- Email: [email protected]
Accelerator plasma-target-based fusion neutron source
Fusion Engineering and Design 41 (1998) 461 – 467
Accelerator plasma-target-based fusion neutron source G.H. Miley a, Y. Gu a, J. DeMora a, M. Ohnishi b,* b
a Uni6ersity of Illinois, Fusion Studies Laboratory, Urbana, IL 61801, USA Institute of Ad6anced Energy, Kyoto Uni6ersity, Gokasho, Uji, Kyoto 611, Japan
Abstract The University of Illinois inertial electrostatic confinement (IEC) device provides 107 2.5-MeV D – D neutrons per second (n s − 1), when operated with a steady-state deuterium discharge at 70 kV (G.H. Miley et al., Inertial electrostatic confinement neutron/proton source, in: M. Haines, A. Knight (Eds.), 3rd Int. Conf. Dense Z-pinches, AIP Conf. Proc. 299, AIP Press, New York, 1994, pp. 675 – 689). Being compact and lightweight, the IEC potentially represents an attractive portable neutron source for activation analysis applications (R.A. Anderl et al., Development of an IEC neutron source for NDE, 16th IEEE/NPSS Symp. Fusion Engineering, IEEE, Piscataway, NJ, 1996, pp. 1482–1485). The plasma discharge in the IEC is unique, using a spherical grid in a spherical vacuum vessel with the discharge formed between the grid and the vessel wall, while the cathode grid also serves to extract high-energy ions. Two key features of the IEC discharge physics are discussed: (1) the formation of ion ‘microchannels’ that carry the main ion flow through grid openings; and (2) the potential well structure formed in the dense central core. © 1998 Elsevier Science S.A. All rights reserved.
1. Introduction Ion-injected inertial electrostatic confinement (EC), originally proposed by P. Farnsworth [1], the inventor of electronic television, was first studied experimentally by Hirsch in the 1960s [2]. Recently, University of Illinois researchers developed a unique single grid version of the IEC and found an unusual ‘Star’ mode discharge involving ion beams (‘microchannels’) that pass through grid openings to the center of the spherical discharge. There are alternate types of IECs—e.g. Hirsch [2] used ion-gun injectors, Barnes et al. used a triple-grid design [3], and Bussard and * Corresponding author. Tel.: + 81 774 383441; fax: + 81 774 383449; e-mail: [email protected]
Krall have proposed a hybrid magnetic–electrostatic version [4]—that generally operate at lower pressures due to added electron injection, but much of the basic physics carries over [5]. The present paper discusses basic characteristics of this single-grid IEC.
2. Spherical IEC neutron source The UI spherical IEC devices employ a spherical vacuum vessel containing a concentric of spherical wire grid. A high fusion rate is generated by intersecting ion beams and the associated potential well structure in a dense plasma region created in the center of the device (Fig. 1). This configuration provides a simple, rugged, low-cost
0920-3796/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0920-3796(98)00148-3
462
G.H. Miley et al. / Fusion Engineering and Design 41 (1998) 461–467
fusion neutron source, operating in the 106 D–D n s − 1 range (or 108 D – T n s − 1 range: the scaling from D–D to D – T simply follows the ratio of the respective fusion cross-sections). Since the D–D reaction results in a release of a MeV proton at the same time a neutron is born, the IEC can also serve as a proton source. Indeed, the extension to an intense 14 MeV proton source is also possible using D– 3He reactions. One key feature which distinguishes the present IEC design from earlier IEC devices is the use of a single grid to produce a gaseous discharge for ion production. The grid simultaneously serves to extract high-energy ions from the discharge. The IECGD avoids the complication of the ion guns (see Hirsch’s design [2]), the goal being the simplest and cheapest design for low-level neutron production. Two devices, IEC-A and IEC-B, were employed in the studies. The IEC-A vessel was fabricated from 48 cm 304 stainless steel. The other vessel, IEC-B, has a 61 cm diameter. The results reported here are for IEC-A, which incorporates nearly the optimum size and design elements for a portable neutron/proton source. Various cathode grids were installed to study the effects of grid size. Typically, cathode grids of 7.6 and 3.75 cm diameter were used in IEC-A using various sizes of T302/304 stainless steel wire: 0.80, 1.04 and 1.30 mm in diameter. All of the grids had a geometric transparency of about 80–97%, with an estimated deviation of less than 3% from exact sphericity. Prior to operation, the IEC is conditioned to remove absorbed gas impurities, using an extended glow discharge operation. The vessel is then pumped down, back-filled to about 5– 20 mtorr, and a 10 – 80 kV electric potential is applied to the cathode to initiate the glow discharge. The voltage and pressure are related by the traditional Paschen voltage breakdown relation, where the voltage is a function of a pressure– length product (the distance from the grid to the vessel wall versus the grid diameter) [6,7]. The diagnostics employed include pressure sensors and current and voltage meters on the cathode power supply. A BF3 proportional coun-
Fig. 1. Basic components of an IECGD.
ter, placed 40 cm from the IEC chamber and surrounded by a 9 cm thick polyethylene cylinder (for thermalization of neutrons), is used to measure neutron source strength.
