Aligning the Low Working Distance Window Option

LOW WORKING DISTANCE MODULE CARE

The low working distance module has multiple thin windows.  Care should be taken while cleaning and using these windows as they could break or crack easily. The windows can be cleaned just as standard optics with a lens tissue and isopropyl alcohol, but the window should be gently brushed without pressure when cleaning.

The low working distance module has multiple windows which must be aligned in close proximity. Care should be taken while adjusting the height of these windows to avoid contacting the windows with objects in the cryostat.  Contact could result in added heat loads, dislodging of the windows resulting in degraded cooling of the windows, a system vacuum leak, or damaging/breaking of the windows.

Overview

The Low Working Distance Module is an optional tool for the Cryostation which allows samples to be positioned for low temperature experiments at a distance of roughly 1mm or more from regular room temperature optical components.  The main components (shown below, top to bottom) are the outer vacuum window and holder, the brass radiation window and aperture, and the adapter ring.

Figure 1: Exploded View of Components (left), Cutaway Assembled View of Components and Vacuum Lid (right)

The outer vacuum window is intended to seal the sample space and allow a high vacuum to be pulled within the system.  The outer window is comprised of a stainless steel support which holds a smaller thin window.  The support creates a large thin area which allows room temperature optics to be positioned directly above the window itself.  The radiation aperture is used in a similar way to support a thin radiation window, while allowing samples much larger than the window to be positioned directly beneath the radiation window.  The radiation window is held only by thermal grease, minimizing the thickness of the overall window assembly. 

For certain applications, the radiation window can be removed, leaving a 6mm aperture.  An additional heat load and higher sample temperatures will result due to the added radiation from room temperature on the sample.  The exact heat load and resulting temperatures will vary depending on the sample and system setup.  When possible, it is recommended to use the radiation window to block radiation from room temperature to the sample.

INSTALLING THE LOW WORKING DISTANCE MODULE

The low working distance module is intended to be installed in the CRYOSTATION™ as a tool to allow working distances of less than 1mm.   Before installing the components, ensure that the windows are clean, and free of any debris or residue.  The vacuum window holder replaces an outer window on the system.  When replacing this window, the o-ring should be inspected for debris and vacuum grease applied if needed. 

The radiation aperture is bonded to the adaptor ring using VGE varnish, supplied with the system.  The radiation window is attached to the radiation aperture with thermal grease.  Care should be taken to maintain only a thin layer of grease between the two components. The N-grease should be applied thinly to the brass washer, not the 9mm radiation shield window. The 9mm radiation shield window can then carefully be placed onto the brass washer. The adaptor ring is then installed into the 30mm window ring using thermal grease.  The 30mm window ring can then replace the existing radiation window and window ring.  Ensure a small amount of thermal grease is applied to the threads before installing the 30mm window ring.

OPERATION OF THE LOW WORKING DISTANCE MODULE

The operation and use of the low working distance module is comprised of adjustment and alignment of the module components to obtain the required working distance.  It is very important to take care while aligning these components to ensure that they do not touch during operation. The two windows colliding could cause a leak, or compromise the integrity of the windows. As the module is designed to allow low working distances, it is important to consider the motion of the internal components within the Cryostation. This motion is due to the compression of o-rings within the system as well as the thermal contraction of internal metal components. This compression occurs in two stages, first the o-rings compress when the system is initially evacuated to between 1-2 Torr, this causes the largest shift of the vacuum lid. As the system cools and the vacuum pressure falls to less than 1mTorr, this additionally compresses the vacuum lid, but there is much less compression than the initial system evacuation.  The radiation shield contracts as the system cools from room temperature to its base temperature.  The contraction of the radiation shield results in a small downward motion of the radiation shield. The sample mount will also slightly contract. If the distances can be determined at room temperature, then the user can image the sample at low temperatures.

Alignment

The system motion can be compensated for in the initial system alignment.

  1. Always start with the radiation window and vacuum window further apart than the expected contraction distance. This is to avoid a collision of the optics. The distance can be adjusted by turning the radiation window ring into the radiation shield.
  2. Next, evacuate the system to between 1-2 Torr and watch to see how the distance between the windows changes. Release vacuum immediately if they get too close.
  3. Repeat the above steps until the required spacing is achieved.
  4. Next, the spacing between the sample and the radiation window can be monitored and the amount of required adjustment noted.
  5. Image the sample at room temperature and ensure that the sample can be viewed. If it can be imaged, then ensure nothing is touching and cool down. If it cannot be imaged, repeat steps 1-4.

Measuring the spacing between these components is often difficult; however, one simple technique for observing the spacing is to place a small mark on each window with a marker.  Typical locations for marking the windows are to place a mark on the inside of the vacuum window and on each side of the radiation window.  By simple visual comparison of the spacing between the dots one can quickly arrive at the correct low working distance.