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The scaling of LWFA in the ultra-relativistic blowout regime: Generation of Gev to TeV monoenergetic electron beams W.Lu, M.Tzoufras, F.S.Tsung, C. Joshi, W.B.Mori UCLA, USA L.O. Silva, R.A.Fonseca IST, Portugal

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Outline Motivation. Physical picture : Illustration of what the ultrarelativistic blowout regime looks like, what the fields are, how the electrons behave and evolution in time. Theory : Ideas behind the theory. Description of how the characteristic quantities of this regime relate to each other. Scaling laws : Scaling of beam energy, beam charge and energy conversion efficiency with laser and plasma parameters. Comparison between the theory and published (as well as unpublished) results, both experimental and simulation. Extrapolation to exotic cases. The possibilities of building single stage 10Gev, 100Gev and even TeV laser electron accelerator and additional issues need to be addressed. Conclusion.

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Motivation Recent results 3 Nature papers (September 2004) where monoenergetic electron beams with energy 70~170MeV by using 10TW 30fs class lasers were measured. Phys. Rev. Lett. by Tsung et al. (September 2004) where monoenergetic beam with energy 260 MeV by using a 13TW 50fs laser were observed. How can we scale this regime to higher energy and better beam quality?

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Questions we try to answer …… Is there a consistent physical picture behind all the experiments and simulations? What is the condition for self-injection of the electron beam? What are the energy, charge and efficiency scaling and their scalabilities? What is the condition for self-guided laser propagation? What is the optimal conditions to choose the parameters? What determines the beam quality ( energy spread, spot size and emittance) ?

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Physical picture Geometry - fields The ponderomotive force of the laser pushes the electrons out of the laser’s way. The particles return on axis after the laser has passed. The region behind the pulse is void of electrons but full of ions (ion channel). The resulting structure moves with the speed of laser’s group velocity, supporting huge accelerating fields and strong focusing force. The ponderomotive force of the laser pushes the electrons out of the laser’s way. The particles return on axis after the laser has passed. The region behind the pulse is void of electrons but full of ions (ion channel). The resulting structure moves with the speed of laser’s group velocity, supporting huge accelerating fields and strong focusing force.

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Physical picture Evolution of the nonlinear structure The front of the laser pulse interacts with the plasma. As a result it loses energy (Local pump depletion) and etches back. The shape and size of the accelerating structure slightly change. Electrons are self-injected in the ion channel at the tail of the ion channel due to the accelerating and focusing fields. The trapped electrons slightly elongate the back of the spheroid. The front of the laser pulse interacts with the plasma. As a result it loses energy (Local pump depletion) and etches back. The shape and size of the accelerating structure slightly change. Electrons are self-injected in the ion channel at the tail of the ion channel due to the accelerating and focusing fields. The trapped electrons slightly elongate the back of the spheroid.

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Physical picture Evolution of the nonlinear structure The blowout radius remains nearly constant as long as the laser power doesn’t vary much. Small oscillations due to the slow laser envelope evolution have been observed. Beam loading eventually shuts down the self injection. The laser energy is depleted as the accelerating bunch dephases. The laser can be chosen long enough so that the pump depletion length is matched with the dephasing length.

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Theory the spherical ion channel and the constant wake slope A spherical ion channel for ultra- relativistic blowout A fully nonlinear theory for the blowout regime for both beam and laser driver can show that for large blowout radius (ultra-relativistic blowout ),the ion channel will become a sphere. A constant wakefield slope (1/2) The wakefield depends linearly on the distance from the center of the ion channel, and has a deep spike near the tail.

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Theory Choosing the laser parameters - matched profile Matched laser spot size For given laser power P, there is a matched laser spot size W 0, which is approximately equal the blowout radius R b Laser For given laser power P and given plasma density n p, this matching condition gives: Ponderomotive : Ion channel : Balance of forces : Approximately :

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Theory Condition for self injection The condition for self injection 1.In the ultra-relativistic blowout regime ( k p R b >>1 and spherical ion channel), the plasma electrons will get parallel speed close to c when they reach the axis near the tail of ion channel. 2.When the electrons reach the axis, their initial velocities are typically smaller than the phase velocity of the wakefield. If they can get enough energy before dephaing through a narrow region near the tail of ion channel, which has both strong accelerating field and focusing force, they get trapped and keep gaining energy. Both conditions can be satisfied if the matched a0>4~5: Simulations show that for even very low plasma density like n p =1*10^15 cm-3 (very high wake phase velocity ), trapping can be achieved by this condition

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Absorption by ponderomotive particles 1D like absorption Electron density 100 TW, cm -3 Two types of absorption: By ponderomotive particles 1D like absorption Theory Local pump depletion For a0 around 4~5, these two absorption are comparable. For a0 around 10 or larger, the 1D like absorption dominates.

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Theory Etching velocity, phase velocity of the wake and dephasing length The laser front etches back by local pump depletion. After pump depletion, it diffracts. The etching back velocity V etch is in principle depends on a0 ( for 1D, V etch is independent of a0). More detail calculation can show that the 3d Vetch is close to 1D results even the energy loss mechanism changes when a0 gets large. This yields the dephasing length and the pump depletion length: Due to the laser etching 100 TW, cm -3 The same scaling for L dp and L pd. Typically we can choose to match dephasing and pump depletion.

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Scaling laws Energy gain, charge and energy conversion efficiency Energy gain : or Total charge : or Energy conversion efficiency : or

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Scaling laws Verification of the scaling through simulations As long as the laser can be guided ( either by itself or using shallow plasma density channel), one can increase the laser power and decrease the plasma density to achieve a linear scaling on power.

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Self-guiding condition The laser self-guiding is based on two effects: 1.The main part of the laser is inside a index of refraction channel made by the laser blowout. 2.The laser front keeps etching back, which prevents the leading front from diffraction before pump depletion. A fully nonlinear theoretical analysis based on the index of refraction gives the following critical a0 for guiding: For all the 3D simulations we have done ( np>1*10^18cm-3), a0~4 is enough for guiding. For density like n p = 2*10^17cm-3, this gives a0 aroud 5~6. In the future, 3D simulations will be used to test this condition for low density.

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Parameter designs for Gev,10Gev,100Gev,1Tev P(PW)τ (fs)np (cm- 3 ) W0 (μm)L(m)a0a0 Δn c /n p Q(nC)E(Gev) e % e <20% e <20% e <20% e % e % e % e %

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Conclusions We have developed a theory that allows us to design laser plasma accelerators operating in the ultrarelativistic blowout regime. We have found that a laser with ”matched” profile achieves stable, self- focused propagation for the entire interaction length. Given the power of a laser we can: 1. Pick the density for self-focused propagation. 2. Choose the rest of the laser parameters. 3. Predict the energy of the monoenergetic beam. For these accelerators, since the energy is proportional to the laser power: -we have shown via numerical simulations that nC, GeV electron bunches can be generated by TW lasers. -According to the scaling, TeV laser plasma accelerators will become possible for PW lasers.

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formulas Matched a0 and spot size : Pump depletion length: Dephasing length: Energy gain: Charge: Efficiency: Critical a0 for self-guiding:

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Beam quality and X-ray loss Except the Tev designs, the X ray losses are small comparing with the beam energy. For the Tev designs, the X ray losses are less than 200Gev. X-Ray emission Energy spread For higher laser power and lower plasma density ( longer dephasing length), the uncertainty in the energy shot by shot will decrease.

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