2.1. Modes of operation Glow discharge operation of the IEC is categorized by three distinct discharge ‘modes’: Star, Central Spot, and Halo [8], descriptive of the
Fig. 2. Photograph of the Star mode. Beamlets of ions converge in the center of the chamber, creating a high fusion power density ‘core’.
G.H. Miley et al. / Fusion Engineering and Design 41 (1998) 461–467
Fig. 3. Neutron output for IEC-A with voltage for several ion currents. Count rates are in units of 105 n s − 1.
visual appearances of the light emitted from the discharges. All three modes are reproducible and stable; each is associated with a different potential well structure, hence neutron production rate. The Star mode was used extensively in experiments described here. It is distinguished by microchannels or ‘spokes’ appearing as beams radiating outward from a bright center spot (Fig. 2). This mode is very efficient for neutron production, since the large effective grid transparency allows numerous passes of ions through the center spot before being intercepted by the grid. Equally important, grid sputtering is greatly reduced, which significantly increases the device lifetime. The Star mode is typically obtained at lower operating pressures (B10 mtorr) and higher voltages (\ 30 kV), using a carefully formed grid with good sphericity and high transparency ( \ 95%).
463
Fig. 4. Striking voltage Vs, vs. pd for solid electrode and for alternate electrodes with holes at various orientations. The solid line is based on a correlation from Miley et al. and Hochberg [6,7].
2.2. Steady-state neutron source strengths Typical plots of measured neutron source strength versus cathode current are shown for IEC-A in Fig. 3 for different cathode voltages and currents. The neutron yield increases linearly with current and scales strongly with voltage, in agreement with the ‘beam-background’ model described by Anderl et al. [8]. The scaling with voltage roughly corresponds to the variation of
Table 1 Spherical cathode S used in the IEC experiments Device/grid
IEC-A A1 A2 IEC-B B1 B2
Diameter (cm)
3.7 7.5 15 15
Construction (geometric transparency)
Wire grid, 85% Wire grid, 90% Wire grid, 89% Hollow sphere, 0%
Fig. 5. A SIMION calculation of ion paths in the IEC. The trajectory ‘bundles’ resemble microchannels while a dense ‘core’ is formed by intercepting trajectories at the center of the sphere (the shaded outer region defines the chamber wall).
464
G.H. Miley et al. / Fusion Engineering and Design 41 (1998) 461–467
mized device, designed for production of about 106 D–D (or 108 D–T) n s − 1, are summarized in Table 1. Recent studies involve development of a higher yield model using pulsed power techniques to increase the ion current.
3. Microchannel observations
Fig. 6. SIMION plots of ion trajectories where ions born near the grid fail to form discrete microchannels.
the fusion cross-section with energy. For practical applications, operation is limited to approximately 70 kV. The maximum current is then set by the power heating limits for the grid. As seen from Fig. 3, the ‘desired’ source rate of 106 D–D n s − 1 is achieved at 70 kV with a current of about 15 mA. Based on this data, parameters for an opti-
Fig. 7. A comparison of radial proton density profiles at currents of 10, 20, 40, 60 and 80 mA. The cathode voltage is constant at 15 kV.
This unique microchannel discharge phenomena (Fig. 2), discovered at the University of Illinois, is termed the ‘Star’ discharge mode. These microchannels radiate both outward from the cathode to the anode, and inward to the bright central plasma ‘core’. This implies that the intense microchannel discharge occurs between the anode and the local plasma volume encompassed by holes in the cathode, i.e. that microchannels seek the regions of highest grid transparency. Thus, the discharge characteristics should depend on an ‘effective’ grid transparency (t), as opposed to the geometric transparency. Consequently, the microchannel discharge depends mainly on the potential profile in the grid openings, i.e. on the grid pattern design, rather than on the fractional surface area of the openings. That is, t accounts for the higher transmission probability of ions in the microchannels versus a uniform ‘sheet’-like flow. These microchannels are not initially formed by constricting electromotive forces. Rather, as illustrated by computer simulations described later, they are created through a geometric ‘self-selection’ process. That is, ions born with trajectories hitting grids within a few passes are quickly eliminated, while those passing repeatedly through grid holes survive, leading to increased ionization (‘births’) along these favored trajectories. In order to test this effective transparency hypothesis for microchannels, the cathode was covered with aluminum foil except for two 2 cm diameter holes located 180° apart. Spokes were formed to emanate from both holes, the discharge characteristics for this configuration became similar to those of a transparent cathode, despite the very low geometric transparency of the two-hole cathode. The foil was then replaced and two new holes were cut 90° apart. While the geometric
G.H. Miley et al. / Fusion Engineering and Design 41 (1998) 461–467
transparencies of these two cathodes are equal, due to the formation of microchannels, the effective transparency, t, of the first grid is high, while that of the second is essentially zero. Although the striking voltages (Vs) for this second configuration were well-defined, once initiated, the discharges were unstable and appeared qualitatively different from discharges using transparent cathodes. As shown in Fig. 4, the values of Vs for the 180° hole cathode fall close to that for a solid electrode, considerably higher than for the 90° hole case. In summary, the discharges using a cathode with high effective transparency, such as the 180° hole type cathode, behave much like transparenttype wire-cathode discharges, even if the geometric transparencies differ greatly. In sharp contrast, the discharges using the cathode with low effective transparency behave as solid-cathode discharges. Hence, the discharge characteristics are primarily a function of the effective, rather than the geometric transparency of the cathode.
