that even if the temperature of the sample mount is at room temperature, the radiation shield and other internal components may still be cold.

2. Open the evacuation valve and vent the vacuum (up to one atmosphere) with either dry nitrogen gas or air (if absolutely certain that the system is at room temperature).

3. Remove the O-ring sealed sample chamber top cover (see Figure 5). 4. Remove the four screws holding the radiation shield window flange. Remove the

entire flange, not just the window. (see Figure 6). 5. If needed, move all the probe stations out to clear the space above the sample holder.

CAUTION: EXTREME CARE MUST BE TAKEN TO AVOID CONTACT OF THE PROBE TIP WITH ANY HARD OBJECT OR SURFACE TO AVOID DAMAGING THE TIP.

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Figure 4. Microwave Probe Cable

MW - Connector

Semi-Rigid Cable

Compression Seal for Cable

Rotate / Planarize MW Probe

Compression Seal for MW Probe Tube

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Figure 5. Removing the vacuum chamber top cover

Figure 6. Removing the radiation shield top flange

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Figure 7. Probe planarizer unscrewing and removing

Figure 8. Loosening or tightening probe arm tube compression seal knob and rotating or removing

tube

Compression Seal Knob Probe Arm Tube

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Figure 9. MW Probe Wiring

Figure 10. Attaching MW Probe

MW Probe Holder Rotated by 900

MW Probe

MW Connector

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Figure 11. Leveling Probe Holder

Figure 12. Tightening and loosening probe tip, using hex key

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Figure 13. Typical fiber optic arm

Figure 14. Fiber optic holder position adjustment

Capillary Holder

Fiber Line Bare Polished End Fiber Line

Position Fixing Screw

Teflon Thread (Dental Floss)

Capillary

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Figure 15. Disconnecting Probe Holder Cooling Braid Terminal

6. The sample can now be mounted on to the sample holder. This is typically done with

provided sample mounting strips or alumina washers as well as with a thin layer of Apiezon (up to room temperature) or Krytox (up to 475 K) low out gassing / thermally conductive vacuum greases or GE Varnish (up to room temperature). It is important to establish good thermal contact with the sample mount to ensure cooling of the sample.

7. With the sample in place, make sure that the probe tips on each probe station are not located below the level of the sample. Use the Z axis stage to move them well above but not too much above the sample level. After checking the position of the probe tips, all the probe stations must be moved inwards toward the vacuum chamber.

8. Position all the probes using X and Y stages approximately over the area of test. 9. Install the radiation shield top window flange. 10. Clean the O-ring on the sample chamber cover and apply a very thin layer of vacuum

grease to the O-ring (if it is dry), then install the top cover of the chamber. 11. The system is now ready for operation.

3.5 PREPARATION FOR COOLDOWN The vacuum chamber must be first evacuated to a pressure of less than 10-3 Torr. The chamber must be pumped for several hours, and preferably overnight, to remove all the air and reduce the out-gassing from bellows and other parts of the vacuum space. Evacuation is best done with a good pumping station (e.g., a turbo-molecular or cold

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trapped rotary/diffusion pumping station). Please note that the pressure read by a gauge at the pumping station will typically read lower than the actual pressure inside the probe system vacuum jacket. Since the pumping throughput is limited by the size of the hole in the evacuation valve, one may wish to use a larger port and a larger throughput valve for faster evacuation. After evacuation, the valve must be firmly closed, but care should be exercised to avoid damaging the valve seat with too much pressure. The transfer line supplied with the equipment may occasionally require evacuation. A new transfer line could require a 24 hour evacuation to ensure that the vacuum has not deteriorated during shipment. The same type of vacuum pumping station and pressure should be used as discussed above. When the evacuation of either vacuum space is initiated, always make sure that the pressure on the pump side of the evacuation valve is lower than the pressure in the vacuum space. This is done to avoid drawing oil vapor from pump to the vacuum space. This is especially true if any of the components under vacuum are cold (e.g. when liquid helium is passing through the inner line of the transfer line or in the cold head of the cryostat). Liquid helium could cryopump to a lower pressure than the pumping station in use. It is always preferable to maintain vacuum in the chamber even when it is not in use, and never allow helium gas or moist air inside the vacuum space. If moisture or helium does accidentally enter the space, a pumping system should be attached to the space for several days in order to bring the pressure down to acceptable levels.

Figure 16. Unscrewing MW probe holder from spare MW probe arm

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Figure 17. Removing MW probe holder from spare MW probe arm

Figure 18. Installing or Removing Ring Magnet Spacer

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Figure 19. Installing or Removing Ring Magnet

Figure 20. Installing or Removing Electrical Isolation Ring

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3.6 INITIAL COOLDOWN (with liquid helium)

CAUTION!

