Results on intense beam focusing and neutralization from the neutralized beam experiment Page: 2 of 7
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the beam space-charge potential to the beam velocity.
This condition limits the minimum residual space charge
potential to mevi2 . Previous neutralization
experiments [26, 28] have provided, to some degree,
confirmation of this limit.
Plasma neutralization in NTX was simulated with the
PIC code LSP [13-14]. The low emittance (-307t mm-mr)
of the NTX beam at the entrance to the neutralized region
allows for the beam to be focused to a small spot (1-2mm
radius). Several r-z LSP simulations were run using the
NTX geometry with a nominal 255-keV, 24-mA singly
charged potasium ion beam assuming the beam envelop is
circular. The beam enters the neutralization pipe (z=0)
with a 2-cm outer radius and a 20-milliradian convergence
angle. Figure 1 shows the beam envelope radius for 3
simulations with: perfect neutralization (ballistic), no
neutralization (vacuum), and a MEVVA source generated
plasma (plasma plug) descibed in next section with a
maximum 1010 cm 3 density. With no neutralization, the
simulation gives a 1.64cm radius at this distance. With
the perfect neutralization, we calculate yields a 1-mm
RMS spot at focus (z=100cm). Including the MEVVA
plasma yields a spot only slightly larger than ballistic
(1.35mm at z = 100cm). In this case, the plasma electrons
provide a source of co-moving electrons with a 96%
0 25 50 75 100
FIG 1. A comparison of beam envelop for simulations
with: perfect neutralization (the lower line), no
neutralization or vacuum (the top line) and MEVVA
source or plasma plug (middle line).
III. DESCRIPTION OF NTX BEAMLINE
NTX consists of three major sections: a potassium
source chamber , a magnetic transport section with
four pulsed quadrupoles , and a one-meter long
neutralization drift section with plasma plug . Figure
2 shows a sketch of the NTX beamline. A thorough
description of the design and characterization of this NTX
beam line has been submitted recently for publication.
We now describe the major sections of NTX.
A. Ion source
The K+ beam is produced on a standard hot-plate
source , with the perveance being determined by
passing the beam through a metal aperture after the diode.
Pulsed power is provided by a Marx generator that was
used in the Multiple Beam Test Experiment (MBE-4)
. A timed crowbar switch on NTX produces pulses
with 0.5 --- 1 ps rise time and a 10-ps "flat-top".
Quadrupoles Cathode arc RF plasma source
Source chamber Plasma plug Dansibo
m - 4-1 n-
Beam source Magnetic-transport section Neutralization drift section
FIG 2. A schematic of NTX beamline setup.
B. Magnetic Beam transport
The section consists of four pulsed quadrupole
magnets separated by short drift regions. The quadrupole
fields are chosen to obtain a beam with 1-m focal length
(20-mm radius and 20-mr convergence angle) at the
entrance to the neutralization region. The choice of a 60-
cm half-lattice period and 2.4-m total length is a scaled
version of a driver design.
C. Plasma source and focusing section
Figure 3 shows (a) schematic of a 1-m long
neutralization section indicated the location of the
different plasma sources (b) the neutralization section on
NTX and (c) the cathode arc plasma source. We now
present results using cathode arc plasma source referred to
as the MEVVA plasma plug throughout the article. The
plasma density of the MEVVA plasma plug can be
estimated by noting that the ion current is given generally
by j,=zen,v,, where j, is the ion current density, z is the
average charge state number (1.7), e is the elementary
charge, n, is the ion number density, and v, is the average
ion velocity (1.54x105 m/s) in the direction of the
collector, which is here identical with the plasma flow
velocity. With an area of collection of about 10-2 m2, one
obtains n, ; 1.8 x 1010 cm3 for the average plasma
density inside the metal shield at about 250 s after arc
triggering, at a pulse-forming network (PFN) charging
voltage of 2.0 keV. We find that the NTX cathode-arc
source produces plasmas with densities in the 101 - 1011
cm ranges and that the plasma density is proportional to
the discharge voltage up to 2.5 keV.
D. Optical imaging technique for beam profile
Non neutralized and neutralized beam were recorded
using modern optics. We have used glass and ceramic
(98% alumina) as scintillator materials. Charge
neutralization was provided by a high-transparency (80-
. . . . . . . . . . ji b
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Roy, P.K.; Yu, S.S.; Eylon, S.; Henestroza, E.; Anders, A.; Bieniosek, F.M. et al. Results on intense beam focusing and neutralization from the neutralized beam experiment, article, October 31, 2003; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc736749/m1/2/: accessed December 16, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.