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Diagnosing the rear surface

Interpreting angular divergence measurements Chris Ridgers, Mark Sherlock, Robert Kingham Roger Evans. Diagnosing the rear surface. Probe rear-surface of the target. Target. Rear-surface structure. Laser. Observer. Real divergence. Apparent divergence.

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Diagnosing the rear surface

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  1. Interpreting angular divergence measurementsChris Ridgers,Mark Sherlock,Robert KinghamRoger Evans

  2. Diagnosing the rear surface Probe rear-surface of the target Target Rear-surface structure Laser Observer Real divergence Apparent divergence Large-scale structures on rear - apparent angular divergence larger than actual divergence

  3. Temperature and density profiles Temperature profile wider than beam because… Te …Ohmic heating saturates nf

  4. Rear-surface sheath field Electric field and potential profiles wider than nf  Ey Sheath: nf Ion spot POOR indicator of angular divergence

  5. Diagnosing the rear surface Fast-electron number-density (radiation from back) Good – but measure OTR and CTR Background temperature (Planckian spectrum) Over estimate Sheath electric field (ion spot size)? very large overestimate

  6. K-a measurements Electrons at max K-a cross section are nearly isotropic Large K-a spots for thin targets implies very large divergence Thicker targets sample electron source at higher energies – more directional K-a needs to included in model

  7. Energy distributions from solid targets The initial burst of electrons are the only ones able to escape so the  measured energy is lower ( these electrons don’t lose (much) energy in the sheath because they are largely responsible for generating the  sheath ). )

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