4. Simulation of the microchannels Simulations of the Star mode have also been made using SIMION, an electric field and ion trajectory program (D. Nahl, INEEL, personal communication, 1995). Electrodes in SIMION are defined point-by-point, and then the program solves Laplace’s equation using a finite-difference method to determine the electric potentials created by the electrodes. These potentials then determine the ion trajectories. This code does not determine the electric potentials produced by a plasma itself. However, numerical estimates of such plasma effects (mainly manifested by distortion of the potential surface in grid holes) indicate the change in microchannel trajectories (versus those closer to the grid which result in losses anyway) is small. Charge–exchange effects are not included in SIMION either. The trajectories calculated thus represent the combination of fast ion and neutral transport, but neglect losses due to fast neutral escape. While this omission introduces an error in the number of ion recirculations, i.e. calculated in ion losses, the general features of trajectory of the
465
paths are not drastically affected. Fig. 5 shows a sample plot of the ion trajectories in the singlegrid IEC. Here the tightly packed region of ion trajectories in the hollow cathode region closely resembles the ion microchannels of the Star mode.
4.1. Microchannel formation The graphical output of SIMION adds further insight into the microchannel formation [9]. The cathode grid produces concave equipotential surfaces in the grid opening, whose shapes are due to the potential of the individual grid wires. These surfaces defocus ions as they approach the grid from the outside. However, if an ion is not excessively defocused so it passes to the opposite side, it approaches convex potential surfaces which refocus the ion toward the center of the microchannel as it exits the grid. The net result of this defocusing/focusing phenomena is that ions that do not initially collide with the grid wires, and do not pass close enough to the grid wire that their trajectories are overly distorted, continue to travel within the microchannel trajectory. Hence, ions near the center of the opening are the only ones that survive multiple passes (‘self-selection’). Subsequent ionization is enhanced along microchannels, further amplifying the effect. The ‘self-selection’ mechanism also serves to narrow the ion beam energy spread. Microchannels are preferentially filled with ions born near the chamber wall (anode), where the electric potential is greater and the perturbation of the electric field due to the grid wires is small. Conversely, if an ion starts near the grid, where a smaller electric potential accelerates it toward the center, the ion may have insufficient momentum as it passes through the grid to avoid significant attraction by the wire, hence a deflection of its trajectory occurs. Since such ions are rapidly lost, the majority of ions in the microchannel have higher energies. Thus, this mechanism results in a relatively narrow energy spread for the ions in the microchannels. Figs. 5 and 6 illustrate the significance of the starting (birth) point of the ions. The ions shown in Fig. 5 start near the wall and form microchannels similar to the experimental observations. In
466
G.H. Miley et al. / Fusion Engineering and Design 41 (1998) 461–467
contrast, in Fig. 6, the ions start close to the grid. In the latter case, the ions do not form separate and distinct microchannels; instead they travel in large, loop-like orbits and are quickly lost by collision with the grids. These trajectories are similar to those obtained by Moses [10]. His simulations concentrated on ions born close to the grid, which resulted in their de-focusing. Consequently, he suggested a second focusing grid was needed to maintain the microchannels. However, since the present calculations with ions born closer to the wall reproduce the experimentally observed microchannels, the complication of a second, refocusing grid, while an interesting option, does not appear necessary. A second fundamental physics aspect of the IEC involves electric potential surfaces formed in the dense core plasma created by the intersection of the microchannels. As originally postulated by Farnsworth [1] and Hirsch [2], this structure can ultimately form a trap for ions, increasing their confinement time. Experimental studies of the structure are discussed next.
5. Potential well structure measured by proton detection For high fusion rates in an IEC system, it is necessary to achieve a potential well structure in the center core that can trap ions [1,2]. Such a structure was originally termed ‘Poissors’ due to the spatially oscillatory nature of the potential in an ideal system [1]. In experimental IEC devices such as IEC-A or IEC-B, the spread of ion energies combined with collisions will dampen the oscillating structure, resulting in a double potential well structure formed by a virtual anode and a virtual cathode [11]. There is, however, considerable debate about whether thermalization of the ion and electron populations will destroy the well [12]. While Hirsch [2] reported a potential well measurement earlier, his results have been disputed. Consequently, measurements were carried out in the present IEC device using a unique collimated proton detector to confirm existence of the well. A microchannel collimator with an array of micrometer-diameter channels was used to obtain the necessary spatial resolution [13].
