Interpreting angular divergence measurements

Chris Ridgers,Mark Sherlock,

Robert KinghamRoger Evans

Diagnosing the rear surface

Probe rear-surface of the target

Large-scale structures on rear - apparent angular divergence larger than actual divergence

Target

Laser

Observer

Rear-surface structure

Apparent divergence

Real divergence

Temperature and density profiles

Temperature profile wider than beam because…

Tet

Te 3 / 2

…Ohmic heating saturates

Te

nf

Rear-surface sheath field

Electric field and potential profiles wider than nf

Ion spot POOR indicator of angular divergence

nf

Ey

Sheath:

E y n f1/ 2

const

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

K- measurements

Electrons at max K- cross section are nearly isotropic

Large K- spots for thin targets implies very large divergence

Thicker targets sample electron source at higher energies – more directional

K- needs to included in model

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

Energy distributions from solid targets