RF Ion Source-Driven IEC Design and Operation
G. H. Miley, Y. Yang, J. Webber, Y. Shaban, and H. Momota
Department of Nuclear, Plasma, and Radiological Engineering
Univ.of Illinois, Urbana-Champaign, 103 S. Goodwin, Urbana, IL 61801,USA, e-mail:ghmiley@uiuc.edu
Abstract. The next step needed to achieve higher neutron yields and improved neutron production efficiency with Inertial Electrostatic
Confinement (IEC) sources require operation with an external ion source so that the reaction chamber pressure is controlled separately for
the source pressure. This paper discusses the design and operation of a radiofrequency (RF) ion gun designed for this purpose.
I.
Introduction
The IEC (Inertial Electrostatic Confinement) concept has
been demonstrated to offer a very attractive compact MeV
neutron source in the 10 8-9 neutron/sec range1,2. Changes in
operation of the same unit can provide protons or intense keV
(a)
(b)
x-rays. In this case, the IEC replaced a Cf-252 source to
provide “on-off” capability. If the IEC yield can be increased
by several orders of magnitude, a number of exciting new
applications become practical. These include use as a neutron
source3, a proton beam source4 (both of which have a plethora
of uses in private industry, medicine, and international
security5), a power and/or propulsion system for spacecraft6,
Figure 1. (a) Photograph of the spherical unit in operation.
and a small teaching device for plasma physics7. The IEC for
The grid and dense core region, just visible in the front port
radioisotope production for medical applications holds promise
opening, are enlarged in the photograph in (b) to show the
8
for commercialization as well. A typical proton flux required
“star mode” ion beams passing through the grid openings.
11
2
for isotope production is ~ 10 ρ/cm -sec, achievable from use
3
9
of fuels such as D-He in the IEC .
This paper presents recent progress research using a unique
external ion source, ILLIBS (Illinois Ion Beam Source). ILLIBS employs a RF-driven plasma in a graded magnetic field
configuration. Use of an external source allows initiation of the plasma discharge below the normal (Paschen relation)
discharge break-down region where losses due to charge exchange are greatly reduced, improving the neutron production
efficiency and allowing higher yields. Wide ranges of sub-breakdown deuterium pressures (0.4 to 2 mTorr) were studied.
With 100 Watt input RF power, ILLIBS provided a high ion current extraction efficiency and a deuterium ion flux of 6 x 10 18
ions/(cm2-sec) at 65 mA with a well collimated beam diameter of ~3 mm. With the ion gun on, a neutron rate of 2 x 107 n/sec
was achieved with grid voltage and current at 75 kV and 15 mA respectively, at 1.2 mTorr. These results suggest that using
the D-T gas mixture and increasing the ion current in this unit, would give a neutron rate of 7.3 x 1012 n/sec at 1.2 mTorr, 75
kV and 1.5-A ion current.
The UIUC IEC uses a transparent cathode grid inside a vacuum vessel (anode) in a spherical configuration or alternately,
a hollow cathode-anode configuration in cylindrical geometry1,3,10. The spherical unit employed in present experiment is
illustrated in Fig. 1a. It used a single grid for simplicity, although multigrid versions offer somewhat better efficiency. The
ILLIBS RF ion source was connected to this unit (shown later in Fig. 4). A photograph of the spherical unit during operation
is shown in Fig. 1b (ion gun off). The potential field associated with this type of grid causes the “star mode” ion beams that
are clearly visible in the photo. This mode is advantageous since ion beams avoid grid collisions by passing through the
center of the grid openings. Wide ranges of geometric dimensions are possible. However, once the electrode spacing is
selected, without use of an external ion source, the voltage and pressure are fixed by the Paschen breakdown relation10.
The spherical unit offers a “point-type” source. The cylindrical version offers similar neutron rates, but provides a “linelike” source extending up to ~25 cm length. The latter is most advantageous for applications requiring uniform coverage of
broad surface areas, e.g. analysis of materials carried on a conveyor belt or for the suitcase inspection at airports. Another
important advantage of the IEC is that it can be converted to an attractive tunable x-ray source with minimum alteration of the
apparatus by reversing electrode polarities and adding electron emitters at the vessel ports 11.
II.
Pulsed Operation
Pulsing the IEC is attractive for higher neutron fluxes since at very high peak currents, the neutron yield scales as the as
the current squared3. Thus, the high peak currents obtained by pulsing provide efficient use of the input power while the
power-supply plus IEC chamber “package” can be quite compact. Both the spherical and cylindrical versions have been
operated with a capacitive- type pulsed power supply12. Pulse lengths of order of 50 μs were employed, slightly exceeding,
the ion cathode-anode recirculation time. In this mode, time-average neutron rates of ~ 108 D-D neutron/s were obtained at
about 50 Hz while the extension to even higher yields appears feasible, using the RIDO concept 13.
III.
