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Application of a magnetized coaxial plasma gun for formation of a high-beta field-reversed configuration
Fusion Engineering and Design 81 (2006) 2843–2847
Application of a magnetized coaxial plasma gun for formation of a high-beta field-reversed configuration T. Nishida, T. Kiguchi, T. Asai ∗ , T. Takahashi, Y. Matsuzawa, T. Okano, Y. Nogi College of Science and Technology, Nihon University, 1-8 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-8308, Japan Available online 17 August 2006
Abstract We have tested a field-reversed configuration (FRC) formation with a spheromak injection for the first time. In this method, initial pre-ionized plasma is injected as a magnetized spheromak-like plasmoid into the discharge chamber prior to main field reversal. The FRC plasma with an electron density of 1.3 × 1021 m−3 , a separatrix radius of 0.04 m and a plasma length of 0.8 m was produced successfully in initial background plasma of about 1.6 × 1019 m−3 by spheromak injection. The density is about one third of the conventional formed by the z-ionized method. © 2006 Elsevier B.V. All rights reserved. Keywords: Field-reversed configuration (FRC); Magnetized coaxial plasma gun (MCPG); Spheromak; Field-reversed theta-pinch (FRTP)
1. Introduction A field-reversed configuration (FRC) is a highly elongated compact toroid which is formed without a toroidal field and can confine the plasma at an extremely high β value [1]. The plasma consists of two distinct regions: a closed-field-line torus and openfield-line sheath plasma. This open-field-line structure is regarded as a natural divertor. FRC plasma thus has significant potential as a future reactor core. Moreover, due to the above-described physical characteristics, FRC plasma is a focus of research interest for its potential use as a fueling tool for a large ∗ Corresponding author. Tel.: +81 3 3259 0894; fax: +81 3 3259 0896. E-mail address: [email protected] (T. Asai).
0920-3796/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2006.07.033
fusion reactor core like ITER [2]. However, the control of plasma parameters, especially density, is problematic when using the conventional of FRC formation method with gas-fill and z or theta pre-ionization. zpre-ionization (z-PI) is the most commonly used and effective ionization method, especially in gas-fill systems [3]. However, comparatively higher filling gas pressure is required for good initiation of discharge and thus is not applicable to a gas-puff system. Moreover, the particle retention factor (ratio of total particle inventory at equilibrium phase to that of the initial filled gas) is approximately 40%, and the neutral particle density is increased in the closed field region. The theta preionization (θ-PI) method uses electrodeless ionization which has the advantages of permitting the application of a system with gas puffing and reduces contamination by impurities [4]. However, the energy conversion effi-
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T. Nishida et al. / Fusion Engineering and Design 81 (2006) 2843–2847
ciency is lower than for electrode discharge. Dilution of the preheating plasma by compression of the bias field was developed as a control method of the electron density, and a lower density FRC was successfully formed, which is required as a target of neutral beam injection in the conceptual FRC reactor design of ARTEMIS [5]. Although the core plasma density is one order lower than without the dilution plasma, the neutral particle density at the open-field-line region is relatively increased. Furthermore, in FRC injection to a reactor core, trailing neutral gas has an adverse effect because it risks cooling the target plasma. Therefore, the development of a new formation technique without gas-fill and with higher ionization efficiency (i.e., reducing excess gas which is not used for FRC formation) is an important issue at the current stage of FRC studies. In this study, a magnetized coaxial plasma gun (MCPG) is employed to initiate the initial plasma in the form of a magnetized plasmoid of a spheromak which is then injected into the main discharge chamber prior to the FRC formation. An FRC is then formed using the field-reversed theta-pinch (FRTP) method. The spheromak is formed in a small region of the MCPG. Only the ionized-closed-field part is accelerated by the Lorenz force and injected into the main chamber. The injected spheromak has a magnetic configuration itself; therefore, the initial plasma is positioned in the center region of the chamber and isolated from the chamber wall. This feature has the potential to reduce the background neutral gas pressure and wall-plasma interaction in the pre-ionized plasma. Moreover, in the conventional method, the breakdown neutral density determines the lower limit of the plasma density. In contrast, the initial plasma is generated in an MCPG and then injected and decompressed into a lower density in the present technique, potentially enabling exploration of the lowdensity parameter region of FRCs.
