| 1. |
- Brunsell, Per, et al.
(författare)
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Feedback Stabilization of Multiple Resistive Wall Modes
- 2004
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 93:22, s. 225001-
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Tidskriftsartikel (refereegranskat)abstract
- Active feedback stabilization of multiple independent resistive wall modes is experimentally demonstrated in a reversed-field pinch plasma. A reproducible simultaneous suppression of several nonresonant resistive wall modes is achieved. Coupling of different modes due to the limited number of the feedback coils is observed in agreement with theory. The feedback stabilization of nonresonant RWMs also has an effect on tearing modes that are resonant in the central plasma, leading to a significant prolongation of the discharge pulse.
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| 2. |
- Brunsell, Per. R., et al.
(författare)
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Resistive wall modes in the EXTRAP T2R reversed-field pinch
- 2003
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 10, s. 3823-
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Tidskriftsartikel (refereegranskat)abstract
- Resistive wall modes (RWM) in the reversed field pinch are studied and a detailed comparison of experimental growth rates and linear magnetohydrodynamic (MHD) theory is made. RWM growth rates are experimentally measured in the thin shell device EXTRAP T2R [P. R. Brunsell , Plasma Phys. Controlled Fusion 43, 1 (2001)]. Linear MHD calculations of RWM growth rates are based on experimental equilibria. Experimental and linear MHD RWM growth rate dependency on the equilibrium profiles is investigated experimentally by varying the pinch parameter Theta=B-theta(a)/ in the range Theta=1.5-1.8. Quantitative agreement between experimental and linear MHD growth rates is seen. The dominating RWMs are the internal on-axis modes (having the same helicity as the central equilibrium field). At high Theta, external nonresonant modes are also observed. For internal modes experimental growth rates decrease with Theta while for external modes, growth rates increase with Theta. The effect of RWMs on the reversed-field pinch plasma performance is discussed.
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| 3. |
- Brunsell, Per R., et al.
(författare)
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Reversed field pinch operation with intelligent shell feedback control in EXTRAP T2R
- 2006
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 46:11, s. 904-913
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Tidskriftsartikel (refereegranskat)abstract
- Discharges in the thin shell reversed field pinch (RFP) device EXTRAP T2R without active feedback control are characterized by growth of non-resonant m = 1 unstable resistive wall modes (RWMs) in agreement with linear MHD theory. Resonant m = 1 tearing modes (TMs) exhibit initially fast rotation and the associated perturbed radial fields at the shell are small, but eventually TMs wall-lock and give rise to a growing radial field. The increase in the radial field at the wall due to growing RWMs and wall-locked TMs is correlated with an increase in the toroidal loop voltage, which leads to discharge termination after 3-4 wall times. An active magnetic feedback control system has been installed in EXTRAP T2R. A two-dimensional array of 128 active saddle coils (pair-connected into 64 independent m = 1 coils) is used with intelligent shell feedback control to suppress the m = 1 radial field at the shell. With feedback control, active stabilization of the full toroidal spectrum of 16 unstable m = 1 non-resonant RWMs is achieved, and TM wall locking is avoided. A three-fold extension of the pulse length, up to the power supply limit, is observed. Intelligent shell feedback control is able to maintain the plasma equilibrium for 10 wall times, with plasma confinement parameters sustained at values comparable to those obtained in thick shell devices of similar size.
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| 4. |
- Drake, James Robert, et al.
