Highly efficient acceleration and collimation of high-density plasma Jan Badziak Institute of Plasma Physics and Laser Microfusion Warsaw, Poland

Co - workers S. Borodziuk, T. Pisarczyk, T. Chodukowski Institute of Plasma Physics and Laser Microfusion Warsaw, Poland E. Krousky, K. Masek, J. Skala, J. Ullschmied PALS Research Centre ASCR, Prague, Czech Republik

For short-wavelength (UV, XUV) radiation is maximized: Acceleration by Laser Plasma Ablation All the fusion schemes currently considered employ laser-induced ablative acceleration (the “rocket effect”) to accelerate and compress plasma. Energetic efficiency of the ablative acceleration (AA): - absorption coefficient - hydrodynamic efficiency  Only a small part (~ 10%) of laser (X-rays) energy is utilized for acceleration. The low energetic efficiency of AA is one of the main reasons for the huge laser energy required for high-gain fusion: > 1MJ for central hot spot ignition  300kJ for ignition with an external trigger We have tried to find a more efficient acceleration scheme which could change this unfavorable situation

Laser- Induced Cavity Pressure Acceleration (LICPA) In the LICPA scheme, a thin target (a macroparticle) placed in a cavity is irradiated by a laser beam through a small hole in the cavity wall and accelerated forward by the pressure produced and accumulated in the cavity by hot plasma expanding from the target. The cylindrical scheme (the LICPA gun) The conical/spherical scheme (the LICPA accelerator and compressor)

A simplified scheme of the PALS experiment The laser parameters: L =  m,  L = 0.3ns, d L = 200  m, E L  J, I L  (0.5 – 2)  W/cm 2 The volume of crater produced in the Al target (the set-up M) was a measure of energy deposited in the target by the high-density forward-moving plasma (and indirectly, a measure of the plasma kinetic energy). The three-frame interferometry gave us information about velocity and density of the plasma. The ion diagnostic enabled to estimate temperature of the hottest part of the plasma and to conclude about ion velocities and the ion flux composition. The results obtained with the LICPA scheme were compared with the ones obtained for ablative acceleration (both with and without the cylindrical channel) and for a direct laser-Al target interaction.

Craters produced in the Al target by a direct laser-target interaction (L-T) as well as by high-density plasma driven by LICPA or ablative acceleration (AA) and guided in the cylindrical channel E L  130J, I L  1.3  W/cm 2, d L  200  m; L ch = 2mm, d ch = 0.3mm L - T LICPAAA The craters produced with LICPA are considerably larger and deeper than in the case of ablative acceleration or a direct laser-target interaction

The volume and depth of craters produced in the AL target by high-density plasma driven by LICPA or ablative acceleration (AA) as a function of laser energy (intensity) LICPA AA (rocket effect) The volume of crater produced in the Al target (energy deposited in the Al target) by the plasma driven by LICPA is more than 30 times greater than in the case of ablative acceleration. The LICPA crater volume increases quickly with laser energy.

The volume and depth of craters produced in the AL target by high-density plasma driven by LICPA or by a direct laser-target interaction (L-T) as a function of laser energy (intensity) LICPA L-T The volume of crater produced in the Al target (energy deposited in the Al target) by the plasma driven by LICPA is 13 – 15 times greater than in the case of the direct laser-target interaction

E L  130J, I L  1.3  W/cm 2, L ch = 2mm, d ch = 0.3mm Interferograms of plasma produced in the LICPA scheme Backward-expanding plasma at the cavity input Forward-accelerated plasma at the output of 2-mm channel A front part of high-density plasma guided in the channel moves with the velocity ~ 2 x 10 7 cm/s

The volume and depth of craters produced in the AL target by high-density plasma driven by ablative acceleration and guided in the cylindrical channel with the Al or Au wall as a function of laser energy (intensity) The plasma guided in the channel with the Au wall produces larger and deeper craters than in the case of the Al channel wall. The craters produced by freely expanding plasma (without the channel) are usually much smaller than the ones produced with the channel. Al cylinderAu cylinder

The volume and depth of craters produced in the AL target by high-density plasma driven by ablative acceleration and guided in the conical channel with the Al or Au wall as a function of laser energy (intensity) Au cone 12 o E L = 173J The forward-moving plasma can be efficiently guided and collimated in the conical channel, especially when using the channel with the Au wall

Can the LICPA scheme be useful for laser fusion ? ? The ignition of DT fuel compressed by the quasi-spherical (cone- guided) implosion is triggered by the ignitor created due to collision of two (or more) DT macroparticles accelerated and compressed by the LICPA mechanism in conical channels. The application of LICPA makes it possible to increase the accelerated DT mass several times. Using counter-propagating macroparticles enables us to decrease the macroparticle velocity required for ignition by a factor 2 (to the velocity ~(5-8) x 107cm/s comparable to that already demonstrated). Impact Ignition Fusion ? DT fuel is accelerated and compressed by LICPA inside a spherical cavity. Central Hot Spot Fusion

Conclusions A novel scheme of high-density plasma (a macroparticle) acceleration using laser-induced cavity pressure (LICPA) has been proposed and demonstrated. The energetic efficiency of LICPA can be about order of magnitude higher than that of the conventional ablative acceleration using the “rocket effect” (due to higher both and ). High-density plasma driven by LICPA can be effectively guided and collimated in mm-length cylindrical or conical channels. The LICPA scheme seems to be scalable to laser intensities and energy fluencies relevant to laser fusion. The preliminary results suggest that LICPA has a great potential to be useful for both fusion-related and non-fusion applications.