2. Experimental setup and diagnostics The FRC plasma is produced in a 1.5 m-long thetapinch coil with a radius of 0.17 m on NUCTE-III [5]. A vacuum vessel made of a transparent quartz tube 2 m in length and 0.256 m in diameter is connected through a stainless steel tube to the vacuum system. The MCPG is mounted at the left end of the vacuum vessel, as shown in Fig. 1.
Fig. 1. A schematic view of magnetized coaxial plasma gun and a part of NUCTE-III device.
The MCPG consists of a formation and an acceleration region. The formation region has an inner electrode and an outer electrode. A fast solenoid valve for hydrogen gas puffing and a gun bias coil are mounted on the middle of the outer electrode. A gun bias field with a flux of 0.074 mWb and a duration time of 200 s is driven by a 400 F capacitor with a charging voltage of 200 V. The electrodes are extended to the end of the theta-pinch coil and work as an accelerator. By the application of a 22.5 kA-gun current and a pulse width of 15 s between the inner and outer electrodes, which is driven by a 20 kV-15 F capacitor, magnetized plasmoids are produced and rapidly accelerated. Plasmoids with a velocity of about 40 km/s are injected into the vacuum vessel filled with a bias field of 0.032 T and a raising time of 90 s. The strength of the bias field also controls the process of plasmoid injection. A main reversed confinement field with a strength of 0.44 T, a 3 s rising and a 120 s decay time are applied when the bias field strength rises to its maximum value. The discharge time sequence of the MCPG pre-heating method is shown in Fig. 2. The behavior of the injected plasmoid and the separatrix radius and length of the formed FRC plasma are measured by a one-turn loop and magnetic probe array. The electron density at z = −0.44 m is diagnosed using a 3.39 m helium–neon interferometer. Forty-five optical detectors are used to observe the emission of light from the pre-ionized and FRC plas-
T. Nishida et al. / Fusion Engineering and Design 81 (2006) 2843–2847
Fig. 2. Operation sequence of FRC formation using the MCPG method.
mas. Each channel of an optical detector consists of a collimator, optical fiber, optical band pass filter and photomultiplier. A convex-plane lens is employed on the collimator and can observe line-integrated emissions along the optical axis of the collimator. Observed wavelengths are λ = 550 ± 5 nm (bremsstrahlung) and λ = 656 ± 5 nm (Balmer series-␣) which is selected by the band path optical filter. The magnetic structures of the injected plasmoids, preheating and FRC plasma are observed using a twodirection (z, θ) magnetic probe array which is inserted from a branch pipe of the vacuum vessel at z = −0.33 m.
3. Experimental results 3.1. Pre-heating plasma using the MCPG method In the MCPG method, an initial pre-heating plasma is produced by injection of the magnetized plasmoid into the vacuum vessel into which 2 mTorr deuterium gas is injected and to which a bias field of 0.032 T is applied. The pressure of the puffing gas is equivalent to about 5 mTorr of gas-fill. Fig. 3 shows the magnetic structure of a plasmoid as measured by the twodirectional internal magnetic probe array. The toroidal field reversed at about y = 0.20 m and the poloidal field
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Fig. 3. Toroidal magnetic field Bt and poloidal magnetic field Bp profiles of magnetized plasmoid formed by the MCPG.
