dyna mics of neutralizing electrons and focusability of intense ion beams
DESCRIPTION
Dyna mics of neutralizing electrons and focusability of intense ion beams. A.F. Lifschitz a , G. Maynard a and J.-L. Vay b a LGPG, Universit ė Paris Sud, Orsay, France b LBNL, Berkeley, USA. Introduction. - PowerPoint PPT PresentationTRANSCRIPT
Dynamics of neutralizing electrons and focusability of intense ion beams
A.F. Lifschitza, G. Maynarda and J.-L. Vayb
aLGPG, Universitė Paris Sud, Orsay, FrancebLBNL, Berkeley, USA
Introduction
Even when the beam is globally neutral, neutralization is not perfect due to the transversal electron temperature → finite screening length
The limit to the neutralization due to finite Te is relevant when:
a) global neutralization is good (f ≥90 %)
b) transversal temperature is high (Te≥10 keV)
Electron transversal temperature is determined by:
a) heating by compression
b) flow of electrons into the beam
beam electrostatic potential → neutralization degree
c) heat exchange with the beam surrounds
This work
Fully-electromagnetic 2-½ PIC simulations (BPIC code) including:
a) beam ionization by collision with background gas
b) background gas ionization by collision with beam ions and electrons
We study the parallel evolution of the temperature and neutralization:
1. Isolated beam
2. Beam interacting with a finite size plasma created by gas ionization
3. Beam interacting with a electron-source-like plasma
Isentropic process →
Electrons behave as an ideal gas under a adiabatic bidimensional compression →
2.5 MeV Xe+ , Ib=2.5 kA
rb0=5 cm, Lb=50 cm (8 ns)
Lf=3 m Isolated beam
Temperature evolution
Departures from 2D compression
Close the focal point:1) Large gradients of density and temperature
2) Electron temperature uncorrelated with density
3) Transfer of energy from radial to axial direction
Isolated beam
Neutralization
Good values for the neutralization can be obtained assuming:
a) infinite beam
b) electrons in thermal equilibrium
Assuming
→
Solutions of 1D Poisson-Boltzmann equation:
Isolated beam
Neutralization by gas ionization
Beam interacting with a finite size plasma
t<(σ ng vb )-1 Ne / Nb« 1 t>(σ ng vb )-1
Plasma and beam compete for picking-up electrons
+ gas density
+ neutralization
Compression overcomes flow-cooling only in the focal region
Temperature evolution
Heat transfer to the plasma tail
Beam interacting with a finite size plasma
More neutralization & less heating
Beam interacting with a e-source-like plasma
SummaryIsolated beam:• Isolated beam behaves as a 2D-adiabatic system.
• Neutralization values are close to infinite beam in thermal equilibrium.
• Departures from 2D compression only visible at the focal region.
Beam interacting with gas ionization plasma:• Neutralization degree proportional to background gas density for early
times and independent for later times due to plasma pick-up.
• Cooling by electron flow into the beam more significant than compression except in the focal region
• Heat transfer to the plasma tail reduces electron temperature inside the beam
Beam interacting with an external plasma:• Gas ionized tail close to an electron source improves beam neutralization
and reduces heating by compression
Neutralization
Initial evolution of temperature is determined by neutralization evolution
Long term neutralization t>(σ ng vb )-1
Short term neutralization t<(σ ng vb )-1 Ne « Nb
neutralization limit for interaction with a electron-source-like plasma
approximation for gas ionization plasma
Independent of gas density
Isolated beam