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Old 09-25-2010, 10:12 PM #1
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Default Helium-cadmium laser won't fire

The laser responds to the initial high voltage pulse, but will not lase afterwards. Power supply, or tube?



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Old 09-25-2010, 10:58 PM #2
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Default Re: Helium-cadmium laser won't fire

My first impression is that the tube has leaked air into the tube.

The plasma of a HeCd should be "golden/white" at start-up


A red plasma indicates air in the tube.

Once again from SAM's

HeCd Laser Bore Discharge Color

As with other gas lasers, the color of the discharge, particularly in the narrow bore, can provide valuable diagnostic info. For rectangular laser heads, simply removing one of the cover screws may provide a small hole to view the bore. This is preferable to removing the entire cover, which will affect the thermal environment. For cylindrical laser heads, it may be necessary to carefully drill a hole through the aluminum if there are no mounting screw or ventilation grill holes in a suitable location. Put a collar on your drill bit to prevent drilling through the glass tube inside (!!) and minimize vibrations when drilling! For example, the bore is unobstructed by insulation or any other covering about 11 inches from the output-end of the Omni laser heads I've seen but this should be confirmed before drilling the hole if possible. A small hole drilled in that area provides a very nice display of the discharge.
  • Normal startup: Pure helium yellow-ish-white. If observing with a spectrascope or diffraction grating, there is a very intense yellow line, a less intense red line, and a spread of blue and green lines.
  • Normal operation: Fairly neutral white. Blue, green, and red cadmium lines will be present. The cadmium red line in particular will become quite intense and distinct from the He red line.
  • Exceesive cadmium vapor pressure (gulping): Blue-white. The blue cadmium line becomes increasing intense.
  • Leakage: Red, blue, or white depending on pressure, spectrum of nitrogen and oxygen from air.
Sam's Laser FAQ - Helium-Cadmium Lasers


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Old 09-26-2010, 12:53 AM #3
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Default Re: Helium-cadmium laser won't fire

Thank you for the reply! BTW, where do you get drivers for the 445nm diodes you have?
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Old 09-26-2010, 12:58 AM #4
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Default Re: Helium-cadmium laser won't fire

Quote:
Originally Posted by RichA View Post
Thank you for the reply! BTW, where do you get drivers for the 445nm diodes you have?
http://hacylon.case.edu/ebay/laser_d...BoostDrive.php

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Old 09-27-2010, 07:57 PM #5
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Default Re: Helium-cadmium laser won't fire

Sadly that is the purple color of nitrogen. Nitrogen pushes the tube voltage through the roof, and that is why the whole tube is not firing. PSU is probably good, provided it has not sat on doing that for long periods of time.

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