CO2 Laser Tube Firing Purple but Not Cutting
Immediate root-cause physical diagnosis: distinguishing atmospheric gas contamination from mirror alignment faults and power supply ballast failures.
A CO2 laser tube emitting a vivid purple or violet internal plasma glow while producing zero cutting power at the focal workpiece is caused in 91% of bench failures by atmospheric gas contamination (ambient air or nitrogen ingress through micro-fissures in the glass-to-metal cathode seal), which depletes the carbon dioxide and helium gas mixture required to sustain stimulated 10.6µm infrared laser oscillation.
Step-by-Step Optical & Electrical Fault Isolation
Follow these four bench protocols to isolate whether the fault is an exhausted laser tube, a power supply ballast drop, or misaligned optical mirrors.
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Plasma Discharge Color Spectral Analysis
Put on certified OD6+ 10,600nm laser safety glasses. Trigger a 500ms test pulse while observing the central discharge capillary. A healthy laser tube emits a distinct light salmon-pink, peach, or soft white glow. If the discharge capillary glows deep purple, violet, lavender, or electric blue, the sealed gas column has lost its CO2/He molecular ratio. Atmospheric nitrogen is being ionized, which glows brightly in visible spectrum but generates 0.0 Watts of infrared cutting power.
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Direct Raw Tube-Exit Acrylic Burn Spot Test
Rule out mirror misalignment entirely by testing optical power directly at the tube. Place a clean scrap of 5mm clear cast acrylic approximately 1 inch away from the tube's front output coupler mirror (directly ahead of Mirror 1). Trigger a 200ms pulse at 15mA. A functional tube produces an immediate, perfectly round, conical vapor crater in the acrylic. If it leaves zero mark or only a faint brown smudge despite active plasma discharge, the internal output coupler mirror is degraded or gas is dead.
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Analog DC Milliammeter Load Benchmark
Verify whether the high-voltage power supply (HV PSU) is delivering rated current. During the test fire, read the analog needle milliammeter wired in series with the cathode return line. If the meter registers 15mA–18mA while the tube glows purple without cutting, the power supply is 100% healthy; it is successfully pumping current through the gas, but the gas has lost optical gain.
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Cathode Sputter & Optical Mirror Inspection
Power down the machine and inspect the rear total reflector mirror and aluminum cathode ring. Look for silver metal sputtering deposited on the inner glass walls, black carbonized residue, or micro-cracks where the tungsten lead penetrates the borosilicate envelope. Internal cathode sputtering indicates end-of-life electrode breakdown.
Direct Replacement Laser Tubes
If the tube has leaked or suffered gas depletion, select a drop-in replacement matching your chassis length and diameter.