Emerging Terawatt Picosecond Co2 Laser Technology and Possible Applications in Accelerator Physics

The first terawatt picosecond (TWps) CO2 laser is under construction at the BNL Accelerator Test Facility (ATF). TWps-CO2 lasers, having the order of magnitude longer wavelength than the well-known table-top terawatt solid state lasers, offer new opportunities for the strong-field physics research. For processes based on electron quiver motion, such as laser wakefield acceleration (LWFA), the advantage of the new class of lasers is due to a gain of two orders of magnitude in the ponderomotive potential for the same peak power. The large average power capability of CO2 lasers is important for the generation of hard radiation through Compton back-scattering of the laser off energetic electron beams, as well as for other applications. Among them are: LWFA modules of a tentative electron-positron collider, g-g (or g-lepton) collider, a possible "table-top" source of high-intensity xrays and gamma rays and the generation of polarized positron beams. 1 EMERGING TWps-CO2 LASER TECHNOLOGY An important physical parameter that enables generation and amplification of picosecond laser pulses is the gain spectral bandwidth. Methods to produce picosecond and sub-picosecond CO2 laser pulses have been developed. One of them is semiconductor optical switching [1]. Using this method, subpicosecond slices out of a multi-nanosecond CO2 laser output may be produced using conventional mode-locked solid-state laser. Pressure broadening of the CO2 gain spectrum at ~10 atm into a 1 THz wide quasi-continuum permits amplification of 0.5 ps laser pulses. Note that gas lasers are free from optical nonlinearity. This permits direct amplification of multi-terawatt l=10 mm laser beams without a sophisticated pulse chirping technique which is necessary for terawatt solid state lasers. For a tL=1 ps pulse propagating in a 10-atm amplifier, the estimated small-signal gain is 3-4%/cm and the extractable specific energy is ~20 mJ/cm. Taking into account that the total discharge volume may exceed 10 l, the possibility of extraction of as much as 100 J of energy in a picosecond pulse from a reasonably compact CO2 laser amplifier looks realistic. However the damage threshold of the output window that is at the level of 0.5 J/cm. For an optical window of the ~100 cm size, the extractable energy is 30-50 J which still permits ~30 TW peak power at a 1-ps laser pulse duration. Thus, to attain terawatt peak power, a ~10-atm, ~10l CO2 amplifier is required. To maintain a uniform discharge under such conditions, the following requirements should be satisfied: a) strong penetrating preionization, b) ~1 MV voltage applied to the discharge, and c) the energy load of several kilojoules deposited in a relatively short, £300 ns, time interval. The first laser with such parameters is under construction at the Brookhaven ATF. Fig.1 shows the principal optical diagram of the ATF TWps CO2 laser system. The 1 MW, 100 ns pulse produced by a 1-atm CO2 laser oscillator is sliced at a semiconductor switch controlled by the picosecond Nd:YAG laser. The high power will be attained via regenerative amplification and four additional passes through the preamplifier followed by three passes in the 10-atm, 10-l final amplifier with the beam expansion to its full 10-cm aperture [2]. Figure 1: Optical diagram of the ATF TWps-CO2 laser The ATF laser, besides its role in user’s experiments, will serve as a test bench for proof-of principle evaluation of the TWps-CO2 technology for such strong field physics applications as high-gradient laser accelerators and high-intensity Compton x-ray sources. For that purpose, ATF also provides a highbrightness 50-MeV electron beam from a photocathode RF linac synchronized within subpicoseconds to the CO2 laser pulse. 2 LASER WAKEFIELD e-e COLLIDER Progress in the exploration of particle interactions relies upon the development of a new-generation of accelerators on a TeV energy scale. One of the prospective approaches is a linear e-e collider based on a highgradient laser acceleration. The enthusiasm that drives research in this area is based on ultra-high electric fields attainable upon the tight focusing of terawatt laser beams. 645 0-7803-4376-X/98/$10.00  1998 IEEE This may permit reduction of the accelerator length by orders of magnitude. Among the known laser acceleration techniques, the laser wakefield acceleration (LWFA) [3] is considered as the most promising. The LWFA method is based on the ponderomotive charge separation and a relativistic wake formation when a short laser pulse propagates in underdense plasma. The amplitude of the accelerating field, Ea, due to the charge separation in a plasma wave is [ ] [ ] E V cm a a n cm a e / = + èç ø÷ 2 2 3 1 , (1) where ne is electron density in plasma, and a is the dimensionless laser vector-potential a= eE mc L / w =0.3EL[TV/m]l[mm]. (2) From Eqs.(1) and (2) we see that Eaμl 2 for a<<1 and Eaμl for a>>1. This is due to the stronger ponderomotive potential of plasma electrons oscillating in a lowerfrequency electromagnetic field. There are still tradeoffs to attain higher Ea with short-wavelength lasers via tighter laser focusing and using higher ne. The analysis of two options to build the 2.5 TeV multi-stage plasmachanneled LWFA linac using CO2 or a solid state lasers is presented in Table 1. Both design options are aimed to attain a luminosity L=10 cms [4] defined as L = ^ N f e 2 2 4 z ps , (3) where Ne is the number of particles per bunch, z is a number of bunches per train, f is the linac repetition rate, and s ^ is the e-beam cross-section at the interaction. The parameters entering Table 1 are chosen according to the following prime considerations: 1. The 50 TW peak laser power foreseeable with stateof-the art laser technology, and the laser pulse duration close to the experimentally demonstrated minimum. 2. The plasma channels are filled with a 100% ionized hydrogen at the density 05 . ne . The normalized emittance of the electron beam at 2.5 TeV due to gas scattering described by Highland formula [5] is: [ ] [ ] [ ] ( ) n e a m rad n cm E MeV m . / » ́ 3 10 15 3 2 .(4) 3. The channel length for every accelerator stage is ~30z 0 , where z 0 =prL /l is the Rayleigh length. A 20 cm long evacuated dead space is assumed between the accelerating channels. Note that optics of the same focal length are used for both lasers. 4. The maximum number of particles per bunch is calculated under the condition that space charge field of the electron bunch does not effect the wakefield structure: ( ) N n c e e p £ w 3 . We see that both design approaches illustrated in Table 1 demonstrate the LWFA capabilities to attain the desired 2.5 TeV electron energy in a compact multi-stage accelerator. However, the calculated requirements to the laser driver for two cases are essentially different. Table 1. Prospective Multi-Stage 2.5 TeV LWFA