High current density negative ion source for beam line transport study Page: 1 of 3
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FERMILAB-Conf-01/205 July 2001
HIGH CURRENT DENSITY NEGATIVE ION SOURCE
FOR BEAM LINE TRANSPORT STUDIES
Vadim Dudnikov, Charles W. Schmidt, Ray Hren and James Wendt
Fermi National Accelerator Laboratory, Batavia, IL 60510, USA
The Fermilab Electron Cooling Program requires a 20-
m solenoidal region to interact 8-GeV antiprotons with an
escorting beam of 4.3-MeV electrons to improve the
phase-space quality of the antiproton beam. The
solenoidal section with additional transport lines to take
and return a 0.5-A electron beam from an electrostatic
accelerator, for energy recovery, must be precisely
aligned and adjusted. For the initial setup and study, and
later testing of this line, a 12.4-keV H- beam can be used
to simulate the 4.3 MeV electron beam. For this purpose a
high-brightness H- ion source has been developed and
tested. The source, a semiplanatron type, with a hollow
cathode discharge and spherical cathode focusing of the
emitted ions to the emission aperture has given an
emission current density up to 0.7 A/cm2. Continuous
operation of 4 weeks has been demonstrated. Such an
optimized source could have many applications for
tandem accelerators, ion beam lithography and ion
For efficient cooling of antiprotons an escorting
electron beam should have very low angular spread,
Aa<0.08 mrad . The beam line optics should have low
aberrations to suppress any electron beam heating. To
detect the low level of aberration an ion beam with a
small emittance and high brightness is needed, as for ion
lithography . H- beams with high brightness have been
produced from pulsed Surface Plasma Sources (SPS) with
a charge-exchange conversion giving cooling to the ions
. For DC production of a negative ion beam with a high
emission current density, a SPS with a hollow cathode
discharge and spherical geometrical focusing of the
negative ions was developed . An H- beam with an
intensity 2.5 mA has been extracted from a 1mm-diameter
aperture. The corresponding emission current density is
0.3 A/cm2. Some further modifications of this type of SPS
were made and tested for use in such beam line
2 SPS CONFIGURATION
For field characterization of the transport bending
magnets a small ion beam is used and formed by two
collimators with a millimeter aperture separated by 3 m.
A similar set of apertures detects the exit beam position
and angle from the magnet. An ion beam from the source
following a solenoidal lens and mass analyzer needs to
* Work supported by the U.S. Department of Energy under contract
pass through a series of 1 mm apertures over a distance of
8 m and have an intensity higher than 1 nA for easy
detection. More detail of such a test system is discussed in
. In early tests a semiplanatron SPS similar to , built
in cooperation with BTG, was used. A basic configuration
of this SPS is shown in Fig.la. Some modifications are
presented in Fig. lb and ic.
5 4 1
Figure 1. Modifications of the Hollow Cathode
Discharge region in a DC SPS. a - perpendicular channel;
b - closed perpendicular channel; c - closed 45-degree
channel. (discharge plasma is red).
A glow discharge in the magnetic field is supported by
a voltage between the cathode (1) and anode (5) separated
by a ceramic insulator (4). The diameter of the cylindrical
cathode body is 16 mm. The discharge, localized in the
cylindrical channel of the hollow cathode (6), is 3 mm in
diameter. The working gas (H2) is delivered to the hollow
cathode region through channel (3) from the gas system.
The gas flow and pressure is controlled by a mechanical
leak. Cesium is delivered to the discharge region through
channel (2) from a small oven filled with pellets of
cesium chromate and titanium powder mixture. Heating
produces a pure cesium vapor from the mixture. The
plasma drifts in the crossed field (ExB) along the groove
(7) to the spherical surface of the emitter (9). The
direction of the plasma drift is determined by the direction
of the magnetic field. The emitter's surface (9) is
bombarded by positive ions and neutrals from the plasma.
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Dudnikov, Vadim & Wendt, Charles W Schmidt and James. High current density negative ion source for beam line transport study, article, July 25, 2001; Batavia, Illinois. (digital.library.unt.edu/ark:/67531/metadc722942/m1/1/: accessed November 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.