The collimated proton detector allows the measurement of the proton generation rate from the proton branch in the D–D fusion as a function of space. To examine the well structure, the proton rate is pivotally scanned across the center of the dense core at a fixed voltage and current. The profiles at different voltages and currents are systematically measured. From this information, using the Abel inversion technique, radial proton density profiles are solved to provide a clear view of the evolving profiles, among which we find the characteristic signature of the profile that marks the existence of a double potential well. A composite of the profiles for various currents at a fixed voltage (15 kV) is shown in Fig. 7. This figure shows the comparative magnitudes among the different currents and presents a clear view of the formation of the potential structure with increasing current (a threshold for the formation of the double well is predicted to occur for a perveance of 2.2 mA kV − 3/2 [14]). The evolution shown is consistent with the trends predicted by the simulations with a one-dimensional Vlasov–Poisson solver [11]. This result is also qualitatively in agreement with Hirsch’s earlier measurement [2]. The ‘definitive signal’ for formation of a double well is the sharp increase in proton production at the center line (r=0) which signifies a high fusion reaction rate due to trapped ions in this region. The formation of this well is initiated at 15 kV, 10 mA, while a primitive double well appears at 15 kV, 20 mA. Then, as the current is increased further, the features of the double well become exceedingly prominent. These results indicate a threshold perveance value of about 1.5 mA kV − 2/3, slightly lower than that predicted by Gu and Miley [14].
6. Summary and conclusions The experiments and simulations described here, employing the unique Star discharge mode involving microchannels discovered at the University of Illinois, have enhanced understanding of the physics of the single-grid type IEC. Specifically, it has been shown that discharges in the Star mode are governed by the effective, rather
G.H. Miley et al. / Fusion Engineering and Design 41 (1998) 461–467
than the geometric grid transparency. Further, the formation of microchannels is explained by a ‘natural’ selection process associated with defocusing of non-channeled ions by the grid potential surface distortion and the favoring of higher energy ions born near the anode. Present single-grid IEC devices, capable of providing a 107 n s − 1 steady-state source with a simple, rugged, low-capital-cost device, are of strong interest for various non-destructive evaluation and neutron activation analysis applications [8]. Further study of both the physics and engineering of these devices is necessary, however, to insure long lifetime operation with minimum maintenance. The extension to higher neutron yields is also under study to further expand the range of applications.
[4]
[5]
[6]
[7]
[8]
[9]
Acknowledgements This work was supported under INEEL contract c CC-S-622904-002-C and DASA contract c 50513-729-41706629. The assistance of other members of the IEC group at Illinois is appreciated. Helpful discussions with R. Hirsch (R.H. Assoc.), R.W. Bussard (MC2 Corp.), J. Hartwell (INEL), R. Nebel and D. Barnes (LANL), are gratefully recognized.
[10]
[11]
[12]
[13]
References [1] P. Farnsworth, US Patent No. 3,258,402 (June 28, 1966). [2] R.L. Hirsch, Inertial-electrostatic confinement of ionized fusion gases, J. Appl. Phys. 38 (1967) 4522. [3] D.C. Barnes, R.A. Nebel, et al., Inertial electrostatic
.
[14]
467
confinement experiments at low working pressure, Rep. NTTR-101, Nambe Tech Corporation, Santa Fe, NM, 1995. R.W. Bussard, N.A. Krall, Inherent characteristics of fusion power systems: physics, engineering and economics, Fusion Technol. 26 (1994) 1326 – 1336. G.H. Miley, J. Javedani, R. Nebel, J. Nadler, Y. Gu, A. Satsangi, P. Heck, Inertial electrostatic confinement neutron/proton source, in: M. Haines, A. Knight (Eds.), 3rd Int. Conf. Dense Z-pinches, AIP Conf. Proc. 299, AIP Press, New York, 1994, pp. 675 – 689. G.H. Miley, Y. Gu, J.M. DeMora, R.A. Stubbers, T.A. Hochberg, J.H. Nadler, R.A. Anderl, Discharge characteristics of the spherical IEC device, IEEE Trans. Plasma Sci., to be published. T.A. Hochberg, M.Sc. Thesis, Dept. Nuclear Engineering, University of Illinois at Urbana – Champaign, IL, USA, 1992. R.A. Anderl, J.K. Hartwell, J.H. Nadler, J.M. DeMora, R.A. Stubbers, Development of an IEC neutron source for NDE, 16th IEEE/NPSS Symp. Fusion Engineering, IEEE, Piscataway, NJ, 1996, pp. 1482 – 1485. J.M. DeMora, R.A. Stubbers, R.A. Anderl, Study of ion microchannels and IEC grid effects using the SIMION code, 16th IEEE/NPSS Symp. Fusion Engineering, IEEE, Piscataway, NJ, 1996, pp. 1486 – 1489. R. Moses, Enhancing IEC performance with focusing grids, Physics of Spherical Continuous Inertial Fusion Workshop, LANL, Santa Fe, NM, January 11 – 13, 1995. I. Tzonev, G.H. Miley, R.A Nadler, A computational study of the convergence of large angular momentum, high current ion beams in an IEC device, Int. Conf. Phenomena in Ionized Gases XXII, vol. 4, 1995, pp. 197 – 198. W.M. Nevins, Can inertial electrostatic confinement work beyond the ion – ion collisional time scale?, Phys. Plasmas 2 (10) (1995) 3804 – 3819. Y. Gu, Measurement of proton density profiles in the spherical IEC fusion device, Ph.D. Thesis, Dept. Electrical and Computer Engineering, University of Illinois, IL, USA, 1997. Y. Gu, G.H. Miley, Pulsed IEC neutron generation, Proc. 10th IEEE Int. Pulsed Power Conf., Albuquerque, NM, USA, July 10 – 13, 1995, P3-55.
