What Is The Pre-ionisation Zone Of A Laser Tube?

The pre-ionisation zone of a CO2 DC glass laser tube is commonly referred to in the tube manufacturers literature and specifications but never explained. Asking the tube manufacturers for information usually results in the comment that it offers good engraving. So I decided to do some research and try and answer the question once and for all. 

So, what is the pre-ionisation zone of a laser tube? The pre-ionisation Zone of a glass DC CO2 laser tube is an operating region that occurs between 4~6mA current or roughly 9~16% laser power. It is characterised by an unstable pale violet plasma beam with a hissy sound. This zone is ideal for precise cutting of thin materials with minimal burning or charring.

Many laser users may not have even come across the term “pre-ionisation zone” before, but it can be an important tool when used in the correct circumstances. Tube manufacturers also call it “High Frequency Impact Engraving”.

Image showing the pre-ionisation Zone of a laser Beam
Incomplete beam at cathode present within the pre-ionisation zone

When Does the Pre-ionisation Zone Start?

The pre-ionisation zone starts when the current in the glass CO2 laser tube reaches around 4mA, which generally equates to a laser power setting of around 9%. 

The zone continues until a current of around 6mA / 16% laser power is reached and the laser beam stabilises. Please note: These values can vary across laser tube powers, manufacturers and even tubes of the same type. 

When the laser tube is in the pre-ionisation zone, you will hear a “hissy” sound coming from the laser tube. If you take a look at the laser tube when it is operating in this zone, the usual steady purple full-length plasma you would see at higher powers is replaced by a pale,  incomplete and jittery plasma.  You will see the plasma is concentrated at the anode (low voltage) end of the tube and jumping around. You will also notice that the plasma does not extend the full length of the tube. As you increase the power closer to the 16% upper threshold, the plasma will become longer and longer until it extends the full distance to the Cathode (high voltage) end.  

Please take suitable precautions if you intend to defeat the safety interlocks and view the laser tube in operation. Always wear appropriate safety glasses when operating your laser in Class IV mode.

What Actually Happens?

The laser power supply provides a high voltage at the Anode (14~30KV) which causes electrons to move through the tube. At low current levels (<4mA) there is insufficient energy to generate a plasma and there is no laser output. As the current increases, the electrons start travelling end to end in the tube but there is still insufficient energy to keep the whole reaction going. So you end up with lots of tiny spikes of lazing action comprising of very high current but lasting just nanoseconds. 

How To Find The Pre-Ionisation Zone Of Your Laser Tube.

Safety First: Wear safety glasses whenever you operate your laser machine is Class IV mode (i.e. when looking at the laser tube in operation. Set up a job with a duration of about 10~15 seconds, and run it at various .

Start at the lowest percentage power that your machine operates at. This is typically between 8 and 11%. While the job is running you should hear a hissy noise from your laser tube. Taking a look at the laser tube itself, you should see a very pale, flickering violet glow that is just at the low voltage (cathode) end of the tube. This is the start of your Pre-ionisation zone.

Increasing the power in 1% steps, will result in the glow travelling further along the length of the tube and the flickering stabilising. The hissing sound will continue throughout the pre-ionisation zone.

The end of the zone is signified by the cessation of the hissing sound and the generation of a strong stable violet glow along the full length of the laser tube. This will typically occur at between 14~16% power.

What Are The Benefits Of The Pre-Ionisation Zone?

This mode is ideal for cutting thin organic materials such as paper and card or thin laser safe plastics as it has enough punch to cut through these materials without the associated heat damage that you would get at higher laser powers. Resulting in clean burn / scorch / melt free cutting.

There is a reason why tube manufacturers refer to this zone as “High Impact Frequency Engraving”. It offers some of the best engraving performance for “binary*” materials such as glass, slate and other mineral materials including ceramics. It’s also a great zone to use for photo engraving.

*A binary material is one that only displays a single colour when hit by the laser beam. Passing over the same spot will not change the colour, only the depth of the mark.

Will The Pre-Ionisation Zone Damage My tube?

No, utilising the pre-ionisation zone of your laser tube will not cause any damage to your tube. Even though there are spikes of high current occurring in this zone, the duration over which they are active is incredibly short and will not put any stress on your tube.

Conclusions Regarding the Pre-ionisation Zone Of A Laser Tube

The pre-ionisation zone is a great tool to use when used for cutting thin delicate materials. These typically exhibit burn marks when cut at higher power / higher speeds. Using the pre-ionisation zone can make a big difference in product presentation. It’s easy to use and then record the settings in your library for future use.

Setting aside an hour to do some pre-ionisation zone engraving trials on a variety of materials is time well spent and will improve the consistency of your engraving projects.

Interested to know what laser power your tube provides within this zone? It’s very quick and easy to check using a laser power meter.

What Next?

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