IEC and RF Source Setup
The IEC employed here has a good vacuum pumping allowing < 10 7 torr prior to the deuterium fill. In order to handle
higher power levels, the cathode was constructed of a tungsten-rhenium alloy (uses a 75:25 percent of W to Re). The ILLIBS
ion source employs RF power (inductively coupled) to excite a deuterium gas fill (primarily driving electrons). The details of
ILLIBS are shown in Fig. 2. The antenna is made from ~ 4 meters length of coaxial insulator high voltage wire, wound
around a glass tube containing the fill gas. The RF signal (13.56 MHz) is applied to one end of the antenna while the other
end is grounded. The oscillating magnetic field created within the coil is directed along its axis and induces a vortex electric
field. Then, the negative potential on the IEC grid serves to extract ions from the RF discharge.
magnetic focusing
stainlesslens
steel
flange
ceramic
(insulator)
negative
potential
positive
part of vacuum
wall,
chamber
coaxial copper
resonator
D2 gas
feed
magnetic differential
coils
helical
antenna RF
generator
Figure 2. Typical setup of the Illinois Ion Beam Source, ILLIBS. The main
components are: magnetic-index dc coils, coaxial
cooper shielding, antenna, front magnetic focus coil, and floating exit nozzle. The three back coils are connected in series.
Figure 3. The ILLIBS Ion Source of Fig. 3 connected to the IEC.
IV.
Neutron Yield Studies
A series of RF gun experiments were conducted with the 55-cm diameter spherical IEC using a 10-cm inner cathode grid
with 98 opening holes. Each hole has an equal area of 3.75 cm2. A photo of the ILLIBS ion source connected to the
spherical IEC is shown in Fig. 3. To characterize this configuration, a series of neutron counts were obtained at different
pressures and cathode voltages. The RF (13.5 MHz) input power was fixed at ~ 100 W.
neutron rate per total input power
(n/J)
10 5
50 kV
60 kV
70 kV
80 kV
10
4
10 3
10 2
0.4
0.6
0.8
1.0
IEC pressure (mTorr)
1.2
Figure 4. Neutrons production efficiency as a function of pressure for different voltages. The
power input is the sum of the RF power, the inductive coupling power (91 W), the power supplied to
the magnets (21 W, giving 0.015 Tesla) and the product of VI on the grid.
In these experiments, the desired IEC chamber pressure and voltage (below breakdown levels) was first set prior to
extracting the deuterium ions from the ILLIBS source. The measured neutron rate per unit input power is plotted in Fig 4
versus the cathode voltage for different pressures. The net cathode current (total current reading with ion gun on) increases
with an increase of the cathode voltage at constant pressure, and with an increase of IEC chamber pressure at the constant
voltage. At full gun operation a D-D neutron yield, of 2 x108 n/sec would be achievable at 150 mA with cathode voltage at
75 kV. Maximum efficiencies of ~10 4 n/J were obtained. The fact that this occurs at higher pressures is deceptive since the
ion currents were low, and beam background fusion still dominated. At very high currents, where beam-beam fusion
dominates, lower pressures would be used, but Fig. 4 does not include this region.
V.
Multi Beam Mode
The Star mode microchannels shown earlier in Fig. 1b form under Pashen discharge conditions inside the IEC. With the
present operation (below breakdown) the incident ion beam initially forms a single ion beam inside the gird region. This
beam in effect replaces the star mode microchannels. Multi-ion beams are generated as pressure, voltage and magnetic fields
are increased. Then a local breakdown occurs within the ion beam path inside the grid pressure. The primary element in this
breakdown process is an electron avalanche. Electrons experience acceleration toward the wall of the IEC chamber (positive
potential). Meanwhile, the ions oscillate within the original ion beam volume. Because of the space-charge limitations,
newly generated ions within the original beam will tend to deflect from the initial beam and split into a secondary one. For
optimum operation, sufficiently high ion currents are needed to create beams within all grid holes, simulating Star mode
operation. For the present setup, it is estimated that such operation would require six RF guns of the type used here. These
guns would be located symmetrically around the IEC similar to the configuration Hirsch employed in early experiments 1.
VI.
Discussion and Conclusion
The ILLIBS RF ion source described here was found to be well suited for injection into the IEC. An ion flux of 6 x 1018
ions/(cm2-sec) was achieved at 65 mA with a beam diameter of 2.89 mm. The ion beam power extraction efficiency was
found to be ~21.7 (mA/cm2)/W, a very high value compared to a typical ion source.
Raw Neutron Production Rate vs. Total Input Power
Unaided SS
Thermal Ion Gun
Raw Neutron Production Rate (n/s)
2.50E+07
Electron Emitters
2.00E+07
1.2 mTorr (ILLIBS)
Power (Thermal Ion Gun)
Power (Electron Emitters)
1.50E+07
Power (1.2 mTorr (ILLIBS))
1.00E+07
5.00E+06
0.00E+00
0
500
1000
1500
2000
2500
Total Input Power (W)
Figure 5. Comparison of neutron production rate for various ion source modes. The ILLIBS offers the highest efficiency,
particularly at higher input power.