.
Accelerator plasma-target-based fusion neutron source G.H. Miley a, Y. Gu a, J. DeMora a, M. Ohnishi b,* b
a Uni6ersity of Illinois, Fusion Studies Laboratory, Urbana, IL 61801, USA Institute of Ad6anced Energy, Kyoto Uni6ersity, Gokasho, Uji, Kyoto 611, Japan
Abstract The University of Illinois inertial electrostatic confinement (IEC) device provides 107 2.5-MeV D – D neutrons per second (n s − 1), when operated with a steady-state deuterium discharge at 70 kV (G.H. Miley et al., Inertial electrostatic confinement neutron/proton source, in: M. Haines, A. Knight (Eds.), 3rd Int. Conf. Dense Z-pinches, AIP Conf. Proc. 299, AIP Press, New York, 1994, pp. 675 – 689). Being compact and lightweight, the IEC potentially represents an attractive portable neutron source for activation analysis applications (R.A. Anderl et al., Development of an IEC neutron source for NDE, 16th IEEE/NPSS Symp. Fusion Engineering, IEEE, Piscataway, NJ, 1996, pp. 1482–1485). The plasma discharge in the IEC is unique, using a spherical grid in a spherical vacuum vessel with the discharge formed between the grid and the vessel wall, while the cathode grid also serves to extract high-energy ions. Two key features of the IEC discharge physics are discussed: (1) the formation of ion ‘microchannels’ that carry the main ion flow through grid openings; and (2) the potential well structure formed in the dense central core. © 1998 Elsevier Science S.A. All rights reserved.
1. Introduction Ion-injected inertial electrostatic confinement (EC), originally proposed by P. Farnsworth [1], the inventor of electronic television, was first studied experimentally by Hirsch in the 1960s [2]. Recently, University of Illinois researchers developed a unique single grid version of the IEC and found an unusual ‘Star’ mode discharge involving ion beams (‘microchannels’) that pass through grid openings to the center of the spherical discharge. There are alternate types of IECs—e.g. Hirsch [2] used ion-gun injectors, Barnes et al. used a triple-grid design [3], and Bussard and * Corresponding author. Tel.: + 81 774 383441; fax: + 81 774 383449; e-mail: [email protected]
Krall have proposed a hybrid magnetic–electrostatic version [4]—that generally operate at lower pressures due to added electron injection, but much of the basic physics carries over [5]. The present paper discusses basic characteristics of this single-grid IEC.
2. Spherical IEC neutron source The UI spherical IEC devices employ a spherical vacuum vessel containing a concentric of spherical wire grid. A high fusion rate is generated by intersecting ion beams and the associated potential well structure in a dense plasma region created in the center of the device (Fig. 1). This configuration provides a simple, rugged, low-cost
0920-3796/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0920-3796(98)00148-3
462
G.H. Miley et al. / Fusion Engineering and Design 41 (1998) 461–467
fusion neutron source, operating in the 106 D–D n s − 1 range (or 108 D – T n s − 1 range: the scaling from D–D to D – T simply follows the ratio of the respective fusion cross-sections). Since the D–D reaction results in a release of a MeV proton at the same time a neutron is born, the IEC can also serve as a proton source. Indeed, the extension to an intense 14 MeV proton source is also possible using D– 3He reactions. One key feature which distinguishes the present IEC design from earlier IEC devices is the use of a single grid to produce a gaseous discharge for ion production. The grid simultaneously serves to extract high-energy ions from the discharge. The IECGD avoids the complication of the ion guns (see Hirsch’s design [2]), the goal being the simplest and cheapest design for low-level neutron production. Two devices, IEC-A and IEC-B, were employed in the studies. The IEC-A vessel was fabricated from 48 cm 304 stainless steel. The other vessel, IEC-B, has a 61 cm diameter. The results reported here are for IEC-A, which incorporates nearly the optimum size and design elements for a portable neutron/proton source. Various cathode grids were installed to study the effects of grid size. Typically, cathode grids of 7.6 and 3.75 cm diameter were used in IEC-A using various sizes of T302/304 stainless steel wire: 0.80, 1.04 and 1.30 mm in diameter. All of the grids had a geometric transparency of about 80–97%, with an estimated deviation of less than 3% from exact sphericity. Prior to operation, the IEC is conditioned to remove absorbed gas impurities, using an extended glow discharge operation. The vessel is then pumped down, back-filled to about 5– 20 mtorr, and a 10 – 80 kV electric potential is applied to the cathode to initiate the glow discharge. The voltage and pressure are related by the traditional Paschen voltage breakdown relation, where the voltage is a function of a pressure– length product (the distance from the grid to the vessel wall versus the grid diameter) [6,7]. The diagnostics employed include pressure sensors and current and voltage meters on the cathode power supply. A BF3 proportional coun-
Fig. 1. Basic components of an IECGD.
ter, placed 40 cm from the IEC chamber and surrounded by a 9 cm thick polyethylene cylinder (for thermalization of neutrons), is used to measure neutron source strength.
