First results from a double Vlasov model for negative ion extraction from volume sources - the possibility of an enhanced transverse space charge limit Page: 4 of 19
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FIRST RESULTS FROM A DOUBLE VLASOV
MODEL FOR NEGATIVE ION EXRACTION
FROM VOLUME SOURCES -
The Possibility of an Enhanced
Transverse Space Charge Limit
J. H. Whealton, D. K. Olsen, and R. J. Raridon
Oak Ridge National Laboratory
Abstract. A new negative ion source extraction model has been
formulated and implemented which explicitly considers the motion of
positive ions and the volume generation of negative ions. It is found
that (1) for high-beam currents, the beam current is limited by a
transverse space charge limit, meaning that an increase in negative ion
density at the extraction sheath will result in a lower beam current (this
result is universally observed at high beam currents); (2) there is a
saddle point with a potential barrier preventing most volume produced
negative ions from being extracted (the combination of 1 and 2
indicates that most of the negative ions being created do not find their
way into the beam); (3) introduction of cesium may cause, most
importantly, an increase in the transverse space charge limit (there is
an abundance of experimental data supporting this effect); (4) cesium
may also result in an increase in the fraction of volume produced
negative ions which are extracted; (5) cesium may also result in a
reduction of extracted electrons by dint of a less negative bias on the
plasma electrode with respect to the adjacent plasma, thus allowing the
transverse space charge limit budget to be taken up wholly by the ions.
(The combination of 3-5 represents the way an actual increase in the
beam current can be achieved); (6) a strong ion time scale sheath
instability due to violation of Bohm criteria produces an anomalous ion
temperature that increases with beam current routinely seen in
experiments; and (7) introduction of cesium may result in a reduction in
this instability. These insights may lead to improvements in volume
negative ion sources, and the most important finding of an increased
Research sponsored by the LDRD Program of Oak Ridge National Laboratory managed
by Lockheed Martin Energy Research Corp. for the U.S. Department of Energy under
Contract No. DE-AC05-960R22464.
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Whealton, J.H.; Olsen, D.K. & Raridon, R.J. First results from a double Vlasov model for negative ion extraction from volume sources - the possibility of an enhanced transverse space charge limit, article, December 1, 1997; Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc693320/m1/4/: accessed March 28, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.