Flashtube


A flashtube produces an electrostatic discharge with an extremely intense, incoherent, full-spectrum white light for a very short time. A flashtube is a glass tube with an electrode at each end and is filled with a gas that, when triggered, ionizes and conducts a high-voltage pulse to make light. Flashtubes are used most in photography; they also are used in science, medicine, industry, and entertainment.

Construction

The lamp consists of a hermetically sealed glass tube which is filled with a noble gas, usually xenon, and electrodes to carry electric current to the gas. Additionally, a high voltage power source is necessary to energize the gas as a trigger event. A charged photoflash capacitor is usually used to supply energy for the flash, so as to allow very speedy delivery of very high electrical current when the lamp is triggered.

Glass envelopes

The glass envelope is most commonly a thin tube, often made of fused quartz, borosilicate or Pyrex, which may be straight, or bent into a number of different shapes, including helical, "U" shape, and circular. In some applications, the emission of ultraviolet light is undesired, whether due to production of ozone, damage to laser rods, degradation of plastics, or other detrimental effects. In these cases, a doped fused silica is used. Doping with titanium dioxide can provide different cutoff wavelengths on the ultraviolet side, but the material suffers from solarization; it is often used in medical and sun-ray lamps and some non-laser lamps. A better alternative is a cerium-doped quartz; it does not suffer from solarization and has higher efficiency, as part of the absorbed ultraviolet is reradiated as visible via fluorescence. Its cutoff is at about 380 nm. Conversely, when ultraviolet is called for, a synthetic quartz is used as the envelope; it is the most expensive of the materials, but it is not susceptible to solarization and its cutoff is at 160 nm.
The power level of the lamps is rated in watts/area, total electrical input power divided by the lamp's inner wall surface. Cooling of the electrodes and the lamp envelope is of high importance at high power levels. Air cooling is sufficient for lower average power levels. High power lamps are cooled with a liquid, typically by flowing deionized water through a tube in which the lamp is encased. Water-cooled lamps will generally have the glass shrunk around the electrodes, to provide a direct thermal conductor between them and the cooling water. The cooling medium should flow also across the entire length of the lamp and electrodes. High average power or continuous-wave arc lamps must have the water flow across the ends of the lamp, and across the exposed ends of the electrodes as well, so the deionized water is used to prevent a short circuit. Above 15 W/cm2 forced air cooling is required; liquid cooling if in a confined space. Liquid cooling is generally necessary above 30 W/cm2.
Thinner walls can survive higher average-power loads due to lower mechanical strain across the thickness of the material, which is caused by a temperature gradient between the hot plasma and cooling water,. For this reason, thinner glass is often used for continuous-wave arc-lamps. Thicker materials can generally handle more impact energy from the shock wave that a short-pulsed arc can generate, so quartz as much as 1 mm thick is often used in the construction of flashtubes. The material of the envelope provides another limit for the output power; 1 mm thick fused quartz has a limit of 200 W/cm2, synthetic quartz of same thickness can run up to 240 W/cm2. Other glasses such as borosilicate generally have less than half the power loading capacity of quartz. Aging lamps require some derating, due to increased energy absorption in the glass due to solarization and sputtered deposits.

Electrodes and seals

The electrodes protrude into each end of the tube, and are sealed to the glass using a few different methods. "Ribbon seals" use thin strips of molybdenum foil bonded directly to the glass, which are very durable, but are limited in the amount of current that can pass through. "Solder seals" bond the glass to the electrode with a solder for a very strong mechanical seal, but are limited to low temperature operation. Most common in laser pumping applications is the "rod seal", where the rod of the electrode is wetted with another type of glass and then bonded directly to a quartz tube. This seal is very durable and capable of withstanding very high temperature and currents. The seal and the glass must have the same coefficient of thermal expansion.
For low electrode wear the electrodes are usually made of tungsten, which has the highest melting point of any metal, to handle the thermionic emission of electrons. Cathodes are often made from porous tungsten filled with a barium compound, which gives low work function; the structure of cathode has to be tailored for the application. Anodes are usually made from pure tungsten, or, when good machinability is required, lanthanum-alloyed tungsten, and are often machined to provide extra surface area to cope with power loading. DC arc lamps often have a cathode with a sharp tip, to help keep the arc away from the glass and to control temperature. Flashtubes usually have a cathode with a flattened radius, to reduce the incidence of hot spots and decrease sputter caused by peak currents, which may be in excess of 1000 amperes. Electrode design is also influenced by the average power. At high levels of average power, care has to be taken to achieve sufficient cooling of the electrodes. While anode temperature is of lower importance, overheating the cathode can greatly reduce the lamp's life expectancy.