(författare)
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Experimental and theoretical studies of active control of resistive wall mode growth in the EXTRAP T2R reversed-field pinch
- 2005
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 45:7, s. 557-564
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Tidskriftsartikel (refereegranskat)abstract
- Active feedback control of resistive wall modes (RWMs) has been demonstrated in the EXTRAP T2R reversed-field pinch experiment. The control system includes a sensor consisting of an array of magnetic coils (measuring mode harmonics) and an actuator consisting of a saddle coil array (producing control harmonics). Closed-loop (feedback) experiments using a digital controller based on a real time Fourier transform of sensor data have been studied for cases where the feedback gain was constant and real for all harmonics (corresponding to an intelligent-shell) and cases where the feedback gain could be set for selected harmonics, with both real and complex values (targeted harmonics). The growth of the dominant RWMs can be reduced by feedback for both the intelligent-shell and targeted-harmonic control systems. Because the number of toroidal positions of the saddle coils in the array is half the number of the sensors, it is predicted and observed experimentally that the control harmonic spectrum has sidebands. Individual unstable harmonics can be controlled with real gains. However if there are two unstable mode harmonics coupled by the sideband effect, control is much less effective with real gains. According to the theory, complex gains give better results for (slowly) rotating RWMs, and experiments support this prediction. In addition, open loop experiments have been used to observe the effects of resonant field errors applied to unstable, marginally stable and robustly stable modes. The observed effects of field errors are consistent with the thin-wall model, where mode growth is proportional to the resonant field error amplitude and the wall penetration time for that mode harmonic.
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| 5. |
- Yadikin, Dimitry, et al.
(författare)
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Advanced feedback control methods in EXTRAP T2R
- 2006
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 13:7, s. 072109-
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Tidskriftsartikel (refereegranskat)abstract
- Previous experiments in the EXTRAP T2R reversed field pinch device have shown the possibility of suppression of multiple resistive wall modes (RWM). A feedback system has been installed in EXTRAP T2R having 100% coverage of the toroidal surface by the active coil array. Predictions based on theory and the previous experimental results show that the number of active coils should be sufficient for independent stabilization of all unstable RWMs in the EXTRAP T2R. Experiments using different feedback schemes are performed, comparing the intelligent shell, the fake rotating shell, and the mode control with complex feedback gains. Stabilization of all unstable RWMs throughout the discharge duration of t(d)approximate to 10 tau(w) is seen using the intelligent shell feedback scheme. Mode rotation and the control of selected Fourier harmonics is obtained simultaneously using the mode control scheme with complex gains. Different sensor signals are studied. A feedback system with toroidal magnetic field sensors could have an advantage of lower feedback gain needed for the RWM suppression compared to the system with radial magnetic field sensors. In this study, RWM suppression is demonstrated, using also the toroidal field component as a sensor signal in the feedback system.
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| 6. |
- Yadikin, Dimitry
(författare)
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Feedback control of resistive wall modes in the reversed field pinch
- 2004
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A wide range of unstable current driven MHD modes is present in the re- versed τeld pinch (RFP) conτguration. An ideally conducting wall facing the plasma can stabilize the ideal MHD modes. In the presence of a resistive wall characterized by the wall time τw, fast mode rotation with the frequency exceeding the inverse wall time gives stabilization for resistive MHD modes. The ideal MHD modes in the RFP are non-rotating modes and can not be stabilized by the resistive wall. Instead they are converted into resistive wall modes (RWM) growing with a growth rate proportional to the inverse of the wall time τw. EXTRAP T2R is an RFP device equipped with a thin resistive wall having the wall time shorter than the plasma pulse duration τw < τp. This feature allows the study of non-resonant non-rotating resistive wall modes. Resistive wall modes dynamics has been studied in EXTRAP T2R . RWM growth rates has been measured and compared with linear MHD stability calculations. Quantitative agreement is observed. In the case τw < τp the RWM can cause discharge degradation and should be stabilized. Active feedback is the way to stabilize the RWM in the RFP. An intelligent shell scheme is one possible feedback scenario. An active feed- back system including a set of sensors and discrete active coils is installed in EXTRAP T2R. The intelligent shell tries to keep the magnetic flux zero at the positions of the sensor. The analog PID controller for the intelligent shell feedback scheme has been studied. A model of the active control system was developed and comparison with the experimental results showed good agree- ment. Encouraging experimental results on the active feedback stabilization of multiple RWMs in the RFP plasmas were obtained.
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| 7. |
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