has a peak at y = 0.20 m, confirming that a spheromaklike magnetic structure is produced by the MCPG. The separatrix radius and the length of the plasmoid are about 0.06 m and 0.40 m, respectively. Average density of the plasmoid is about 2.5 × 1020 m−3 from Figs. 3 and 6(b). Fig. 4 shows time evolution of the axial radiation profiles, which are emitted from the initial plasma produced by the MCPG method. The solid and the dotted lines are the line-integrated radiation intensity of bremsstrahlung and the Balmer-␣ line spectrum, respectively. The strong emission of the Balmer-␣, which indicates plasma formation, appears at the front of the plasmoid. This indicates that the injected plasmoid ionizes filled neutral particles while penetrating into the vacuum vessel. The intensity of the bremsstrahlung, which depends on the electron density, increases at the back side of the plasmoid. We assume that the plasmoid is translated while the plasmoid dissipates the magnetic and kinetic energy, and the dissipated energy is used for the ionization and heating of the initial plasma. It seems that the plasmoid is annihilated at around the center of the theta-pinch coil. The initial plasma is then formed only on the left side of the theta-pinch coil. The incident range of the plasmoid with an injection velocity of 40 km/s is about 0.5–0.6 m. The density of the initial plasma for FRC formation becomes about 1.6 × 1019 m−3 .
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Fig. 5. Time evolution of poloidal magnetic field profile.
Fig. 4. Time evolution of axial emission profiles. Solid line and closed circles indicates bremsstrahlung and dotted line and open circles Balmer-␣.
3.2. FRC plasma formation by MCPG method Fig. 5 shows time evolution of the y-profile of the Bp field measured using the internal magnetic probe array. The reversed compression field is applied at t = 0. The poloidal magnetic field is reversed till 4 s. It is thus confirmed that FRC plasma is produced by the MCPG method. The liftoff flux is about 0.4 mWb and 25% of the initial bias flux. The separatrix radius at 2 s is about 0.04 m as estimated by the poloidal field profile. Time evolution of the separatrix radius at z = 0.45 m and line integrated electron density at z = −0.43 m are indicated in parts (a) and (b) of Fig. 6, respectively. The FRC plasma is compressed radially up to 2 s. The separatrix radius becomes about 0.04 m. It can be seen from the time evolution of the axial profile of the separatrix radius, shown in Fig. 7, that the radial compression is uniform over the initial plasma formed region and the plasma length is about 0.8 m. The plasma
Fig. 6. Time evolution of separatrix radius at z = −0.43 m (a) and line integrated electron density ne dl (b).
T. Nishida et al. / Fusion Engineering and Design 81 (2006) 2843–2847
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4. Summary We have proposed a new startup method for FRC plasma using a magnetized coaxial plasma gun. Preliminary experiments were performed on the NUCTE-III device. As a result of injection of a spheromak-like plasmoid with radius 0.06 m, length 0.40 m, density 2.5 × 1020 m−3 and velocity 40 km/s, a preheating plasma for FRC formation was produced, with density of about 1.6 × 1019 m−3 . The formation of an FRC plasma was confirmed by application of a reversed field. The configuration was maintained for about 10 s. The density was about 1.3 × 1021 m−3 , approximately one third of that achieved using the z-discharge pre-ionized method. We confirm the capability of low-density FRC formation using the MCPG method. To form a long-lived FRC plasma with higher density and low background neutral particles, further experimental investigation is necessary to optimize the MCPG system and operation conditions.
References
Fig. 7. Time evolution of axial separatrix radius profile.
density becomes about 1.3 × 1021 m−3 and is about one third that of an FRC plasma formed by the z-discharge method. The plasma then shrinks axially and disrupts at 10 s.
[1] M. Tuszewski, Field reversed configurations, Nucl. Fusion 28 (1988) 2033–3092. [2] A.L. Hoffman, P. Gurevich, J. Grossnickle, J.T. Slough, Inductive field-reversed configuration accelerator for tokamak fueling, Fusion Sci. Technol. 36 (1999) 109–125. [3] J.T. Slough, A.L. Hoffman, Experimental study of the formation of field-reversed configurations employing high-order multipole fields, Phys. Fluids B 2 (1990) 797–808. [4] M. Tuszewski, D.P. Taggart, R.E. Chrien, D.J. Rej, R.E. Siemon, B.L. Wright, Axial dynamics in field reversed theta pinche II: stability, Phys. Fluids B 3 (1991) 2856–2870. [5] Y. Ohkuma, M. Urano, M. Nakamura, Y. Narushima, T. Takahashi, Y. Nogi, Production of a low density field reversed configuration plasma, Nucl. Fusion 38 (1998) 1501–1509.
