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Coherence and Ultrashort Pulse Laser Emission Part 11

Tham khảo tài liệu 'coherence and ultrashort pulse laser emission part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 392 Coherence and Ultrashort Pulse Laser Emission Fig. 6. Time dependency of the energy distribution of a xenon plasma heated with a Nd-YAG laser pulse with an initial electron density of ne0 1018 cm-3. lnf n fỂ - zz ei-z 19 I r z t J Jo where aei is the absorption coefficient and z is the length. The main mechanism of the absorption of probe radiation in the absence of absorption bands and lines is as already mentioned in 2.2 the inverse bremsstrahlung. During the quick heating-up of the electrons due to the laser pulse no expansion work is achieved. The amount of heat -Ql is supplied to the electrons per time unit. Standarized to the volume with the absorption coefficient of the electrons for inverse bremsstrahlung aei the heat source strength Pl results Pl r z t V - L KeihÁr z t . 20 The electron temperature changes agreeable to d dt toJB aeilL r z t . 21 For the absorption coefficient aei equation 7 is used. Because of the proportionality of the absorption coefficient to n2 under certain circumstances a possible rest ionisation from the previous laser pulse may play an important role. A high repetition rate and the effect of the magnetic field on the remaining ionisation can have a positive influence on the absorption of the plasma in this issue. For the thermal conduction out of the central area of the discharge the continuity equation obtains Interaction of Short Laser Pulses with Gases and Ionized Gases 393 Fig. 7. Time dependency of the energy distribution of a xenon plasma heated with a Nd-YAG laser pulse with an initial electron density of ne0 1019 cm-3. di XJw 0. 22 The heat flux obtains Jw -K Te 23 where K is the thermal conductivity. The temporal change of the electron temperature due to thermal conduction obtains dTe d . V KVTe . 24 dt 3nekB For the thermal conductivity we use the expression k T 4 104 9 - 8 Z - 3 w TkB B . . 25 eữ e Z ln A Here e0 ist the elementary charge of an electron. The term ln A describes the so-called Coulomb-logarithm which .