Gases and fill pressure

Depending on the size, type, and application of the flashtube, gas fill pressures may range from a few kilopascals to hundreds of kilopascals. Generally, the higher the pressure, the greater the output efficiency. Xenon is used mostly because of its good efficiency, converting nearly 50% of electrical energy into light. Krypton, on the other hand, is only about 40% efficient, but at low currents is a better match to the absorption spectrum of Nd:YAG lasers. A major factor affecting efficiency is the amount of gas behind the electrodes, or the "dead volume". A higher dead volume leads to a lower pressure increase during operation.

Operation

The electrodes of the lamp are usually connected to a capacitor, which is charged to a relatively high voltage, using a step up transformer and a rectifier. The gas, however, exhibits extremely high resistance, and the lamp will not conduct electricity until the gas is ionized. Once ionized, or "triggered", a spark will form between the electrodes, allowing the capacitor to discharge. The sudden surge of electric current quickly heats the gas to a plasma state, where electrical resistance becomes very low. There are several methods of triggering.

External triggering

External triggering is the most common method of operation, especially for photographic use. The electrodes are charged to a voltage high enough to respond to triggering, but below the lamp's self-flash threshold. An extremely high voltage pulse,, the "trigger pulse", is applied either directly to or very near the glass envelope. The short, high voltage pulse creates a rising electrostatic field, which ionizes the gas inside the tube. The capacitance of the glass couples the trigger pulse into the envelope, where it exceeds the breakdown voltage of the gas surrounding one or both of the electrodes, forming spark streamers. The streamers propagate via capacitance along the glass at a speed of 1 centimeter in 60 nanoseconds. The triggering can be enhanced by applying the trigger pulse to a "reference plane", which may be in the form of a metal band or reflector affixed to the glass, a conductive paint, or a thin wire wrapped around the length of the lamp. If the capacitor voltage is greater than the voltage drop between the cathode and the anode, when the internal spark streamers bridge the electrodes the capacitor will discharge through the ionized gas, heating the xenon to a high enough temperature for the emission light.

Series triggering

Series triggering is more common in high powered, water-cooled flashtubes, such as those found in lasers. The high-voltage leads of the trigger-transformer are connected to the flashtube in series,, so that the flash travels through both the transformer and the lamp. The trigger pulse forms a spark inside the lamp, without exposing the trigger voltage to the outside of the lamp. The advantages are better insulation, more reliable triggering, and an arc that tends to develop well away from the glass, but at a much higher cost. The series-triggering transformer also acts as an inductor. This helps to control the flash duration, but prevents the circuit from being used in very fast discharge applications. The triggering can generally take place with a lower voltage at the capacitor than is required for external triggering. However, the trigger-transformer becomes part of the flash circuit, and couples the triggering-circuit to the flash energy. Therefore, because the trigger-transformer has very low impedance, the transformer, triggering-circuit, and silicon controlled rectifier must be able to handle very high peak-currents, often in excess of 1500 amps.

Simmer-voltage triggering

Simmer-voltage triggering is the least common method. In this technique, the capacitor voltage is not initially applied to the electrodes, but instead, a high voltage spark streamer is maintained between the electrodes. The high current from the capacitor is delivered to the electrodes using a thyristor or a spark gap. This type of triggering is used mainly in very fast rise time systems, typically those that discharge in the microsecond regime, such as used in high-speed, stop-motion photography or dye lasers. The simmering spark-streamer causes the arc to develop in the exact center of the lamp, increasing the lifetime dramatically. If external triggering is used for extremely short pulses, the spark streamers may still be in contact with the glass when the full current-load passes through the tube, causing wall ablation, or in extreme cases, cracking or even explosion of the lamp. However, because very short pulses often call for very high voltage and low capacitance, to keep the current density from rising too high, some microsecond flashtubes are triggered by simply "over-volting", that is, by applying a voltage to the electrodes which is much higher than the lamp's self-flash threshold, using a spark gap. Often, a combination of simmer voltage and over-volting is used.