Application of a magnetized coaxial plasma gun for formation of a high-beta field-reversed configuration T. Nishida, T. Kiguchi, T. Asai ∗ , T. Takahashi, Y. Matsuzawa, T. Okano, Y. Nogi College of Science and Technology, Nihon University, 1-8 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-8308, Japan Available online 17 August 2006
Abstract We have tested a field-reversed configuration (FRC) formation with a spheromak injection for the first time. In this method, initial pre-ionized plasma is injected as a magnetized spheromak-like plasmoid into the discharge chamber prior to main field reversal. The FRC plasma with an electron density of 1.3 × 1021 m−3 , a separatrix radius of 0.04 m and a plasma length of 0.8 m was produced successfully in initial background plasma of about 1.6 × 1019 m−3 by spheromak injection. The density is about one third of the conventional formed by the z-ionized method. © 2006 Elsevier B.V. All rights reserved. Keywords: Field-reversed configuration (FRC); Magnetized coaxial plasma gun (MCPG); Spheromak; Field-reversed theta-pinch (FRTP)
1. Introduction A field-reversed configuration (FRC) is a highly elongated compact toroid which is formed without a toroidal field and can confine the plasma at an extremely high β value [1]. The plasma consists of two distinct regions: a closed-field-line torus and openfield-line sheath plasma. This open-field-line structure is regarded as a natural divertor. FRC plasma thus has significant potential as a future reactor core. Moreover, due to the above-described physical characteristics, FRC plasma is a focus of research interest for its potential use as a fueling tool for a large ∗ Corresponding author. Tel.: +81 3 3259 0894; fax: +81 3 3259 0896. E-mail address: [email protected] (T. Asai).
0920-3796/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2006.07.033
fusion reactor core like ITER [2]. However, the control of plasma parameters, especially density, is problematic when using the conventional of FRC formation method with gas-fill and z or theta pre-ionization. zpre-ionization (z-PI) is the most commonly used and effective ionization method, especially in gas-fill systems [3]. However, comparatively higher filling gas pressure is required for good initiation of discharge and thus is not applicable to a gas-puff system. Moreover, the particle retention factor (ratio of total particle inventory at equilibrium phase to that of the initial filled gas) is approximately 40%, and the neutral particle density is increased in the closed field region. The theta preionization (θ-PI) method uses electrodeless ionization which has the advantages of permitting the application of a system with gas puffing and reduces contamination by impurities [4]. However, the energy conversion effi-
2844
T. Nishida et al. / Fusion Engineering and Design 81 (2006) 2843–2847
ciency is lower than for electrode discharge. Dilution of the preheating plasma by compression of the bias field was developed as a control method of the electron density, and a lower density FRC was successfully formed, which is required as a target of neutral beam injection in the conceptual FRC reactor design of ARTEMIS [5]. Although the core plasma density is one order lower than without the dilution plasma, the neutral particle density at the open-field-line region is relatively increased. Furthermore, in FRC injection to a reactor core, trailing neutral gas has an adverse effect because it risks cooling the target plasma. Therefore, the development of a new formation technique without gas-fill and with higher ionization efficiency (i.e., reducing excess gas which is not used for FRC formation) is an important issue at the current stage of FRC studies. In this study, a magnetized coaxial plasma gun (MCPG) is employed to initiate the initial plasma in the form of a magnetized plasmoid of a spheromak which is then injected into the main discharge chamber prior to the FRC formation. An FRC is then formed using the field-reversed theta-pinch (FRTP) method. The spheromak is formed in a small region of the MCPG. Only the ionized-closed-field part is accelerated by the Lorenz force and injected into the main chamber. The injected spheromak has a magnetic configuration itself; therefore, the initial plasma is positioned in the center region of the chamber and isolated from the chamber wall. This feature has the potential to reduce the background neutral gas pressure and wall-plasma interaction in the pre-ionized plasma. Moreover, in the conventional method, the breakdown neutral density determines the lower limit of the plasma density. In contrast, the initial plasma is generated in an MCPG and then injected and decompressed into a lower density in the present technique, potentially enabling exploration of the lowdensity parameter region of FRCs.