2.1. Modes of operation Glow discharge operation of the IEC is categorized by three distinct discharge ‘modes’: Star, Central Spot, and Halo [8], descriptive of the
Fig. 2. Photograph of the Star mode. Beamlets of ions converge in the center of the chamber, creating a high fusion power density ‘core’.
G.H. Miley et al. / Fusion Engineering and Design 41 (1998) 461–467
Fig. 3. Neutron output for IEC-A with voltage for several ion currents. Count rates are in units of 105 n s − 1.
visual appearances of the light emitted from the discharges. All three modes are reproducible and stable; each is associated with a different potential well structure, hence neutron production rate. The Star mode was used extensively in experiments described here. It is distinguished by microchannels or ‘spokes’ appearing as beams radiating outward from a bright center spot (Fig. 2). This mode is very efficient for neutron production, since the large effective grid transparency allows numerous passes of ions through the center spot before being intercepted by the grid. Equally important, grid sputtering is greatly reduced, which significantly increases the device lifetime. The Star mode is typically obtained at lower operating pressures (B10 mtorr) and higher voltages (\ 30 kV), using a carefully formed grid with good sphericity and high transparency ( \ 95%).
463
Fig. 4. Striking voltage Vs, vs. pd for solid electrode and for alternate electrodes with holes at various orientations. The solid line is based on a correlation from Miley et al. and Hochberg [6,7].
2.2. Steady-state neutron source strengths Typical plots of measured neutron source strength versus cathode current are shown for IEC-A in Fig. 3 for different cathode voltages and currents. The neutron yield increases linearly with current and scales strongly with voltage, in agreement with the ‘beam-background’ model described by Anderl et al. [8]. The scaling with voltage roughly corresponds to the variation of
Table 1 Spherical cathode S used in the IEC experiments Device/grid
IEC-A A1 A2 IEC-B B1 B2
Diameter (cm)
3.7 7.5 15 15
Construction (geometric transparency)
Wire grid, 85% Wire grid, 90% Wire grid, 89% Hollow sphere, 0%
Fig. 5. A SIMION calculation of ion paths in the IEC. The trajectory ‘bundles’ resemble microchannels while a dense ‘core’ is formed by intercepting trajectories at the center of the sphere (the shaded outer region defines the chamber wall).
464
G.H. Miley et al. / Fusion Engineering and Design 41 (1998) 461–467
mized device, designed for production of about 106 D–D (or 108 D–T) n s − 1, are summarized in Table 1. Recent studies involve development of a higher yield model using pulsed power techniques to increase the ion current.
3. Microchannel observations
Fig. 6. SIMION plots of ion trajectories where ions born near the grid fail to form discrete microchannels.
the fusion cross-section with energy. For practical applications, operation is limited to approximately 70 kV. The maximum current is then set by the power heating limits for the grid. As seen from Fig. 3, the ‘desired’ source rate of 106 D–D n s − 1 is achieved at 70 kV with a current of about 15 mA. Based on this data, parameters for an opti-
Fig. 7. A comparison of radial proton density profiles at currents of 10, 20, 40, 60 and 80 mA. The cathode voltage is constant at 15 kV.
This unique microchannel discharge phenomena (Fig. 2), discovered at the University of Illinois, is termed the ‘Star’ discharge mode. These microchannels radiate both outward from the cathode to the anode, and inward to the bright central plasma ‘core’. This implies that the intense microchannel discharge occurs between the anode and the local plasma volume encompassed by holes in the cathode, i.e. that microchannels seek the regions of highest grid transparency. Thus, the discharge characteristics should depend on an ‘effective’ grid transparency (t), as opposed to the geometric transparency. Consequently, the microchannel discharge depends mainly on the potential profile in the grid openings, i.e. on the grid pattern design, rather than on the fractional surface area of the openings. That is, t accounts for the higher transmission probability of ions in the microchannels versus a uniform ‘sheet’-like flow. These microchannels are not initially formed by constricting electromotive forces. Rather, as illustrated by computer simulations described later, they are created through a geometric ‘self-selection’ process. That is, ions born with trajectories hitting grids within a few passes are quickly eliminated, while those passing repeatedly through grid holes survive, leading to increased ionization (‘births’) along these favored trajectories. In order to test this effective transparency hypothesis for microchannels, the cathode was covered with aluminum foil except for two 2 cm diameter holes located 180° apart. Spokes were formed to emanate from both holes, the discharge characteristics for this configuration became similar to those of a transparent cathode, despite the very low geometric transparency of the two-hole cathode. The foil was then replaced and two new holes were cut 90° apart. While the geometric
G.H. Miley et al. / Fusion Engineering and Design 41 (1998) 461–467
transparencies of these two cathodes are equal, due to the formation of microchannels, the effective transparency, t, of the first grid is high, while that of the second is essentially zero. Although the striking voltages (Vs) for this second configuration were well-defined, once initiated, the discharges were unstable and appeared qualitatively different from discharges using transparent cathodes. As shown in Fig. 4, the values of Vs for the 180° hole cathode fall close to that for a solid electrode, considerably higher than for the 90° hole case. In summary, the discharges using a cathode with high effective transparency, such as the 180° hole type cathode, behave much like transparenttype wire-cathode discharges, even if the geometric transparencies differ greatly. In sharp contrast, the discharges using the cathode with low effective transparency behave as solid-cathode discharges. Hence, the discharge characteristics are primarily a function of the effective, rather than the geometric transparency of the cathode.
