1. Coupling Laser Wakefield Acceleration (LWFA) and Direct Laser Acceleration (DLA) to Increase the Maximum

Energies of Electron Beams The use of tabletop accelerators may enable useful lower-energy applications such as in food sterilization, materials science, nuclear medicine, and structural biology. Recently, there have been efforts to couple two advanced accelerator concepts—Laser Wakefield Acceleration (LWFA) and Direct Laser Acceleration (DLA)—to increase the maximum energies of electron beams produced by a LWFA given the limitations of a particular laser system [1,2]. Coupling these two mechanisms aids in maximizing the electron energy gain because of the DLA of the electrons that experience betatron motion in the laser pulse polarization plane.

2. Femtosecond Terawatt Ti:sapphire Laser System

Acknowledgements

6. Future Research of Direct Laser Acceleration

4. Variance in Electron Beam Quality

Approach: Better diagnose DLA Applications: Plasma-based particle accelerators, THz radiation, Imaging

Microseconds to Femtoseconds conversion

Pulse duration

3. Laser Wakefield Acceleration

  I. Intense laser propagation through plasma

In LWFA, if the laser pulse overlaps the trapped electrons then DLA can cause amplified betatron oscillations due to the transverse field of the laser. The transverse momentum of electrons is converted into longitudinal momentum.

References [1] Shaw, J. L., Tsung, F. S., Vafaei-Najafabadi, N., Marsh, K. A., Lemos, N., Mori, W. B., & Joshi, C. (2014). Role of direct laser acceleration in energy gained by electrons in a laser wakefield accelerator with ionization injection. Plasma Physics and Controlled Fusion, 56(8), 084006. doi:10.1088/0741-3335/56/8/084006 [2] Zhang, X., Khudik, V. N., & Shvets, G. (2015). Synergistic Laser-Wakefield and Direct-Laser Acceleration in the Plasma-Bubble Regime. Physical Review Letters, 114(18). doi:10.1103/PhysRevLett.114.184801

I would like to thank Jessica Shaw, Dr. Nuno Lemos, and especially Professor Chandrashekhar Joshi for an irreplicable opportunity. I am grateful for all of their guidance and enthusiasm throughout the summer. I would also like to thank William Herrera and Harsha Kittur for their support and leadership in the High School Summer Research Program.

Full-width, half-maximum

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(A)

At ~40 fs there is no significant direct laser acceleration. At ~45 fs significant direct laser acceleration starts. At ~60 fs significant direct laser acceleration is at its best due to the laser overlapping the trapped electrons.

(B)

In previous experiments aiming to demonstrate the presence of DLA, changing the plasma density induced a lot of second-order effects that made it difficult to cleanly separate out the effect of DLA.  Therefore, we are trying to keep the length of the bubble fixed by keeping a constant plasma density, and then we will change the length of the laser to control the degree of overlap.

Controlling the degree of overlap results in the electrons microbunching, increased acceleration gradients, and increased transverse motion—important for stronger X-rays. Pulse duration was optimal at around 60 femtoseconds. Although, experimentation with direct laser acceleration is still in its early stages the experimentation of diagnosing DLA will continue, as DLA can possibly act as the main mechanism of acceleration in the future.

II. Lighter electrons expelled outward

III. Plasma wake results from Coulomb force of ion column attracting electrons

V. Ion column causes electrons to undergo betatron oscillations

IV. Trapped electrons accelerate

(B) Pulse duration ~60 fs after conversion

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(A) Pulse duration ~45 fs after conversion

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For DLA to occur, the laser pulse length has to be long enough for the laser to overlap the trapped electrons. By altering this laser pulse length with a constant plasma density, we can control this degree of overlap and therefore the presence and strength of DLA.