(Color online) Sequences of interferograms and electron density dis- tributions of plasma stream for 20 l m PS disk and RAS targets applied. E l 1⁄4 500 J. 

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... that hydrodynamic efficiency e g was equal $ (30–50)% in this case. The most spectacular parameter in the game with impact fusion ignition is, no doubt, the velocity of impactor, but the energy transfer to the fusion target should be efficient and that means that mass and density of impactor also must be high enough. So, it is very important to look for an appropriate so- lution. Figure 3 illustrates the acceleration of 10 l m Al disk. It refers to two shots made for similar experimental parameters (the same disk thickness and energy E l 1⁄4 500 J and very similar target construction). The only difference was that the second sequence of shadowgrams illustrates the case when the exit of the channel was covered with thin (1.0 l m) Al foil. Comparison of these two shots shows that acceleration process with the covered channel target is more effective. The disk leaving the channel seems to have a higher velocity, which may be a little peculiar because there are no physical reasons to have a higher velocity for the macroparticle accelerated in the covered channel. The explanation of this phe- nomenon is quite simple. The cold plasma leaving the open channel quickly loses its density expanding into hemisphere. The reference laser beam of interferometer does not see rap- idly increasing part of low density plasma. In the case of the covered channel that expansion is much slower, energy of expanding plasma is transferred to the covering foil (of solid density), and it means higher density and higher energy density of macroparticle. A thin foil covering the target causes local compression of the outbursting plasma and, to some extent, decreases center mass speed of moving plasma. The movement of the optical shadow boundary seen in the shadowgrams gives information about velocity of a particular density layer but not of the accelerated disk (macroparticle) as a whole. We have two effects that compete each other. Speed of macroparticle (center mass velocity) is a little bit lower, but its density grows. Density gradient is steeper (this is important from the point of view of recorded shadowgrams), and it makes impression that the accelerated foil moves faster than previously (in the case of "open channel" target). And, one more advantage of this target configuration is that during the collision, an ultrahigh-pressure shock wave can be generated in the impacted foil covering the target channel, and the foil is not preheated. 10,11 Two sets of interferograms which illustrate the results of acceleration in “open channel” target (a) and “covered channel” target (b) are presented in Fig. 4. They show the case of 10- l m PS disk accelerated in 1 mm long channel by the laser pulse of energy E l 1⁄4 400 J. In the case (b) the channel was covered by 1 l m PS foil. We can notice that only a central part of the flow is a plasma and it is surrounded with a cold plasma (gas) with refractive index opposite to plasma refractive index. The second experimental session was made with modified RAS targets. Targets used in the case of backward acceleration are shown in Fig. 5. The laser pulse enters the short (200–500 l m) channel, is focused on the laser beam axis in the middle of the hole inside the channel, goes through the hole made in the foil, and irradiates the light ablator placed in the bottom of the cavity chamber. Expanding plasma interacts with the foil and accelerates it. The typical result obtained with a modified RAS target is shown in Fig. 6, where a sequence of two shadowgrams (taken from two different shots) showing the movement of 20- l m PS foil at two selected times (1 and 2 ns after the laser pulse maximum) is presented. The laser beam (E l 1⁄4 500 J) is incident from the right. The depth of the channel target was set to be equal 500 l m. After 1 ns, the optical shadow boundary traveled the distance of 300 l m, relatively to the target surface. Taking into account the fact that the accelerated foil fragment started to move in the channel 500 l m long, one can estimate the average foil fragment velocity to be equal approximately 8 Â 10 7 cm/s. More direct measure- ment of average velocity of the foil fragment can be eval- uated taking into account the progress of the front of the dark region which is 600 l m between the individual frames taken with 1 ns delay in the interval of 1-2 ns. From these numbers the average speed can be found to be 6 Â 10 7 cm/s. Comparable results were obtained for 10- l m Al foil and similar target construction applied. Having in mind that the foil started with zero velocity, it means that maximum velocities v max of accelerated Al and PS foil fragments were > 7 Â 10 7 cm/s. So, they can approach the top result mentioned above (NRL), and, moreover, they were obtained for thicker (higher mass density) foils (10- l m Al and 20- l m PS foils vs. 10.5 l m CH foil). The accelerated cold plasma flowing out of the channel was dense (electron density $ 2 Â 10 20 cm À 3 ) and sharply shaped (Fig. 7). It can be regarded as an additional indirect evidence of the high efficiency of the acceleration process. Concluding, CPA leads (especially in version of backward acceleration) to significantly higher velocities of flyer foils than those obtained in traditional way (ablative acceleration scheme) in similar experimental conditions. The best results obtained are on the level of the world top velocities (NRL Washington, ILE Osaka). Also, the hydrodynamic efficiency of the energy transfer to the flyer foil is much higher. Other important advantages of CPA mentioned earlier: 7 it enables acceleration by laser of very heavy macroparticles (rd f $ 10 À 1 g/cm 2 ) to velocities of $ 1 Â 10 7 cm/s and it is not sensitive to laser light wavelength (RAS version). A very high hydrodynamic efficiency obtained in CPA experiments may be a chance to meet the requirements of the laser This system work was energy supported and construct in part by an the impactor HiPER for project laser fusion under Grant experiments Agreement (more No. convenient 211737, is by the the forward Ministry version). of Sci- Application ence and Higher of the Education “covered channel” MNiSW, target Poland gives under an evident Grant increase No. N202 of 130639, the density by the of Access accelerated to Research plasma Infrastructure outbursting from activity the in channel, the 7th which Framework is a key Program problem of from the EU the Contract point of view No. 228334, of possible Laserlab applications Europe, in and impact by fast "Research ignition Centre area. of Laser Plasma" LC528 of the Ministry of Education, Youth and Sports of the Czech Republic. This work was supported in part by the HiPER project under Grant Agreement No. 211737, by the Ministry of Sci- ence and Higher Education MNiSW, Poland under Grant No. N202 130639, by the Access to Research Infrastructure activity in the 7th Framework Program of the EU Contract No. 228334, Laserlab Europe, and by "Research Centre of Laser Plasma" LC528 of the Ministry of Education, Youth and Sports of the Czech ...