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Paper review:

Paper review: Accelerating monoenergetic protons from ultrathin foils by flat-top laser pulses in the directed-Coulomb-explosion regime(Bulanov_PRE_2008). Kim Youngkuk. June 18, 2012. Coulomb explosion. V.V. Kulagin Q.E 42 (1) 58-64 (2012).

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Paper review:

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  1. Paper review: Accelerating monoenergetic protons from ultrathin foils by flat-top laser pulses in the directed-Coulomb-explosion regime(Bulanov_PRE_2008) Kim Youngkuk June 18, 2012

  2. Coulomb explosion V.V. Kulagin Q.E 42 (1) 58-64 (2012) Electrons in the main target are totally expelled Remained target body is charged with positive Positive ions are exploded There exist returning electrons. These electrons reduce ion-acceleration efficiency • requirements very thin target (<100nm ?)

  3. Directed Coulomb explosion DCE: Coulomb explosion + Radiation pressure Coulomb explosion Radiation pressure dominant Ion cloud Electrons in the main target are almost expelled And remained ions are forced to accelerate to the laser propagation direction by radiation pressure And then positive ions are exploded to the forward • requirements proper target thickness (~100nm ?)

  4. Super Gaussian beam Same spot radius, same laser energy with ordinary Gaussian More pulse energy is concentrated in spot radius Super Gaussian can expel electrons at more large area Super Gaussian has high ponderomotive force inner spot area Super Gaussian make more flat electric field

  5. Simulation Set Up(paper) • 2D PIC code REMP • Ultrathin double layer (Al + H) • wavelength λ=1μm • mesh size: λ/200 → 5nm • simulation box: 20λ * 10λ→ 4000 * 2000 cell (1 or 2 computers) • 500TW laser → 2.7*10e22 W/cm2 • pulse duration: 30fs • spot size: 1μm • target position: 3.33λ → 3.33μm • linearly polarization: z-porization • density: 400nc and 30nc • thickness: 0.1λ and 0.05λ → 100nm and 50nm • pre-plasma: non • longitudinal pulse profile: gaussian • transverse pulse profile: gaussian or super-gaussian

  6. Simulation Set Up(reproduce) • 2D PIC code REMP • Ultrathin double layer (Al + H) • wavelength λ=1μm • mesh size: λ/200 → 5nm • simulation box: 20λ * 10λ→ 4000 * 2000 cell (1 or 2 computers) →30λ * 10λfor PML set up (1computer) • 500TW laser → 2.7*10e22 W/cm2 • pulse duration: 30fs • spot size: 1μm (at FWHM)( 보통 spot radius를 exp(-1)인지점으로 잡기에 이 부분을 별 생각 없이 넘어가고 sgm=1um로 두었는데 numerical error가 발생하여 펄스의 변형이 심했습니다. 간단한 테스트 결과 sgm=2um 까지는 펄스의 변형이 거의 없습니다. 다시 시뮬레이션을 해야 하는데 일단은 2um로 하려 합니다. 1.2um가 논문에서 제시한 spot size 입니다.) • target position: 3.33λ → 10μm for PML set up • linearly polarization: z-porization • density: 400nc(Al+13) →5200nc(electron) and 30nc(proton and electron of patch) • thickness: 0.1λ and 0.05λ → 100nm and 50nm • pre-plasma: non • longitudinal pulse profile: gaussian • transverse pulse profile: gaussian or super-gaussian

  7. Simulation Result(paper) • During the coulomb explosion, high longitudinal electric field is generated by high-Z ions • low-Z protons are accelerated by electric field • absorbing boundary check

  8. Simulation(reproduce) Ex Ex Gaussian pulse Super gaussian(6th order) 왜 논문과 결과가 다른지 필드 데이터를 살펴보니 펄스 모양이 많이 변형되었습니다…

  9. Simulation (fail) Gaussian pulse Super gaussian Sgm=1um 이고 펄스가 타겟에 입사하기 전의 모습입니다. 펄스의 amplitude 가 많이 낮아지고 폭은 넓어졌습니다. 낮아진 amplitude 때문에 Al 의 가속이 더뎌졌다고 생각됩니다.

  10. Simulation(reproduce) Ex Ex Gaussian pulse Super gaussian(6th order) Spot size = 4um 로 증가시켜서 펄스 모양의 변형을 줄인 다음 시뮬레이션을 하였습니다. T=30T 인 순간으로 둘에게서 큰 차이를 발견할 수 없었습니다. 에너지 분포에도 차이가 전혀 없습니다. 결과도 논문과는 다릅니다.

  11. Simulation(reproduce) Spot size가 작아서 펄스 모양의 변화가 심하다고 하였는데 time step을 줄이니까 sgm=1um 에서도 펄스 변형이 없는 것을 간단하게 테스트 하였습니다. 그래서 time step을 절반으로 줄이고 시뮬레이션을 하였는데 또 펄스 변형이 일어나서 제가 프로그램 수정 중에 무슨 실수를 하였는지 살펴보아야 합니다. 또 하나는 electron 밀도를 400*13nc 로 두면서 super particle 을 각각 입자마다 다르게 설정을 하였는데 혹시나 이것이 영향을 주었는지도 살펴보려 합니다.

  12. Simulation Result(paper) • Super Gaussian expels electrons more rapidly and more broadly because of high ponderomotive force in the spot area • flat-top beam efficiently prevents the electrons from returning to the evacuated region from radial direction due to higher ponderomotive force • In the Gaussian beam case the positive charge builds up only when the accelerated portion of the foil is separated from the target • Proton energy saturates as the longitudinal field goes to zero

  13. Simulation Result(paper) • peak-like energy distribution is typical for Coulomb explosion • Δε/ε=3% (Gaussian) • Δε/ε=3.6% (super Gaussian) • Author suggested flat electric field make monoenergetic beam, but he don’t show that • maximum energy increases 40%

  14. Conclusion • flat-top beams will enhance DCE regime through the increase of the Coulomb explosion stage effect • 40% energy gain enhancement • flat-top beam evacuate electrons from a larger area • flat-top beam more efficiently prevents the electrons from returning to the evacuated region in radial direction

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