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Sodium discharge from low pressure sodium lamps

Strictly speaking, the low-pressure sodium lamp is the only discharge lamp that can be called an ion pump, which occurs in every voltage half cycle. The so-called ion pump characteristic refers to the tendency of a gas (sodium vapor) that is easily ionized to migrate to the tube wall when the current increases, thereby comprehensively improving the electrical characteristics and radiation performance of the lamp. To explain this effect in detail, it is necessary to first understand the situation of the burning lamp during the stable period and the entire half cycle.

The current in the lamp stops flowing once every half cycle, but a small amount of sodium ions that disappear quickly remain in the discharge path; it is precisely because of the existence of these residual sodium ions that the current is restored when the appropriate voltage is established in the next half cycle. The current change in the lamp is close to a sine curve. Although the arc voltage decreases to a lower value within 2 to 3 milliseconds, it rises to a peak at 5 milliseconds; there is almost no light output before 4 milliseconds, but it reaches the highest light output at 6 milliseconds. The light efficiency curve is also very interesting, especially the small peak that appears in the center.

The current can be seen to increase first in the center of the discharge tube through the stroboscopic observer. As the current begins to increase, the lamp voltage drops, which is a typical unrestrained discharge. In the early stages of the cycle, the light output is low, and the discharge can be seen to fill the entire discharge tube. The free electrons generated are more active than positive ions of the same charge, and when the electrons reach the tube wall, they establish a negative wall charge. At this time, a radial electric field is generated, causing the positive ions to migrate to the tube wall, and the ions and electrons recombine at or near the tube wall, thereby increasing heat loss and increasing the density of sodium vapor near the tube wall. This process is the same in most low-pressure discharges, but it is not noticed because the gas in the arc tube diffuses rapidly to maintain the balance of gas pressure. In low-pressure sodium lamps, the neon pressure may be 5000 times the sodium vapor pressure. In this case, the diffusion rate of sodium is actually slower than the migration rate of sodium ions in the radial electric field.

The above process increases the sodium vapor density near the tube wall 3 milliseconds after the start of each half cycle of the Dewar low-pressure sodium lamp; although the increase is not large, the effect is very important because the excitation and ionization of sodium atoms near the tube wall are more frequent than other parts of the tube. As a result of this increasing effect, the entire light output of the lamp is actually radiated from the area within 1 mm of the tube wall. This phenomenon is most obvious when observing the curved part of the inner wall of the crescent-shaped cross-section tube with a stroboscopic observer.

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