4. Simulation of the microchannels Simulations of the Star mode have also been made using SIMION, an electric field and ion trajectory program (D. Nahl, INEEL, personal communication, 1995). Electrodes in SIMION are defined point-by-point, and then the program solves Laplace’s equation using a finite-difference method to determine the electric potentials created by the electrodes. These potentials then determine the ion trajectories. This code does not determine the electric potentials produced by a plasma itself. However, numerical estimates of such plasma effects (mainly manifested by distortion of the potential surface in grid holes) indicate the change in microchannel trajectories (versus those closer to the grid which result in losses anyway) is small. Charge–exchange effects are not included in SIMION either. The trajectories calculated thus represent the combination of fast ion and neutral transport, but neglect losses due to fast neutral escape. While this omission introduces an error in the number of ion recirculations, i.e. calculated in ion losses, the general features of trajectory of the
465
paths are not drastically affected. Fig. 5 shows a sample plot of the ion trajectories in the singlegrid IEC. Here the tightly packed region of ion trajectories in the hollow cathode region closely resembles the ion microchannels of the Star mode.
4.1. Microchannel formation The graphical output of SIMION adds further insight into the microchannel formation [9]. The cathode grid produces concave equipotential surfaces in the grid opening, whose shapes are due to the potential of the individual grid wires. These surfaces defocus ions as they approach the grid from the outside. However, if an ion is not excessively defocused so it passes to the opposite side, it approaches convex potential surfaces which refocus the ion toward the center of the microchannel as it exits the grid. The net result of this defocusing/focusing phenomena is that ions that do not initially collide with the grid wires, and do not pass close enough to the grid wire that their trajectories are overly distorted, continue to travel within the microchannel trajectory. Hence, ions near the center of the opening are the only ones that survive multiple passes (‘self-selection’). Subsequent ionization is enhanced along microchannels, further amplifying the effect. The ‘self-selection’ mechanism also serves to narrow the ion beam energy spread. Microchannels are preferentially filled with ions born near the chamber wall (anode), where the electric potential is greater and the perturbation of the electric field due to the grid wires is small. Conversely, if an ion starts near the grid, where a smaller electric potential accelerates it toward the center, the ion may have insufficient momentum as it passes through the grid to avoid significant attraction by the wire, hence a deflection of its trajectory occurs. Since such ions are rapidly lost, the majority of ions in the microchannel have higher energies. Thus, this mechanism results in a relatively narrow energy spread for the ions in the microchannels. Figs. 5 and 6 illustrate the significance of the starting (birth) point of the ions. The ions shown in Fig. 5 start near the wall and form microchannels similar to the experimental observations. In
466
G.H. Miley et al. / Fusion Engineering and Design 41 (1998) 461–467
contrast, in Fig. 6, the ions start close to the grid. In the latter case, the ions do not form separate and distinct microchannels; instead they travel in large, loop-like orbits and are quickly lost by collision with the grids. These trajectories are similar to those obtained by Moses [10]. His simulations concentrated on ions born close to the grid, which resulted in their de-focusing. Consequently, he suggested a second focusing grid was needed to maintain the microchannels. However, since the present calculations with ions born closer to the wall reproduce the experimentally observed microchannels, the complication of a second, refocusing grid, while an interesting option, does not appear necessary. A second fundamental physics aspect of the IEC involves electric potential surfaces formed in the dense core plasma created by the intersection of the microchannels. As originally postulated by Farnsworth [1] and Hirsch [2], this structure can ultimately form a trap for ions, increasing their confinement time. Experimental studies of the structure are discussed next.
5. Potential well structure measured by proton detection For high fusion rates in an IEC system, it is necessary to achieve a potential well structure in the center core that can trap ions [1,2]. Such a structure was originally termed ‘Poissors’ due to the spatially oscillatory nature of the potential in an ideal system [1]. In experimental IEC devices such as IEC-A or IEC-B, the spread of ion energies combined with collisions will dampen the oscillating structure, resulting in a double potential well structure formed by a virtual anode and a virtual cathode [11]. There is, however, considerable debate about whether thermalization of the ion and electron populations will destroy the well [12]. While Hirsch [2] reported a potential well measurement earlier, his results have been disputed. Consequently, measurements were carried out in the present IEC device using a unique collimated proton detector to confirm existence of the well. A microchannel collimator with an array of micrometer-diameter channels was used to obtain the necessary spatial resolution [13].
