Characteristics of a RF-Driven Ion Source for a Neutron Generator Used For Associated Particle Imaging Page: 4 of 6
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The ion species of the plasma were analyzed using
a mass spectrometer that filtered each species
according to mass. A plot of the atomic fraction vs.
power and pressure for deuterium and hydrogen is
shown in Fig. 3.
Atomic ion fraction vs. RF power
0.85 . m ) -
---D1 (13.56 MHz,18.5 mT)
---H1 (27.12 MH z,10mT)
50 100 150 23 25 3 35
RF Power (watts)
FIGURE 3. A plot of the atomic ion fraction vs. power for
hydrogen and deuterium at both 13.56 and 27.12 MHz. It
can be seen that the 13.56 MHz rf produces 10-15% higher
atomic fraction than 27.12 MFDz at low powers, but at higher
powers theft dirnc bbecomes rroghl 5%.deedifferenc
decreases from about 7% at 100 watts to 3% at 300 watts.
From this, we can see that the atomic fraction
increases with increasing plasma density and decreases
with electron temperature. It may seem counter-
intuitive that a lower electron temperature would result
in higher atomic ion fraction. However, looking at
table 3, it can be seen that the dissociation energy of
H2 is around 10 eV and the ionization energy of H is
13.6 eV while the ionization energy of H is 15.6 eV.
This means that for partially ionized plasmas, an
increase in the electron temperature would suppress
the dissociation of H2 to H atoms and increase Hf* ion
donation. Therefore, the atomic ion fraction should
increase with increasing electron density and
decreasing electron temperature, which matches the
global model and simulation done in  and .
The highest atomic ion fraction can be achieved using
13.56 MHz with a deuterium fill gas. The atomic ion
fraction also increases with increased RF power and
slightly with pressure. However, due to the design of
the sealed neutron generator, the pressure needs to be
minimized, but the atomic ion fraction is still sufficient
at low pressures (>85%).
TABLE 3. Hydrogen Ion Formation Reactions.
Reactions Electron energy
H2 + e - H2++ 2e
H2 + e - 2H + e
H2 + H2 - H3+ H
0 eV (no electron needed)
The RF frequency used also has an effect on the
atomic fraction. For a RF frequency of 13.56 MHz,
the current density and the atomic ion fraction increase
greatly when the plasma is in the inductively coupled
mode, which is easily achieved at this frequency.
When the frequency was increased to 27.12 MHz, the
plasma could ignite at pressures of less than 10 mT,
but the plasma was capacitively coupled, and the
power must be increased to 200-300 Watts before the
transition to inductively coupled mode occurs and the
power can then be decreased. In the inductively
coupled mode, the plasma is denser, brighter, and has
low sheath voltages, leading to lower electron energies
in the plasma sheath. Capacitively coupled plasmas
are dimmer, less dense (by one or two orders of
magnitude), and have high sheath voltages which
cause greater electron energies in the sheath -.
Therefore, the source should be operated in the
inductively coupled regime because the plasma density
increases by an order of magnitude and the atomic ion
fraction increases by over 25% compared to the
capacitively coupled regime.
Since the final design of the neutron generator will
employ a D-T gas mixture in the ion source, hydrogen
and deuterium were tested to investigate the effect of
gas mixing on atomic fraction. While it is true that a
mixed D/T beam will create some neutrons from the
D-D and T-T reactions, the cross section for D-T is
over 300 times greater, and the impact of the D-D and
T-T neutrons is insignificant due to the lower cross
sections and lower neutron energies. The advantages
of using a mixed D/T beam and a self-loading target is
that the life-time will be longer than using a pure
deuterium beam with a pre-loaded tritium target. For
pure hydrogen gas, there are three possible ion species,
but with a deuterium and tritium mixture, there are 9
possible species, with 7 different masses. With a
hydrogen and deuterium mixture, there are 9 possible
species, and only 6 possible masses. Although this
poses a difficulty in determining the exact species of
all ions, the mass spectrum should identify two large
peaks at mass 1 and 2 and little to no peaks for masses
3 through 6. Fig. 4 shows the plot of the mass
spectrum of the deuterium-hydrogen gas mixture at
100 watts and 20 mT, indicating that the plasma was
dominated by masses 1 and 2, confirming that the high
atomic fraction remains for a gas mixture. Since the
amount of D2+ ions is very small, one can conclude
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Wu, Ying; Hurley, John P.; Ji, Qing; Kwan, Joe & Leung, Ka-Ngo. Characteristics of a RF-Driven Ion Source for a Neutron Generator Used For Associated Particle Imaging, article, August 8, 2008; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc1013252/m1/4/: accessed March 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.