News

What is high-voltage gas discharge? Learn about the evolution from low-voltage discharge to arc discharge in one article.

The fluorescent lamps, mercury lamps, and Xenon Lamps we often talk about all rely on the same principle: gas discharge. "High-pressure discharge" is the type of discharge that occurs when the gas pressure is high. So, what's the difference between it and "low-pressure discharge"?

1. What happens when the gas pressure increases?

When we start with a low-pressure discharge and gradually increase the gas pressure, two distinct changes occur:

1. The gas temperature rises, while the electron temperature drops. Collisions between gas molecules and electrons increase, transferring the electrons' energy to the gas. As a result, the gas gets hotter and the electrons get "cooler." Eventually, their temperatures converge, around 4000-6000 K.

2. The discharge area shrinks. As the temperature rises, the discharge becomes less uniform and more concentrated in a high-temperature core area, forming a temperature gradient around the tube wall. This phenomenon is called shrinkage discharge.

2. How is an arc formed?

When the gas pressure is even higher, the gas and electron temperatures become almost the same, and the entire discharge is in a state called "local thermal equilibrium." At this point, the role of the tube walls becomes irrelevant—the discharge can even proceed freely between the two electrodes, a phenomenon we're familiar with as arc discharge.

However, the arc has a problem: it's unstable. It may bend, tremble, and jump, like a restless flame. Therefore, stabilizing it requires several techniques:

Using the tube walls to limit the discharge path → tube walls stabilize the arc; reducing the distance between the electrodes → electrodes stabilize the arc; using magnetic fields to confine the arc → magnetic fields stabilize the arc.

Generally speaking, long-tube lamps (such as fluorescent lamps) are "tube-wall stabilized," while short-arc light sources (such as mercury and Xenon Lamps) are "electrode stabilized."

3. Why Add a Choke?

High-voltage discharges also share a common characteristic:

Their volt-ampere characteristic is negative—the greater the current, the lower the voltage. This "abnormal" characteristic can easily lead to runaway current, so a choke or high-impedance transformer is needed in series with the circuit to limit the current and ensure stable lamp operation.

Simply put:

High-voltage discharge = high temperature + contraction + arcing + negative voltage drop. This is the core principle of high-voltage gas discharge.

PREVIOUS：Low pressure discharge, electric light source NEXT：Diffusion processes in metals: How do gases "