29 1.12 Four images of the evolution of the supernova SN1987a during. the first ten years since the explosion. The delay between the explosion and the diagnostic laser is 52.8 1lS. The circle is not perfect, probably due to small errors in the position of the wire.
List of Tables
It can be seen that the actual size of the focus is quite large in the horizontal direction. The remainder of the thesis describes two specific gas structures that we have studied in our laboratory.
Gas and plasma structures
Natural and artificial gas structures on our planet
- Mirages
In this case, the energy of the shock is spread as a typical blue light, the so-called Cherenkov radiation, which can be seen in the pools of nuclear reactors. This is due to the fact that mirages involve the presence of many inverted images of the same object.
Structures in the Solar System
If we consider the solar system as a whole, it is contained within a large cocoon created by the particles of the solar wind. This is the energy source of most of the solar system: the fusion of four hydrogen atoms into one helium atom.
Space structures
The energy, SOHO found, is transferred by a magnetic field, but most of the details are still unknown. To prevent this damage, the new science of space meteorology is being developed around the world.
Shocks
One of the most intriguing mysteries is the temperature of the solar corona, a few million degrees above a surface of only a few thousand. So most of the elements that sustain life on Earth were created in a star and ejected during one of the explosions described above.
Gas clouds
The center of the nebula is hidden by another cloud of dust and is only visible in radio waves (www.stsci.edu). But, as described earlier, many times these clouds could have wiped out most of the creatures living on Earth.
Explosions
- Novas
- Supernovas
- Hypernovas
- Planetary nebulae
Some images of the evolution of the supernova SN1987a are shown in Figure 1.12 on the next page. Most of the carbon (the basis of life) and particulate matter (crucial building blocks of solar systems like ours) in the universe is produced and distributed by red giant stars.
Galaxies
If galaxies were surrounded by a large halo of matter containing up to 90% of their mass, this would explain their rotation.
Gas and plasma structures in laborato- rles
- Magnetically confined plasmas
To avoid energy losses due to both open field lines and contamination, the plasma can be arranged in a toroidal shape. In this way, particles with mass m and charge q are free to move parallel to the field lines, but are forced to follow helical Larmor orbits or gyro-radius PL =mV1./qB, where B is the magnetic field strength and v1.the velocity perpendicular to the field lines lines. The resulting charge separation then produces an electric field E, which causes the plasma to drift out of the torus in the Ex B direction.
This reduces the number of coils and usually leads to better access for plasma diagnostics.
Particle acceleration in plasmas
- Beat wave accelerator
- Experimental results
- Laser wake field accelerator
- Experimental results
- Self Modulated Laser Wake Field Accel- erator (SMLWF)
- Experimental results
- Laser guiding
- Experimental results
- Improved laser guiding
- Conclusion
- Production
- Spherical shocks
- Cylindrical shocks
The phase velocity of the plasma wave is equal to the group velocity of the laser in the plasma. This resonant state can be achieved by tuning the plasma density according to fixed frequencies for the laser beams. In the laser wake field accelerator, the plasma wave drives the movement of plasma electrons by the ponderomotive force of the laser light.
The trick to achieving mono energetic beams has been found to be to carefully adjust the interaction length of the laser with the plasma.
Resistance comparison
- Propagation
- The image acquisition system
- The optical setup
- The circuit
From the equation it can be noted that the explosion time does not depend on the radius of the wire. The situation is made more complicated by the energy dissipation of the shocks at the shock front. The electrical/electronic part of the experiment can be divided into two well separated sections.
This pulse is sent to both the oscilloscope and the high-voltage trigger of the spark gap.
Gas pipes
Electron density gradient
The radial gradient of the refractive index is the most important quantity to consider when steering a high-intensity laser pulse through a plasma. The intensity of the laser always falls away from the axis of the beam, and for a Gaussian beam it has been calculated and experimentally proven that self-channeling of the beam occurs if the power exceeds a critical one. Using shock waves, it is possible to preform a channel with a high gradient of the electron density ne, comparable to that of rv5.1022 cm-4 obtained during self-channeling[65].