The collimated proton detector allows the measurement of the proton generation rate from the proton branch in the D–D fusion as a function of space. To examine the well structure, the proton rate is pivotally scanned across the center of the dense core at a fixed voltage and current. The profiles at different voltages and currents are systematically measured. From this information, using the Abel inversion technique, radial proton density profiles are solved to provide a clear view of the evolving profiles, among which we find the characteristic signature of the profile that marks the existence of a double potential well. A composite of the profiles for various currents at a fixed voltage (15 kV) is shown in Fig. 7. This figure shows the comparative magnitudes among the different currents and presents a clear view of the formation of the potential structure with increasing current (a threshold for the formation of the double well is predicted to occur for a perveance of 2.2 mA kV − 3/2 [14]). The evolution shown is consistent with the trends predicted by the simulations with a one-dimensional Vlasov–Poisson solver [11]. This result is also qualitatively in agreement with Hirsch’s earlier measurement [2]. The ‘definitive signal’ for formation of a double well is the sharp increase in proton production at the center line (r=0) which signifies a high fusion reaction rate due to trapped ions in this region. The formation of this well is initiated at 15 kV, 10 mA, while a primitive double well appears at 15 kV, 20 mA. Then, as the current is increased further, the features of the double well become exceedingly prominent. These results indicate a threshold perveance value of about 1.5 mA kV − 2/3, slightly lower than that predicted by Gu and Miley [14].
6. Summary and conclusions The experiments and simulations described here, employing the unique Star discharge mode involving microchannels discovered at the University of Illinois, have enhanced understanding of the physics of the single-grid type IEC. Specifically, it has been shown that discharges in the Star mode are governed by the effective, rather
G.H. Miley et al. / Fusion Engineering and Design 41 (1998) 461–467
than the geometric grid transparency. Further, the formation of microchannels is explained by a ‘natural’ selection process associated with defocusing of non-channeled ions by the grid potential surface distortion and the favoring of higher energy ions born near the anode. Present single-grid IEC devices, capable of providing a 107 n s − 1 steady-state source with a simple, rugged, low-capital-cost device, are of strong interest for various non-destructive evaluation and neutron activation analysis applications [8]. Further study of both the physics and engineering of these devices is necessary, however, to insure long lifetime operation with minimum maintenance. The extension to higher neutron yields is also under study to further expand the range of applications.
[4]
[5]
[6]
[7]
[8]
[9]
Acknowledgements This work was supported under INEEL contract c CC-S-622904-002-C and DASA contract c 50513-729-41706629. The assistance of other members of the IEC group at Illinois is appreciated. Helpful discussions with R. Hirsch (R.H. Assoc.), R.W. Bussard (MC2 Corp.), J. Hartwell (INEL), R. Nebel and D. Barnes (LANL), are gratefully recognized.
[10]
[11]
[12]
[13]
References [1] P. Farnsworth, US Patent No. 3,258,402 (June 28, 1966). [2] R.L. Hirsch, Inertial-electrostatic confinement of ionized fusion gases, J. Appl. Phys. 38 (1967) 4522. [3] D.C. Barnes, R.A. Nebel, et al., Inertial electrostatic
.
[14]
467
confinement experiments at low working pressure, Rep. NTTR-101, Nambe Tech Corporation, Santa Fe, NM, 1995. R.W. Bussard, N.A. Krall, Inherent characteristics of fusion power systems: physics, engineering and economics, Fusion Technol. 26 (1994) 1326 – 1336. G.H. Miley, J. Javedani, R. Nebel, J. Nadler, Y. Gu, A. Satsangi, P. Heck, Inertial electrostatic confinement neutron/proton source, in: M. Haines, A. Knight (Eds.), 3rd Int. Conf. Dense Z-pinches, AIP Conf. Proc. 299, AIP Press, New York, 1994, pp. 675 – 689. G.H. Miley, Y. Gu, J.M. DeMora, R.A. Stubbers, T.A. Hochberg, J.H. Nadler, R.A. Anderl, Discharge characteristics of the spherical IEC device, IEEE Trans. Plasma Sci., to be published. T.A. Hochberg, M.Sc. Thesis, Dept. Nuclear Engineering, University of Illinois at Urbana – Champaign, IL, USA, 1992. R.A. Anderl, J.K. Hartwell, J.H. Nadler, J.M. DeMora, R.A. Stubbers, Development of an IEC neutron source for NDE, 16th IEEE/NPSS Symp. Fusion Engineering, IEEE, Piscataway, NJ, 1996, pp. 1482 – 1485. J.M. DeMora, R.A. Stubbers, R.A. Anderl, Study of ion microchannels and IEC grid effects using the SIMION code, 16th IEEE/NPSS Symp. Fusion Engineering, IEEE, Piscataway, NJ, 1996, pp. 1486 – 1489. R. Moses, Enhancing IEC performance with focusing grids, Physics of Spherical Continuous Inertial Fusion Workshop, LANL, Santa Fe, NM, January 11 – 13, 1995. I. Tzonev, G.H. Miley, R.A Nadler, A computational study of the convergence of large angular momentum, high current ion beams in an IEC device, Int. Conf. Phenomena in Ionized Gases XXII, vol. 4, 1995, pp. 197 – 198. W.M. Nevins, Can inertial electrostatic confinement work beyond the ion – ion collisional time scale?, Phys. Plasmas 2 (10) (1995) 3804 – 3819. Y. Gu, Measurement of proton density profiles in the spherical IEC fusion device, Ph.D. Thesis, Dept. Electrical and Computer Engineering, University of Illinois, IL, USA, 1997. Y. Gu, G.H. Miley, Pulsed IEC neutron generation, Proc. 10th IEEE Int. Pulsed Power Conf., Albuquerque, NM, USA, July 10 – 13, 1995, P3-55.
.