Characteristics of a RF-Driven Ion Source for a Neutron Generator Used For Associated Particle Imaging Page: 3 of 6
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delivered through a spiral, external planar antenna on
the back of the source.
As with all RF-driven sources, the plasma can
either be inductively or capacitively coupled,
depending on the power, pressure, frequency, and the
source material and geometry. Previous studies at
LBNL ,  have shown that sources that use
internal RF antennas and solenoidal external antennas
are more susceptible to capacitively coupling to the
metal walls and the antenna itself, resulting in a
weaker, less dense plasma. However, in the planar
antenna ion source configuration, the capacitive
coupling is limited between the antenna and the
plasma, making it much easier to operate the source in
the inductively coupled mode. Inductively coupled
plasma sources are preferred for the neutron generator
due to their high power efficiency and high plasma
density-the same reasons that make them widely
used in the semiconductor fabrication industry. In this
experiment, RF frequencies of 13.56 and 27.12 MHz
were used to generate the plasma, and both hydrogen
and deuterium have been used as the discharge gases.
The ion species, current density, electron temperature,
and electron density have all been measured for RF
input power ranging from 80 to 300 watts and source
pressures between 5 to 20 mT.
RESULTS AND DISCUSSION
Table 2 shows the ignition and operating pressures
of deuterium at both 13.56 and 27.12 MHz for a fixed
RF power of 100 watts.
Electron temperature vs. RF power (13.56 MHz)
TABLE 2. Comparison of 13.56 and 27.12 MHz RF
Frequency: 13.56 MHz 27.12 MHz
Ignition Pressure (for D2) 30 mT
Minimum Pressure (for D2) 8.5 mT
Ignition Pressure (for H2) 50 mT
Minimum Pressure (for H2) 12.5 mT
*Denotes plasma was capacitively coupled
Both the ignition and operating pressures are lower
for the 27.12 MHz than for the 13.56 MHz plasma, but
the plasma starts in capacitively coupled mode, and
does not transition into inductively coupled mode until
the power is increased to 300 watts. The RF input
power can be reduced after the plasma changes to
inductively coupled mode. The ignition and operating
pressures are lower for deuterium than for hydrogen,
due to the fact that the ion confinement time is longer
as a result of the larger deuterium mass.
The electron density and electron temperature have
been measured with an RF-compensated Langmuir
probe as a function of power for deuterium and
hydrogen at 13.56 MHz, as shown in Fig. 2a and 2b.
The electron density increases with power and
pressure, due to the increased ionization from more
frequent collisions, which occur at higher power and
greater number of neutrals, respectively. The electron
temperature decreases with RF power and pressure due
to the increase in ion and electron density because the
mean free path is lowered and subsequently does not
have enough time to accelerate to higher energies
before colliding with another particle. For deuterium,
electron and ion density are higher and the electron
temperature is lower compared to hydrogen as a result
of the mass difference.
Electron density vs. RF Power (13.56 MHz)
50 100 150 200 250 300 350 50 100 150 200 250 300 350
RF Power (watts) RF Power (watts)
FIGURE 2. The electron temperature (a) and electron density (b) as a function of input if power for deuterium and hydrogen.
It can be seen that the electron density increases with both the power and pressure. The electron density is also lower for
hydrogen, which is expected since the current density is also lower. The electron temperature decreases with increasing pressure
and also with increasing power. The difference in electron temperature between deuterium and hydrogen is negligible.
---- Deuterium (8 mT)
a) --- Deuterium (12 mT)
-4-- Deuterium (18 mT)
-K--- Hydrogen (15 mT)
---- Deuterium (8 mT)
-0-- Deuterium (12 mT)
-0- Deuterium (18 mT)
I --x--Hydrogen (12.5 mT)
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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/m1/3/: accessed March 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.