Flash Tube
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https://en.m.wikipedia.org/wiki/Flashtube
Ориентировочное время чтения: 8 мин
The electrodes of the lamp are usually connected to a capacitor, which is charged to a relatively high voltage (generally between 250 and 5000 volts), 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 plasmastate, where elec…
The electrodes of the lamp are usually connected to a capacitor, which is charged to a relatively high voltage (generally between 250 and 5000 volts), 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, (usually between 2000 and 150,000 volts), the "trigger pulse", is applied either directly to or very near the glass envelope. (Water-cooled flashtubes sometimes apply this pulse directly to the cooling water, and often to the housing of the unit as well, so care must be taken with this type of system.) 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 (170 km/s). (A trigger pulse must have a long enough duration to allow one streamer to reach the opposite electrode, or erratic triggering will result.) 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, (one lead to an electrode and the other to the capacitor), 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 (SCR) 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.
Prepulse techniques
Very rapid rise-times are often achieved using a prepulse technique. This method is performed by delivering a small flash through the lamp just before the main flash. This flash is of much lower energy than the main flash (typically less than 10%) and, depending on the pulse duration, is delivered just a few thousandths to a few millionths of a second before the main flash. The prepulse heats the gas, producing a dim, short-lived afterglow that results from free electrons and ionized particles that remain after the pulse shuts down. If the main flash is initiated before these particles can recombine, this provides a good quantity of ionized particles to be used by the main flash. This greatly decreases the rise time. It also reduces the shock wave and makes less noise during operation, vastly increasing the lifetime of the lamp. It is especially effective on very fast-discharge applications, allowing the arc to expand faster and better fill the tube. It is very often used with simmer voltage and sometimes with series triggering, but rarely used with external triggering. Prepulse techniques are most commonly used in the pumping of dye lasers, greatly increasing the conversion efficiency. However, it has also been shown to increase the efficiency of other lasers with longer fluorescence lifetimes (allowing longer pulses), such as Nd:YAG or titanium sapphire, by creating pulses with almost square waveforms.
Ablative flashtubes
Ablative flashtubes are triggered by under-pressurizing. Ablative flashtubes are typically constructed using quartz tubing and one or both electrodes hollowed out, allowing a vacuum pump to be attached to control the gas pressure. The electrodes of the lamp are connected to a charged capacitor, and then the gas is vacuumed from the lamp. When the gas reaches a low enough pressure (often just a few torr) randomly-ionized particles are able to accelerate to velocities sufficient to begin ejecting electrons from the cathode as they impact its surface, resulting in a Townsend avalanche that causes the lamp to self-flash. At such low pressures, the efficiency of the flash would normally be very low. However, because of the low pressure, the particles have room to accelerate to very high speeds, and the magnetic forces expand the arc so that the bulk of its plasma becomes concentrated at the surface, bombarding the glass. The bombardment ablates (vaporizes) large amounts of quartz from the inner wall. This ablation creates a sudden, violent, localized increase in the internal pressure of the lamp, increasing the efficiency of the flash to very high levels. The ablation, however, causes extensive wear to the lamp, weakening the glass, and they typically need replacement after a very short lifetime.
Ablative flashtubes need to be refilled and vacuumed to the proper pressure for each flash. Therefore, they cannot be used for very high-repetition applications. Also, this usually precludes the use of very expensive gases like krypton or xenon. The most common gas used in an ablative flashtube is air, although sometimes cheap argon is also used. The flash usually must be very short to prevent too much heat from transferring to the glass, but the flashes can often be shorter than a normal lamp of comparative size. The flash from a single ablative flashtube can also be more intense than multiple lamps. For these reasons, the most common use for the lamps is for the pumping of dye lasers.
Variable pulse width control
In addition, an insulated-gate bipolar transistor (IGBT) can be connected in series with both the trigger transformer and the lamp, making adjustable flash durations possible. An IGBT used for this purpose must be rated for a high pulsed-current, so as to avoid over-current damage to the semiconductor junction. This type of system is used frequently in high average-power laser systems, and can produce pulses ranging from 500 microseconds to over 20 milliseconds. It can be used with any of the triggering techniques, like external and series, and can produce square wave pulses. It can even be used with simmer voltage to produce a "modulated" continuous wave output, with repetition rates over 300 hertz. With the proper large bore, water-cooled flashtube, several kilowatts of average-power output can be obtained.
Electrical requirements
The electrical requirements for a flashtube can vary, depending on the desired results. The usual method is to first determine the pulse duration, the maximum amount of energy tolerable at that duration (explosion energy), and the safe amount of operating energy. Then pick a current density that will emit the desired spectrum, and let the lamp's resistance determine the necessary combination of voltage and capacitance to produce it. The resistance in flashtubes varies greatly, depending on pressure, shape, dead volume, current density, time, and flash duration, and therefore, is usually referred to as impedance. The most common symbol used for lamp impedance is Ko, which is expressed as ohms per the square root of amps (ohms(amps ).
Ko is used to calculate the amount of input voltage and capacitance needed to emit a desired spectrum, by controlling the current density. Ko is determined by the internal diameter, arc length, and gas type of the lamp and, to a lesser extent, by fill pressure. The resistance in flashtubes is not constant, but quickly drops as current density increases. In 1965, John H. Goncz showed that the plasma resistivity in flashtubes is inversely proportional to the square root of current density. As the arc develops, the lamp experiences a period of negative resistance, causing both the resistance and voltage to decrease as the current increases. This occurs until the plasma comes into contact with the inner wall. When this happens, the voltage becomes proportional to the square root of current, and the resistance in the plasma becomes stable for the remainder of the flash. It is this value which is defined as Ko. However, as the arc develops the gas expands, and calculations for Ko do not take into account the dead volume, which leads to a lower pressure increase. Therefore, any calculation of Ko is merely an approximation of lamp impedance.
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Trigger circuit for large xenon flash tubes
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https://en.m.wikipedia.org/wiki/Flashtube
Ориентировочное время чтения: 8 мин
The electrodes of the lamp are usually connected to a capacitor, which is charged to a relatively high voltage (generally between 250 and 5000 volts), 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 plasmastate, where elec…
The electrodes of the lamp are usually connected to a capacitor, which is charged to a relatively high voltage (generally between 250 and 5000 volts), 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, (usually between 2000 and 150,000 volts), the "trigger pulse", is applied either directly to or very near the glass envelope. (Water-cooled flashtubes sometimes apply this pulse directly to the cooling water, and often to the housing of the unit as well, so care must be taken with this type of system.) 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 (170 km/s). (A trigger pulse must have a long enough duration to allow one streamer to reach the opposite electrode, or erratic triggering will result.) 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, (one lead to an electrode and the other to the capacitor), 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 (SCR) 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.
Prepulse techniques
Very rapid rise-times are often achieved using a prepulse technique. This method is performed by delivering a small flash through the lamp just before the main flash. This flash is of much lower energy than the main flash (typically less than 10%) and, depending on the pulse duration, is delivered just a few thousandths to a few millionths of a second before the main flash. The prepulse heats the gas, producing a dim, short-lived afterglow that results from free electrons and ionized particles that remain after the pulse shuts down. If the main flash is initiated before these particles can recombine, this provides a good quantity of ionized partic
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Flash Tube