It is evident, from operation with the ion gun that an “auxiliary source driven discharge” mode. This mode occurs when the
chamber pressure is too low to allow a discharge at the cathode potential. Then when ions are injected into the vessel
from the ion gun, a discharge immediately forms with the major microchannel aligned with the axis of the gun. Once the gun
is turned off, this driven discharge is extinguished.
A wide range of deuterium pressures in the sub-breakdown region (0.4 to 2 mTorr) was examined. With a 100 Watt RF
input to the ion gun, a neutron rate of 2 x 10 7 n/sec was measured with the IEC running at 75 kV, 15 mA, 1.2 mTorr. A
neutron rate of 2 x 108 n/sec at 75 kV, 1.2 mTorr is achievable at full gun operation, i.e. at 150 mA. As these results
illustrate, the use of ILLIBS greatly enhances the reaction efficiency for the IEC, one of the main goals of this research. This
confirms the vision that ion injection can provide a route to higher-powered IEC units in the future. Thus, a “next-step” IEC
with neutron yields > 1012 n/sec from D-T fusion reactions appears promising provided a higher current ILLIBS can be built,
and/or more ILLIBS are used.
The benefits of RF ion injection are further illustrated in Fig. 5 where RF is compared to other plasma generation
methods. As shown, the neutron yield at a given power input (neutron production efficiency) is significantly higher with RF
ion injection as other modes of IEC operation such as electron emitters and thermal ion guns. The ILLIBS gun enables IEC
operation over a wide range of deuterium pressures that had not been previously studied.
In conclusion the gun-injected IEC opens the route to significantly higher neutron yields. Such a device, if achieved,
would further expand the potential areas for application for IEC-based neutron sources. New applications such as medical
isotope production would then become attractive since these units could provide localized isotope facilities near the medical
facilities using them. The IEC appears to have a cost advantage, which would make such distributed facilities feasible and
attractive.
VII.
Acknowledgements
This study was supported under NASA grant # NAS8-01010. Vital contributions from J. Nadler, Y. Gu, B Jurczyk, M.
Coventry, L. Wu, and H. Momota are gratefully acknowledged.
VIII.
References
1. G. H. MILEY, J. JAVEDANI, R. NEBEL, J. NADLER, Y. GU, A. SATSANGI, AND P. HECK, “An Inertial
Electrostatic Confinement Neutron/Proton Source." Proc. of Third International Conference on Dense Z-pinches, eds.
Malcolm Haines and Andrew Knight, AIP Conference Proceedings 299, AIP Press, 675-689, (1994).
2. J. SVED, “The Commercial IEC Portable Neutron Source,” Proc. of Trans. of the ANS, 77, 504, (1997).
3. G. H. MILEY, “A Novel 2.5-MeV D-D Neutron Source,” Journal of Brachytherapy International, 1, No. 1, p. 111-121,
(1997).
4. NIETO, M., H. MOMOTA, AND G.H. MILEY, “Use of an IEC Device as a Proton Beam Source,” Proc. of ICONE 8,
Eighth Intern. Conf. on Nuclear Eng., Baltimore, MD, (2000).
5. G.H. MILEY AND L. WU, “IEC-based Neutron Generator for Security Inspection System”, ANS Summer Meeting, San
Diego, CA, (2003).
6. W. E. HAMMOND, M. COVENTRY, J. HANSON, I. HRBUD, G.H. MILEY, and J. NADLER, “IEC fusion: The
future power and propulsion system for space,” Proc. of AIP, V 504 (1), p 1572-1580, (2000).
7. G.H. MILEY, “The IEC Device as a Low Cost Student Experiment with a Burning Fusion Plasma,” Proc. of the Am.
Physical Society, 46, No. 2 (2001).
8. G.L KULCINSKI and J.F. SANTARIUS, “New Opportunities For Fusion In The 21 st Century-Advanced Fuels”, Fusion
Tech., 39, 480-485, (2001).
9. R.P. ASHLEY, G.L. KULCINSKI, J.F. SANTARIUS, S.K. MURALI, G. PIEFER, AND R. RADEL, “Steady-State D3
He Proton Production in an IEC Fusion Device,” Fusion Technology, 39, 546-551, (2001).
10. G. H. MILEY, H. MOMOTA, Y. SHABAN, H. HORA, “Discharge Characteristics of the Spherical Inertial Electrostatic
Confinement (IEC) Device,” IEEE Trans. on Plasma Science, 25, 4, 733-739, (1997).
11. Y. Gu and G.H. Miley, "Spherical IEC Device as a Tunable X-ray Source," Bult. APS, 11, p 1851 (1995).
12. Y. GU, P. HECK, AND G. MILEY, "Ion Focus Via Microchannels in Spherical Inertial-Electrostatic Confinement and
Its Pulsed Experimental Results," 1995 IEEE Intern. Conf. on Plasma Science, IEEE Conf. Rec. 95CH35796, 266-267
(1995).
13. B. JURCZYK, Y. GU, G. MILEY, “Resonant Ion Driven Oscillation (RIDO) Concept,” American Physical Society, 42,
No. 10, p. 1818, (1997).