This lab ties together the principles covered in the first two labs of this course, specifically vacuum purity and residual gas analysis, in a critical application: processing a helium-neon laser tube.
The laser tube will have a local isolation valve (called the tube valve) in series with it as per the main diagram on the turbomolecular pumping system description (found on the main page for this course). This is to ensure the tube may be stored at high vacuum to keep it free from contaminants. Connect the tube valve to the isolation valve for the system if not already done. Open the isolation valve and evacuate the manifold and the connecting vacuum line between the laser tube and the system. Now open the laser tube valve to evacuate it as well. Leave both valves open for the duration of this experiment.
The experiment begins with air being admitted to the tube. This contaminates the tube and electrode surfaces with air gases (your task is to remove these contaminants allowing the tube to operate).
Purity is critical to allowing the relatively low-gain HeNe to operate so a cycle of cleaning the tube is required before the final gas fill. Begin by powering the tube and observing the discharge (if any). SLOWLY open the evacuate valve to lower pressure inside the tube. If a glow discharge begins, continue pumping until it becomes a thin violet glow followed by a dim red glow and finally extinguishes (you can guess the pressure at this point knowing the minimum pressure for a discharge in air from an earlier lab). Pump for a few more minutes then close the evacuate valve on the system to isolate the manifold and tube from the pump.
Fill the laser tube with research-grade helium to a pressure of approximately 2 torr. Start the power supply and observe the tube glowing - the colour may well change as it operates. Allow the tube to run for three minutes. Now, start the RGA and take a sample of the gas in the manifold (keeping the RGA head pressure between 1 and 5 times 10-5 torr as per lab #2 procedure) - call it "first flush". The gas mixture may well show a quantity of undesired impurities as well as the helium injected into the system - many impurities will have evolved from the laser tube itself as it heats. Stop the RGA and turn OFF the filament with the proper icon.
Evacuate the tube to ultimate vacuum again. The flushing procedure must be repeated a minimum of three times to reduce the quantity of impurities in the system. It is not necessary to take an RGA scan each time (only for the first flush).
Now fill the laser tube with the required gas mix determined from your pre-lab research (an estimate of total pressure based on empirical data such as tube dimensions will suffice for this part of the experiment). Close the manifold valve and overfill with He and Ne gases in a suitable proportion (see the Vacuum Notes on the course home page regarding this) - fill with neon first, adjust this pressure if necessary to about 1 torr, then add helium to get the proper ratio. Start the power supply and reduce the pressure gradually until the tube begins to operate (around 4 torr) - use the evacuate valve to reduce the pressure. If the laser fails to operate at all, the current gas mix can be flushed and another attempt made to fill the tube. Assuming the laser begins to operate at all, stop reducing pressure (the tube is presumably at the maximum operating pressure now).
Now that the tube is operating, note the power output (in mW). Reduce the pressure in approximately 0.2 torr increments using the evacuate valve and note the power at each pressure until lasing stops - this allows you to determine the optimal gas pressure for this tube as well as the pressure range over which the laser will operate. The laser has now ceased operating (the pressure is too low) but there is still gas inside the manifold. Run a quick RGA scan to determine the ratio of helium:neon in the tube at this point (Your observations now include the maximum tube operating pressure, minimum pressure, and the He:Ne ratio for which these observations are valid). The RGA analysis on this (working) gas mix allows verification of the tube contents and assessment of impurity levels - call this scan "working mix". In the lab report you will compare it to the original "first flush" - You expect, of course, to find helium and neon gases in the mix but check specifically for the impurities identified in the prelab. Use this data to verify the He:Ne ratio as well.
If necessary, refill the tube and repeat to obtain a table of pressure-vs-output power values.
Repump and fill the tube with the same mixture at a pressure of 1 to 2 torr greater than optimal gas pressure (determined above by observation). Take an RGA scan and save it as "prerun" then operate the tube for 10 minutes continuously. During this time, observe the optical power output on the meter at intervals of 30 seconds and graph it over time. Run a final RGA scan ("postrun") to see what types of gases evolved which caused the power output to drop and if the ratio changed.
Tech Notes: Use the Melles-Griot power supply as it features a higher output voltage (which is also listed on the sticker with the current on the underside so the voltage drop on the ballast resistor can be calculated and hence the tube voltage).
When evaluating a gas mixture, it is often required that a gas component be expressed as a ratio (e.g. He:Ne). In the case of a helium-neon gas mix, the procedure to produce a ratio is to divide the total helium pressure by the total neon pressure (addition of both stable isotopes) to obtain the ratio for helium. If, for example, the RGA showed 2E-5 torr of helium and 2E-6 torr of neon the ratio would be 10:1 for Helium:Neon (which is the most common way to express the gas mixture employed). Note that the total pressure at the RGA is NOT relevant!
Now, if one wanted to know the actual partial pressure of neon in the tube, one can say that one part in ELEVEN is actually neon so if the tube pressure was 2.5 torr (as read from the manifold gauge), there is actually 2.5/11 or 0.227 torr of neon in the tube. This partial pressure is required for calculation of the E/P ratio as is the anode-to-cathode distance and the tube voltage.On E/P Ratios
E/P ratios will be covered extensively in PHTN1400 (Laser Systems) under the topics of the CO2 laser as well as pulsed gas lasers. In a nutshell, the E/P ratio is a characteristic of each gas and relates the electric field (E, in Volts/cm) to pressure (P, in torr) and so has units of "Volts per cm-torr". For each gas, the E/P ratio can be researched or observed (as it is in this lab). If the E/P value for a gas such as neon is known, one can predict the optimal (partial) pressure for that gas in a discharge tube by knowing the voltage across the tube and the length of the discharge path (between anode and cathode).
To determine the E/P ratio for neon, find the partial pressure of neon (by multiplying the total pressure by the ratio found on the RGA): it should at least be reasonably close to that determined in the prelab. Next divide the TUBE voltage (power supply voltage minus ballast resistor voltage drop) by the product of that neon pressure (in torr) times the distance between the anode and cathode (in cm). CHECK THE UNITS. Expect a value between 100 V/(cm-torr) and 300 V/(cm-torr).
For the curious, the concept of the E/P ratio (and the actual number for neon) are covered in this article by Calvert on electrical discharges. It will be covered in the lasers course later in the term.
For this experiment, an abbreviated lab report is required (word processed, never hand-written) as follows. As usual, submit the lab report in a bound folder NOT simply a pile of loose, stapled papers! nor in a large binder. A title page is required and each major question must begin on a new page (so your lab report must be at least ten pages in length, but hopefully longer).
For ALL information provide a footnote as to WHERE you obtained it (e.g. the page on SAM's site, page in Csele, from Calvert's article, etc). No "Magic Numbers" - all values used for comparison must be referenced as to their source!