Highly ionized plasmas are being considered as a medium for
charge neutralizing heavy ion beams in order to focus beyond
the space-charge limit. Calculations suggest that plasma at
a density of 1–100 times the ion beam density and at a
length ∼0.1–2 m would be suitable for achieving a
high level of charge neutralization. An Electron Cyclotron
Resonance (ECR) source has been built at the Princeton Plasma
Physics Laboratory (PPPL) to support a joint Neutralized Transport
Experiment (NTX) at the Lawrence Berkeley National Laboratory
(LBNL) to study ion beam neutralization with plasma. The ECR
source operates at 13.6 MHz and with solenoid magnetic fields
of 1–10 gauss. The goal is to operate the source at pressures
∼10−6 Torr at full ionization. The initial
operation of the source has been at pressures of
10−4–10−1 Torr. Electron
densities in the range of 108 to 1011
cm−3 have been achieved. Low-pressure operation
is important to reduce ion beam ionization. A cusp magnetic
field has been installed to improve radial confinement and reduce
the field strength on the beam axis. In addition, axial confinement
is believed to be important to achieve lower-pressure operation.
To further improve breakdown at low pressure, a weak electron
source will be placed near the end of the ECR source. This article
also describes the wave damping mechanisms. At moderate pressures
(> 1 mTorr), the wave damping is collisional, and at low
pressures (< 1 mTorr) there is a distinct electron cyclotron
resonance.
Ferroelectric Plasma Source for Heavy Ion Beam Charge Neutralization