2. Experimental setup and diagnostics The FRC plasma is produced in a 1.5 m-long thetapinch coil with a radius of 0.17 m on NUCTE-III [5]. A vacuum vessel made of a transparent quartz tube 2 m in length and 0.256 m in diameter is connected through a stainless steel tube to the vacuum system. The MCPG is mounted at the left end of the vacuum vessel, as shown in Fig. 1.
Fig. 1. A schematic view of magnetized coaxial plasma gun and a part of NUCTE-III device.
The MCPG consists of a formation and an acceleration region. The formation region has an inner electrode and an outer electrode. A fast solenoid valve for hydrogen gas puffing and a gun bias coil are mounted on the middle of the outer electrode. A gun bias field with a flux of 0.074 mWb and a duration time of 200 s is driven by a 400 F capacitor with a charging voltage of 200 V. The electrodes are extended to the end of the theta-pinch coil and work as an accelerator. By the application of a 22.5 kA-gun current and a pulse width of 15 s between the inner and outer electrodes, which is driven by a 20 kV-15 F capacitor, magnetized plasmoids are produced and rapidly accelerated. Plasmoids with a velocity of about 40 km/s are injected into the vacuum vessel filled with a bias field of 0.032 T and a raising time of 90 s. The strength of the bias field also controls the process of plasmoid injection. A main reversed confinement field with a strength of 0.44 T, a 3 s rising and a 120 s decay time are applied when the bias field strength rises to its maximum value. The discharge time sequence of the MCPG pre-heating method is shown in Fig. 2. The behavior of the injected plasmoid and the separatrix radius and length of the formed FRC plasma are measured by a one-turn loop and magnetic probe array. The electron density at z = −0.44 m is diagnosed using a 3.39 m helium–neon interferometer. Forty-five optical detectors are used to observe the emission of light from the pre-ionized and FRC plas-
T. Nishida et al. / Fusion Engineering and Design 81 (2006) 2843–2847
Fig. 2. Operation sequence of FRC formation using the MCPG method.
mas. Each channel of an optical detector consists of a collimator, optical fiber, optical band pass filter and photomultiplier. A convex-plane lens is employed on the collimator and can observe line-integrated emissions along the optical axis of the collimator. Observed wavelengths are λ = 550 ± 5 nm (bremsstrahlung) and λ = 656 ± 5 nm (Balmer series-␣) which is selected by the band path optical filter. The magnetic structures of the injected plasmoids, preheating and FRC plasma are observed using a twodirection (z, θ) magnetic probe array which is inserted from a branch pipe of the vacuum vessel at z = −0.33 m.
3. Experimental results 3.1. Pre-heating plasma using the MCPG method In the MCPG method, an initial pre-heating plasma is produced by injection of the magnetized plasmoid into the vacuum vessel into which 2 mTorr deuterium gas is injected and to which a bias field of 0.032 T is applied. The pressure of the puffing gas is equivalent to about 5 mTorr of gas-fill. Fig. 3 shows the magnetic structure of a plasmoid as measured by the twodirectional internal magnetic probe array. The toroidal field reversed at about y = 0.20 m and the poloidal field
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Fig. 3. Toroidal magnetic field Bt and poloidal magnetic field Bp profiles of magnetized plasmoid formed by the MCPG.
