X-ray diode results

The main disadvantage of using the single photon counting CCD to determine the hot electron temperature of plasmas produced by lasers of intensity ~ 10¹⁵ W cm^-2 is the large number of shots required to obtain an analysable spectrum. This method cannot be operated in a single shot mode for this laser intensity regime and the analysis is difficult to do in real time due to image processing requirements. The x-ray diodes offer a method

to deduce the hot electron temperature quickly and on a shot by shot basis. The diodes have been specifically designed to detect and amplify very low radiation levels in different regions of the spectrum and by comparing the 4 signals, a suprathermal temperature can be determined quickly and accurately, and can be automated using a program to read off the oscilloscope traces and perform real time analysis.

Figure 3.18 Comparison between hot electron temperature calculations as the laser burns through the target. The red points are experimental data calculated using R1 and the black points use R2 (see figure 3.10). The vertical dashed line represents the shot number at which the laser has burnt through the 2mm thick aluminium target.

As is discussed in section 3.5, two different ratios between the channels are used to determine the hot electron temperature. A comparison of the calculated temperatures from each ratio can be used as an additional test as to the quality of the result obtained.

Figure 3.18 shows a series of measurements taken, where the laser was repeatedly fired on to the same area of the target disk, until the laser burnt through the 2mm thick aluminium. Figure 3.18 shows, firstly, a good agreement between the calculated

temperatures from the two ratios and secondly, the temperature decreases as the laser burns through the target. As the laser is not re-focussed after each shot, the on target intensity decreases as the focal position of the lens is progressively further away from the target surface due to the ablation of the target. This lower on target intensity causes a

Figure 3.19 Change in diode signal when the laser burns through the 2mm aluminium target. The black lines are the signals on each of the four channels (numbered 1 to 4 as in table 3.1 reading left to right) from the first (in focus) shot

and the red lines are the diode signals on the shot after burn through.

reduction in the hot electron temperature. The reduction in hot electron temperature observed as the laser becomes de-focussed is not as would be expected using the scaling laws in section 2.1.3. One would expect to see a decrease in hot electron temperature until the electron energy was low enough to effectively thermalise within the thermal plasma making it indistinguishable from the thermal population of electrons. This is most likely not observed due to the combination of the de-focussed laser pulse interacting with the sloped walls of the conical cavity, which has been shown to increase hot electron

generation [67], and thermal self-focusing of the laser within the laser produced plasma [68]. This combination results in a smaller reduction in hot electron temperature as the laser de-focuses. This trend is only seen to continue until the point at which the laser burns through the target where the temperature increases to ~ 7 keV, and remains approximately constant and the individual diode signals rapidly increase by a factor of ~ 4 (see figure 3.19). This increase in signal is very sudden (figure 3.20) and occurs on the shot where visible emission is first seen on the rear side of the target indicating the laser has burnt through the target. As the relative signals between the diodes remain the same (similar temperature), this rapid increase in signal is due to the diodes detecting the emission originating from a larger number of hot electrons than before the laser ablates through the target.

Figure 3.20 Variation in diode signal on channel 1 demonstrating the rapid enhancement of hot electron generation upon target burn through.

The cavity the laser produces is approximately conical in shape, shown in figure 3.21, with a front aperture of diameter 0.55 ± 0.02 mm and a rear aperture of 0.18 ± 0.02 mm.

As a result of this open ended conical cavity, or conical frustum, the plasma density profile changes dramatically and as a result changes the mechanisms behind the hot electron generation.

Figure 3.21 Cavity produced by laser ‘burn through’ as viewed from the front surface (a), and the rear surface (b). The thickness of the aluminium target is 2mm.

The laser continues to interact with the sloped walls of the conical frustum producing hot electrons via resonance absorption (see section 2.1.2) as is observed before burn through noted by the small reduction in hot electron temperature and number as the cavity is forming and the laser is de-focussing. As is shown in figure 3.22, the plasma density profile along the laser axis would be expected to change from an approximately exponential profile to a more Gaussian like profile with a sub-critical peak density. When a calorimeter is placed at the rear side of the target, it is found that ~ 3% of the incident laser energy emerges from the hole produced by the laser burning through the target.

This is an indication that the plasma within the cavity, along the laser axis, is sub-critical and can also be shown by estimating the plasma expansion from the ion sound speed, shown in figure 3.22.

Figure 3.22 Scale diagrams demonstrating the laser beam expansion within the conical cavity before (a) and after (b) burn through. The plasma expansion is directed along the blue arrows and the plasma expansion is shown by the blue dots

representing the scale length of the produced plasma. The graph inserts show the expected density profiles schematically.

It has been shown that the interaction of a laser pulse in this irradiance regime with a density profile as shown in figure 3.22(b) can cause a reduction in the irradiance threshold for Stimulated Raman Scattering (SRS) [69]. The result of this would be to increase the volume over which the hot electrons are generated, thus increasing the number of hot electrons as observed.

Work has been carried out more recently investigating the effect of low density foams within a cone-in-shell target for laser fusion by fast ignition [70]. The purpose of the low density foams is to produce near-critical density plasma when they are irradiated by a

laser pulse [70-72]. This near-critical density plasma is seen to enhance laser coupling in the plasma and has been found to produce an approximately 3 fold increase in the number of hot electrons with little variation in hot electron temperature [70,72]. A similar effect is observed in this experiment using a lower irradiance (~ 10¹⁵ W cm^-2) than previously investigated and without the low density foam to produce plasma with reduced mass density. In this experiment, the sub-critical density plasma is produced through the interaction of an expanding laser beam with the sloped walls of an open ended conical cavity. This technique could potentially be scaled to higher irradiance to produce a larger number of hot electrons whilst maintaining hot electron temperature, which increases with irradiance. This is highly desirable for fast ignition ICF as the energy of the fast electrons must be carefully controlled in order to heat the ignition region within an ICF capsule [29,73].