Always use the supplied transfer line when operating with LHe or LN2. Never connect the cryostat inlet port directly to the outlet of

a pressurized LN2 tank without using the supplied transfer line. High pressure from the LN2 tank will result in damage to the

internal structure of the ST-500. 1. Close the flow regulator valve at the bottom of the transfer line storage dewar leg (see

Figure 21). This can be done by turning the knurled brass knob counter-clockwise as observed from above the knob. Do not over tighten the knob.

2. Insert the transfer line storage dewar leg into a storage dewar very slowly, allowing the cryogen vapor to vent out of the dewar. (For easiest adjustment of the transfer rate, the storage dewar should be equipped with a 0 - 5 psi pressure gauge, a venting valve, and a hose adapter for adding helium gas pressure if necessary.) As the leg enters the storage dewar, open the transfer line flow regulator valve about 3 turns, and monitor the transfer line exhaust for a few seconds to confirm that the flow is not restricted. (The flow rate is quite small during the initial phase of the transfer, until the entire inner line becomes cooled.)

3. Insert the bayonet end (short leg) of the transfer line into the cryostat (see Figure 22). Tighten the bayonet connection (knurled nut) to form a gas tight seal. When inserting the transfer line into the inlet of the cryostat, it is advisable to remove the threaded coupling with the associated metal ring & O-ring (Quick-Connect) and slide them on to the male section of the bayonet. This will aid in smooth insertion of the bayonet without tearing the O-ring. When the bayonet is fully inserted, the O-ring, and metal ring can be pushed into their final position and the threaded coupling tightened to complete the gas tight O-ring compression seal for the cryogen fill. It is a good idea to perform this operation a few times and get a feel of how this is done.

4. Continue to insert the transfer line storage dewar leg into a storage dewar. Close and open the valve several times during the first few minutes of transfer, to be sure the valve does not freeze shut during cooldown, and that the flow is not restricted. Once the leg is fully inserted into the storage dewar, raise the leg about 1 cm from the storage dewar bottom to minimize the chance of frozen debris entering the line. After the leg touches the bottom of the dewar, lift it about 1 cm to avoid any frozen debris at the bottom. Seal the transfer line leg against the dewar by tightening the compression fitting on the top of the dewar, and allow the boiling cryogen to build up pressure inside the dewar.

5. In general, heat introduced by conduction down the transfer line leg is sufficient to maintain positive pressure in the storage dewar. Adjust the storage dewar vent valve

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to maintain a storage dewar pressure of 2 - 3 psi (0.3-0.45 atm). (In some cases, helium gas will need to be added to maintain constant pressure in the storage dewar.)

6. After a short wait (typically 5 - 10 minutes) the sample mount will begin to cool rapidly.

7. Once the sample mount temperature reaches about 5 K, the flow regulator can be closed, until the sample temperature begins to increase. Open the regulator just enough to cool the sample mount to ~ 4.5 K. For best operating efficiency, it is best to open the regulator valve just enough to maintain the desired minimum temperature.

8. To speed up the probe station helium cooling, attach a mechanical pump to the gas venting port of the cryostat and evacuate this space continuously during the first phase of the cooldown. This procedure will also remove the air from the inner line and provide a path for cryogen free from air and moisture. Never do this procedure with liquid nitrogen since nitrogen could become solid during pumping.

Figure 21. Liquid helium (or Nitrogen) transfer line

Storage Dewar Leg

Cryostat Leg

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Figure 22. Inserting transfer line bayonet into the cryostat inlet, then tightening the fittings

3.7 INITIAL COOLDOWN (with liquid nitrogen)

CAUTION!

Always use the supplied transfer line when operating with LHe or LN2. Never connect the cryostat inlet port directly to the outlet of

a pressurized LN2 tank without using the supplied transfer line. High pressure from the LN2 tank will result in damage to the

internal structure of the ST-500.

Cooldown with LN2 (see the LN-50 Dewar with Inlet Adaptor on Figure 23) is very similar to operation with LHe, and the procedure above can be used with the following exceptions. 1. Use a LN2 storage dewar pressure of about 5 psi and open the flow regulator valve

about 3 turns. It will take 5 - 10 minutes before the sample mount temperature begins to drop.

2. It is often necessary to add gas pressure to the storage dewar, to maintain constant pressure of 5 psi. Either helium or nitrogen gas can be used for this purpose.

3. Once the temperature reaches about 100 K, close the flow valve completely, and wait until the sample temperature stabilizes or begins to increase. Open the flow valve regulator just enough to reach 77 K. Too large a flow will cause liquid to collect inside the sample mount, making temperature control at higher temperatures difficult or impossible.

