Doppler-Shift Proton Fraction Measurement on a CW Proton Injector Page: 4 of 6
4 p.View a full description of this report.
Extracted Text
The following text was automatically extracted from the image on this page using optical character recognition software:
Doppler-shift Proton Fraction Measurement on a CW Proton Injector
J. H. Kamperschroer (General Atomics),
J. D. Sherman, T. J. Zaugg (Los Alamos National Laboratory),
A. H. Arvin, A. S. Bolt, and M. C. Richards (Savannah River Site)Abstract
A spectrometer/Optical Multi-channel Analyzer has been
used to measure the proton fraction of the cw proton
injector developed for the Accelerator Production of
Tritium (APT) and the Low Energy Demonstration
Accelerator (LEDA) at Los Alamos. This technique,
pioneered by the Lawrence Berkeley National
Laboratory (LBNL), was subsequently adopted by the
international fusion community as the standard for
determining the extracted ion fractions of neutral beam
injectors. Proton fractions up to 95 3% have been
measured on the LEDA injector. These values are in
good agreement with results obtained by magnetically
sweeping the ion beam, collimated by a slit, across a
Faraday cup. Since the velocity distribution of each
beam species is measured, it also can be used to
determine beam divergence. While divergence has not
yet been ascertained due to the wide slit widths in use,
non-Gaussian distributions have been observed during
operation above the design-matched perveance. An
additional feature is that the presence of extracted water
ions can be observed. During ion source conditioning at
75 kV, an extracted water fraction >30% was briefly
observed.
1. Introduction
A diagnostic developed by the magnetic fusion
energy community has been successfully utilized on the
LEDA injector test facility at Los Alamos[1]. The
technique was developed in the late 1970s at LBNL[2] as
a means of determining the composition of neutral beams
used to heat magnetic fusion plasmas. Extracted H+, H2+,
and H3+ interact with background gas to produce fast,
excited hydrogen atoms with energies of E, E/2, and E/3,
where E is the energy of the extracted ions. By
observing the beam at an angle relative to the direction
of propagation, the Doppler-shift separates the light from
the three species. The wavelength of the Doppler-shifted
light is X = X0 (1 - @ cos 0), where %0 is the unshifted
wavelength, P = v/c, and 0 is the angle between the
viewing line of sight and the beam's direction of
propagation. Beam composition is determined from the
quantity of light associated with each Doppler-shifted
line. Cross sections for Balmer-cc (Ha) production from
H+, H2+, and H3+ incident on hydrogen gas have been
measured[3], permitting a quantitative measure of the
beam composition. In addition to the Doppler shift dueto the differing $'s, the light from each species is
broadened due to the cos 0 term. For small beam
divergence, and neglecting the broadening of the
instrument, the line shape is a direct measure of the
velocity distribution. In principle, a quantitative measure
of the divergence can be made[4]. Collisions of extracted
water ions with the background gas produce excited
hydrogen atoms with energy E/18, where E is the
extraction potential. Water contamination becomes
apparent at a level of <1%
Experiments carried out at Los Alamos have been a
successful proof-of-principle test on a proton injector, even
though photon fluences from the injector are orders of
magnitude below that of large fusion ion sources. The
photon production rate is proportional to the beam current,
the background gas density, and the Ha production cross
sections. While the current densities of fusion and APT
ion sources are similar, fusion ion sources have much
larger extraction areas. Fusion ion source currents are of
order 100 A versus 100 mA for the LEDA/APT injector.
Another significant difference is the background gas
density. In neutral beam injectors, the goal is to convert
the extracted ions into neutrals by collisions with
background gas. For a proton injector, it is important to
avoid proton loss via charge exchange to H0. Near the ion
source, the background gas density is -10 times lower than
in a fusion neutral beam injector. The net result is that the
Ha production rate in a proton injector is -10 times lower
than in a fusion neutral beam injector.
Several factors favor the proton injector. The APT ion
source is cw compared to pulse-lengths of a few seconds
for fusion ion sources, long integrations are therefore
possible. Another advantage is that the neutralization and
dissociation of the extracted ion beam, that takes place in
the neutral beam injector before the beam reaches the
observation point, occurs to a much smaller degree in a
proton injector. The problem created by changing the state
of the beam is that the Ha production cross sections are
different for the daughters than for their parents.
Therefore, the composition of the beam at the observation
point must be known to deduce the composition at the
extraction plane. Due to the low gas pressure between the
ion source and observation point in a proton injector, a
beam composed of H+, H2 , and H3+ can be assumed.
Initial data collection occurred at 50 kV, during
experiments supporting a 1.25 MeV cw radio-frequency
quadrupole (RFQ)[5]. The RFQ for LEDA and APT has
been designed to accept a 75 keV proton beam. A small
amount of data was collected after the ion source was
regapped for operation at 75 keV.
Upcoming Pages
Here’s what’s next.
Search Inside
This report can be searched. Note: Results may vary based on the legibility of text within the document.
Tools / Downloads
Get a copy of this page or view the extracted text.
Citing and Sharing
Basic information for referencing this web page. We also provide extended guidance on usage rights, references, copying or embedding.
Reference the current page of this Report.
Kamperschroer, J. H.; Sherman, J. D.; Zaugg, T. J.; Arvin, A. H.; Bolt, A. S. & Richards, M. C. Doppler-Shift Proton Fraction Measurement on a CW Proton Injector, report, December 31, 1998; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc688132/m1/4/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.