has a peak at y = 0.20 m, confirming that a spheromaklike magnetic structure is produced by the MCPG. The separatrix radius and the length of the plasmoid are about 0.06 m and 0.40 m, respectively. Average density of the plasmoid is about 2.5 × 1020 m−3 from Figs. 3 and 6(b). Fig. 4 shows time evolution of the axial radiation profiles, which are emitted from the initial plasma produced by the MCPG method. The solid and the dotted lines are the line-integrated radiation intensity of bremsstrahlung and the Balmer-␣ line spectrum, respectively. The strong emission of the Balmer-␣, which indicates plasma formation, appears at the front of the plasmoid. This indicates that the injected plasmoid ionizes filled neutral particles while penetrating into the vacuum vessel. The intensity of the bremsstrahlung, which depends on the electron density, increases at the back side of the plasmoid. We assume that the plasmoid is translated while the plasmoid dissipates the magnetic and kinetic energy, and the dissipated energy is used for the ionization and heating of the initial plasma. It seems that the plasmoid is annihilated at around the center of the theta-pinch coil. The initial plasma is then formed only on the left side of the theta-pinch coil. The incident range of the plasmoid with an injection velocity of 40 km/s is about 0.5–0.6 m. The density of the initial plasma for FRC formation becomes about 1.6 × 1019 m−3 .
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Fig. 5. Time evolution of poloidal magnetic field profile.
Fig. 4. Time evolution of axial emission profiles. Solid line and closed circles indicates bremsstrahlung and dotted line and open circles Balmer-␣.
3.2. FRC plasma formation by MCPG method Fig. 5 shows time evolution of the y-profile of the Bp field measured using the internal magnetic probe array. The reversed compression field is applied at t = 0. The poloidal magnetic field is reversed till 4 s. It is thus confirmed that FRC plasma is produced by the MCPG method. The liftoff flux is about 0.4 mWb and 25% of the initial bias flux. The separatrix radius at 2 s is about 0.04 m as estimated by the poloidal field profile. Time evolution of the separatrix radius at z = 0.45 m and line integrated electron density at z = −0.43 m are indicated in parts (a) and (b) of Fig. 6, respectively. The FRC plasma is compressed radially up to 2 s. The separatrix radius becomes about 0.04 m. It can be seen from the time evolution of the axial profile of the separatrix radius, shown in Fig. 7, that the radial compression is uniform over the initial plasma formed region and the plasma length is about 0.8 m. The plasma
Fig. 6. Time evolution of separatrix radius at z = −0.43 m (a) and line integrated electron density ne dl (b).
T. Nishida et al. / Fusion Engineering and Design 81 (2006) 2843–2847
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4. Summary We have proposed a new startup method for FRC plasma using a magnetized coaxial plasma gun. Preliminary experiments were performed on the NUCTE-III device. As a result of injection of a spheromak-like plasmoid with radius 0.06 m, length 0.40 m, density 2.5 × 1020 m−3 and velocity 40 km/s, a preheating plasma for FRC formation was produced, with density of about 1.6 × 1019 m−3 . The formation of an FRC plasma was confirmed by application of a reversed field. The configuration was maintained for about 10 s. The density was about 1.3 × 1021 m−3 , approximately one third of that achieved using the z-discharge pre-ionized method. We confirm the capability of low-density FRC formation using the MCPG method. To form a long-lived FRC plasma with higher density and low background neutral particles, further experimental investigation is necessary to optimize the MCPG system and operation conditions.
References
Fig. 7. Time evolution of axial separatrix radius profile.
density becomes about 1.3 × 1021 m−3 and is about one third that of an FRC plasma formed by the z-discharge method. The plasma then shrinks axially and disrupts at 10 s.
[1] M. Tuszewski, Field reversed configurations, Nucl. Fusion 28 (1988) 2033–3092. [2] A.L. Hoffman, P. Gurevich, J. Grossnickle, J.T. Slough, Inductive field-reversed configuration accelerator for tokamak fueling, Fusion Sci. Technol. 36 (1999) 109–125. [3] J.T. Slough, A.L. Hoffman, Experimental study of the formation of field-reversed configurations employing high-order multipole fields, Phys. Fluids B 2 (1990) 797–808. [4] M. Tuszewski, D.P. Taggart, R.E. Chrien, D.J. Rej, R.E. Siemon, B.L. Wright, Axial dynamics in field reversed theta pinche II: stability, Phys. Fluids B 3 (1991) 2856–2870. [5] Y. Ohkuma, M. Urano, M. Nakamura, Y. Narushima, T. Takahashi, Y. Nogi, Production of a low density field reversed configuration plasma, Nucl. Fusion 38 (1998) 1501–1509.