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Figure 23. Liquid Nitrogen Dewar with inlet adapter with ½” O-ring seal to accept the probe station

transfer line 3.8 VARIABLE TEMPERATURE OPERATION After cooldown to helium temperature, the flow valve opening should be adjusted to maintain the lowest temperature of interest. This is done to optimize cryogen consumption. The necessary (minimum) flow required is a function of the heat load into the cold finger. Any supplied temperature controller should be used to raise the sample temperature above helium temperature (refer to the Temperature Controller Manual when necessary). The PID settings and heater power should be adjusted for optimal temperature control at the various temperature ranges. Note that most cryogenic systems can be very accurately controlled without using the D setting (set D to zero). High heater powers will be needed above 80 K, and higher settings of P will also be needed at higher temperatures. Please refer to the accompanying controller manual and the section below for more details.

Pressure Gauge Venting Vent

O-ring Seal Port

NW-40 Flange

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Adjustment in the cryogen flow can be made at any time to optimize its consumption at higher temperatures. This can be done by either decreasing the storage dewar pressure or closing the flow valve gradually. If the heater power required to control the temperature is too high, it is an indication that there is too much cryogen flow. The temperature of the cold finger can be reduced by reducing the pressure at the helium vent port of the cryostat. This is done by connecting a mechanical pump to the vent port. The flow valve may be partially opened (less than ¼ turn), thus developing a pressure gradient at the needle valve and continuously transferring cold helium vapor to the cold finger. This configuration can provide a lower temperature for extended time periods, but at a higher helium consumption rate. 3.9 SHUT DOWN Upon the completion of the experiment, the flow control valve should be closed, and the heater output of the controller turned off. The transfer line leg should be removed from the storage dewar, and may also be removed from the cryostat itself. If the latter is done, make sure that a pressure relief valve is placed at the vent port and the bayonet entry port is sealed. Let the cryostat warm up to room temperature. If the transfer line is left in position for the next cooldown, it is best to keep the transfer line in both the dewar and in the cryostat, leaving the foot valve closed. To prevent air and moisture into the cryogen space, place a low pressure 1 PSIG relief valve at the vent port of the cryostat. The relief valve eliminates any pressure build-up during the warming of the cryostat and at the same time eliminates out side air going into the cryostat. The sample mount and radiation shield should reach room temperature before the vacuum chamber can be opened. This is to prevent any air or moisture from condensing (or freezing) on the radiation shield, test sample, sample holder and probes. An alternative method of rapid warm up is by releasing the vacuum chamber vacuum with dry nitrogen gas. This results in rapid warm up of the internal components, but it will also result condensation of moisture on window and the exterior surface of the chamber and the bellows of the probe manipulators. 3.10 OPTIONAL QUICK SAMPLE OR RING MAGNET CHANGING BY FAST RADIATION SHIELD WARMING UP TO ROOM TEMPERATURE This is an option of quick (within ~ one hour) sample or ring magnet changing by fast warming up the radiation shield to the room temperature. To open the vacuum chamber and change the sample within one hour proceed with the following steps (refer to the Temperature Controller Manual when necessary).

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1. Replace Cable B (Sample Mount Si Diode) with Cable C (Radiation Shield Si Diode) at the Channel B of the Temperature Controller.

2. Connect Controller’s relay green connector to the controller. 3. Connect the radiation shield heater banana plug (labeled E) to the Booster power supply. 4. Set the Channel A of the Controller to 300 K. 5. Make sure that the Channel B relay is set to 300 K. 6. Using the set handles of the Booster Power Supply set it’s current to the maximum value and voltage to zero. 7. Turn the Temperature Controller “Control” on. The sample mount heater ring temperature (Channel A) should start going up. 8. Slowly increase the Booster Power supply voltage to 25 Volts (NOT EXCEED 25 Volts). Watch on the Booster Power Supply current indicator. The current should start running through the circuit and the radiation shield temperature (Channel B) should start going up. Make sure that as soon as the radiation shield temperature reaches 300 K, the relay opens and the booster power supply current drops to zero. 9. Wait until both sample mount and the radiation shield temperatures are stable at 300 K. 10. Proceed with the sample changing as described in Section 3.4.

3.11 OPERATION PRECAUTIONS AND MAINTAINS

CAUTION The large cylindrical magnets are incredibly powerful and should only be handled by people experienced with the power of the strong magnets. They are killer strong and must be handled accordingly. Please be extremely carefully. Two of these stuck together will take most adults full strength to separate. Monster holding power! MAGNETS ARE VERY FRAGILE AND SHOULD NEVER COME IN CONTACT WITH EACH OTHER, OR CLOSE ENOUGH TO BE ATTRACTED TO EACH OTHER. In order to maintain the high performance of the cryogenic system, it is recommended that the following suggestions and precautions be followed: 1. Always maintain utmost cleanliness inside the vacuum chamber. Clean surfaces with

100% alcohol (ethanol), when necessary. Any elastomer seal should be cleaned and lightly quoted with high vacuum grease. Evacuation is best done with either a cold trapped diffusion pump or a turbo-molecular pump.

