Physical Foundations for Acceleration by Traveling Laser Focus

In this method called Traveling Laser Focus (TLF), multi-cell microstructures scaled down to the laser wavelength-size. Each cell in these structures has an opening from the side. Special Electro-Optical device controllably sweeps focused laser spot along these openings in accordance with instant position of accelerated micro-bunch inside the structure. This arrangement reduces the illuminating time for every point on the structure's surface and power required from the laser. Physical limitations considered for mostly important components of the TLF scheme. INTRODUCTION The way of acceleration we are defending for many years based on scaling down dimensions of accelerating structure so, that it becomes resonant with electromagnetic (EM) radiation having micrometer wavelength. Fabrication of such tiny structures is possible due to nano-technology, which demonstrates remarkable achievements. One can see this visiting web-site of any Nano-Factory (Cornell Nano-Scale Science & Technology Facility, CNF, for example, http://www.cnf.cornell.edu/). So accelerating structure made with this technology can work with a Laser source of EM Radiation. Of cause our method is not as trivial as just scaling down the structure. Some tricks are thought here. Namely, excitation of the structure is going locally from the side, following the bunch during its way inside the structure. By this arrangement the surface of the structure becomes illuminated locally for extremely short time. This time comes to be even shorter, than the time between electron-electron collisions in material of structure. High gradient required not only by desire to make accelerator more compact but also by necessity to keep the ratio of wakes to acceleration field at reasonable level. Acceleration rate required brings the laser power close to destruction limit for the structure. So the proper arrangement of excitation of accelerating structure becomes first crucial moment of all acceleration strategy. Second crucial moment is a necessity to prepare and keep tiny emittance required for free pass through structure having dimensions comparable with laser wavelength. Emittance required comes to be . IP phenomenon is third moment here. Physical limitations could manifest itself in sweeping device, material damage, in preparation and keeping tiny emittance and at IP. In this paper we considered some of these limitations. Let us remind on TLF first. rad cm ⋅ ⋅ ≤ −8 10 6π γε THE TLF METHOD In Traveling Laser Focus method, laser radiation focused onto a spot covering the side openings in accelerating structure. Excitation of each cell of the structure is going from this side opening. This laser spot is swept in longitudinal direction so it is running 1 Electronic version available at http://www.lns.cornell.edu/public/CBN/2004/CBN04-6/phys_found.pdf synchronously with the bunch inside the accelerating structure [1]. Special Electrooptical device does this sweep. As the motion of the focused laser spot along the structure is going with the speed of particle, this yields the picture, shown in Fig.1. Evidently, v c / tan = α where c is the speed of light, and v is the speed of the bunch. For electrons or positrons 4 / π α ≅ . Radiation applied to every point of structure during c lt / ~ τ , where is the instant height of the sloped laser bunch, Fig. 1. Electric field in each point of the laser bunch is perpendicular to the line, connecting this point with the center of the sweeping device. The number of cells excited simultaneously is ~ l t l ac f λ / , lf is a spot size, Fig.1. The focal point is following the beam in average. Some mismatches allowable, as the phase of the laser radiation is synchronized once with the particle’s bunch motion. Due to this arrangement, all laser pulsed power acts for generation of accelerating field at the instant particle’s location only. Power reduction and shortening of illuminating time is equal numerically to the number of resolved spots (pixels), associated with the sweeping device. Let us consider what limitations might occur here.