The tail of the shock is the interesting part for laser channeling because the pressure drops almost to vacuum over a short distance.
Experimental set up
- Cylindrical shocks
- Different foci
The laser beam is sent through the tube and produces shadow grams of shock waves. In this case, tube formation does not occur because the shock waves arrive at different times. In principle, given sufficient energy, there is no limit to the length of the tube.
The square tube that focuses the light; the light is refracted towards the center of the image, which appears darker.
The cylindrical gas pipe
Our experiment showed that the larger distance is unusable, because the shocks become too wide and too weak before reaching the center, and the pipe has poor quality, that is, a small density gradient. The smaller diameter is also unusable, in this case because the ends of the wire are too close together and an electric arc occurs between them, rather than causing the wire to explode. It is clear that no channel formation takes place after collision due to the turbulence.
This image was taken before the shocks and therefore the channel does not have the correct density profile for particle acceleration.
Experimental problems
- Debris
The uneven background is due to defects in the lens used to collimate the laser. The circle is not perfect, probably due to small errors in the position of the wire. Since the time between the explosion and the firing of the laser is measured on the oscilloscope, it can only mean that the shocks were faster.
The small dots near the center are focal points created by the intersection of the shock waves.
Conclusions
In other experiments, the capillary walls were coated with lasing medium, which is removed during the first part of the discharge. In both schemes, after a number of strokes, the capillary deteriorates and must be replaced. In another scheme[14], a laser beam is sent through the capillary and, in this case, after a while the capillary mouth melts.
In this way, there would be no need to replace the capillaries, which stop the production every now and then.
Colliding shock lenses
Experimental set-up
- Speed measurement
This type of connection dramatically reduces the impedance of the circuit, but is potentially dangerous. Both examples of capacitors actually have half the voltage applied in the. The resistors (several hundred kiloohms) that divide the voltage are clearly visible, as well as other circuit components.
In order to know the lens size corresponding to each delay, we had to measure the speed of the shock waves.
Experimental results
- Laser anomaly
Assuming that the focal length is directly proportional to the diameter of the lens, the corresponding variation in the focus position can be as high as 2 m. This means that the results should be considered as a lower limit for the quality of the lens. It can be seen that the actual size of the focus is quite large in the horizontal direction.
The focal spot size is comparable to the 0.5 mm expected for a diffraction-limited beam.
Applications
During this series of experiments, it was found that the image in the far field showed two fainter ghosts. After we carefully checked that they were not the result of internal reflection of the mirrors, we placed a 100 ~m pinhole in the telescope and thus obtained a spatial filter. The secondary images immediately disappeared, proving that they were separate beams produced in the laser itself.
The nitrogen laser
The classic laser
The most common nitrogen laser has another advantage in its cheapness and in the fact that it can be easily made from a normal double-sided printed circuit board. The basic idea of the laser is to keep the impedance as low as possible, because a nitrogen laser must have the most energy delivered in the shortest possible time. Then, when the spark gap opens, the high frequencies of the discharge cause the inductor to act as an open circuit, and the discharge occurs across the two electrodes.
As can be easily seen in figure 6.1, the high voltage surface of the laser is in the open air.
The parabolic laser
This places a practical limit on the length of the discharge and therefore on the energy that can be obtained. First, the photons always travel with the best possible gain, and second, other than a practical one, there is no limit to the length of the exhaust pipe. The slope of the discharge channel can be easily calculated, see figure 6.5 on page 177.
The speed of the discharge wave traveling through the laser is equal to toc/.jf., where c is the speed of light and Ethe permittivity of the medium.
Experimental results
The shape of the laser is not the only parameter that affects the energy output. Graph 6.2 on page 180 shows the behavior of the output versus the nitrogen flux (ie the velocity of the gas). As we will see later, the optimal flux depends on the size of the laser.
The first measurements took into account the difference in output between the two ends of the laser.