2. Always maintain vacuum in the transfer line and evacuate it occasionally (more often for new lines). Evacuation should be done while the inner line is at room temperature. Helium gas or moisture should never be allowed in the vacuum space.

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3. Do not heat the sample mount above 475 K (450 K with MW probes and 350 K when a permanent magnet installed). A high vacuum pumping station should be used to continuously evacuate the vacuum space during operation above 325 K.

4. Do not bend the flexible part of the transfer line less than 12” radius. 5. Do not over tighten the foot valve. 6. Keep the end of the foot valve leg about 1 cm above the bottom of the LHe dewar. 7. To remove deposits from Beryllium Copper probe tip, rinse with deionized water

using compressed air to dry excess water from the tip. To remove oxide deposits from the tip, wash the tip in a 10% w/w sodium carbonate solution (Na2CO3) and rinse in de-ionized water. Do not dry the tips. Next dip the tip into a 10% v/v nitric acid solution (HNO3) until a reaction occurs. Rinse the tip in deionized water immediately after the acid reaction starts. Do not let the reaction continue for more than one second. After rinsing, dry excess water with compressed air.

3.12 MICROMANIPULATED PROBE STATIONS The X, Y and Z stage assemblies are precision positioning devices for manipulating probes on the sample. These carry fairly large forces when the vacuum chamber is under vacuum. They should remain as dust and moisture free as far as possible. The stainless steel bellows are fragile and should not be impacted with hard tools or objects, especially if assemblies are moved in and out and a wrench is used to manipulate the bolts on the sliding base plate.

SECTION 4

TEMPERATURE CONTROL 4.1 GENERAL Most temperature controllers used with cryogenic systems are designed to provide a stable temperature for measurements at any temperature within the range of the system. The temperature controller provides a DC heater power to a resistance heater attached to the cold finger of the cryostat. The controller balances the amount of heat input with the cooling power available at the cooling tip to provide a stable temperature at any set point entered by the user. In addition, a controller, with proper feedback, balances its time response against that of the cold finger in order to achieve a stable temperature without oscillations or unnecessary delays. Besides cryostat performance, there are other factors that may affect the heater power supplied by the controller. These factors include extraneous heat loads (for example, heat generated in the measurements, heat carried down electrical leads, and radiated heat loads), and extraneous heat capacities of samples or sample mounts. Cryogenic controllers require a broad range of feedback parameters because cooling power, cryostat response time, and other factors affecting temperature control are temperature dependent and system dependent.

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4.2 FEEDBACK CONTROL Temperature controllers used with cryogenic systems have three term temperature control parameters. The controller utilizes proportional, integral and differential (PID) feedback to optimize heater response. The feed back parameters are set on the front panel of the controller or through the computer interfaces to the controller. Many controllers can be placed in an “auto-tuning” mode, and with the help of an internal set program, seek the best PID values and lock on it until the parameters affecting the PID change. Autotuning is helpful in the initial stage of operating a system but does not necessarily provide the most optimum PID values. Several other parameters affect the overall system gain. These include the heater range (usually low, medium and high), which is set in the same way as the other feedback parameters, the thermometer’s temperature sensitivity, and physical and thermal separation of the heater and the thermometer, all of which are fixed in the cold head. Optimal control parameters and ultimate system performance depend on the entire cryogenic system, and not simply on the controller. 4.3 CONTROL PARAMETERS The optimal feedback parameters P, I, D to be entered into the controller, vary from system to system, and from temperature to temperature. They also depend on the cooling power, thermal mass, heat input to the system. As such, it is difficult to specify at the onset which parameters are optimum for any particular temperature, in any specific system. A guideline, however, would be to observe the initial cooldown and test data supplied with your system. These data indicate the parameter that was used during its initial testing to provide temperature stability within the temperature specification. While these may not be the optimum temperature control parameters, they are probably relatively close to optimum. The second option is to operate, initially, in auto-tuning mode, find the set parameters in this mode and try to optimize from these known set of parameters. A general guide for optimizing the feedback parameters is to start with the feedback parameters used in the cooldown test or set by the controller in the auto-tuning mode. Increase the proportional gain (P) until the temperature starts to oscillate, and then reduce the gain until the oscillation stops. Then increase the integral gain (I) until the system starts to oscillate, and reduce the gain until the oscillation stops. Repeat the same with the proportional gain parameter. Since the parameters are all interdependent, it is not sufficient to optimize each one independently. In the process, if the temperature stability becomes adequate, or better yet, if excursions from the set point become undetectable, there is no need to continue the optimization.