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  • 1.
    Brunsell, Per
    et al.
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Bergsåker, Henric
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Cecconello, Marco
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Drake, James Robert
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Gravestijn, Robert
    KTH, Tidigare Institutioner, Fysik.
    Hedqvist, Anders
    KTH, Tidigare Institutioner, Fysik.
    Malmberg, Jenny A.
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Initial results from the rebuilt EXTRAP T2R RFP device2001Inngår i: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 43, nr 11, s. 1457-1470Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The EXTRAP T2R thin shell reversed-field pinch (RFP) device has recently resumed operation after a major rebuild including the replacement of the graphite armour with molybdenum limiters, a fourfold increase of the shell time constant, and the replacement of the helical coil used for the toroidal field with a conventional solenoid-type coil. Wall-conditioning using hydrogen glow discharge cleaning was instrumental for successful RFP operation. Carbon was permanently removed from the walls during the first week of operation. The initial results from RFP operation with relatively low plasma currents in the range I-p = 70-100 kA are reported. RFP discharges are sustained for more than three shell times. Significant improvements in plasma parameters are observed, compared to operation before the rebuild. There is a substantial reduction in the carbon impurity level. The electron density behaviour is more shot-to-shot reproducible. The typical density is n(e) = 0.5-1 x 10(19) m(-3). Monitors of H-alpha line radiation indicate that the plasma wall interaction is more toroidally symmetric and that there is less transient gas release from the wall. The minimum loop voltage is in the range V-t = 28-35 V, corresponding to a reduction by a factor of two to three compared to the value before the rebuild.

  • 2.
    Brunsell, Per. R.
    et al.
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Malmberg, Jenny-Ann
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Yadikin, Dimitry
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Cecconello, Marco
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Resistive wall modes in the EXTRAP T2R reversed-field pinch2003Inngår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 10, s. 3823-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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)/<B-phi> 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.

  • 3.
    Brunsell, Per R.
    et al.
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Yadikin, Dmitriy
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Gregoratto, D.
    Bolzonella, T.
    Cecconello, Marco
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Drake, James Robert
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Luchetta, A.
    Malmberg, Jenny-Ann
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Manduchi, G.
    Marchiori, G.
    Liu, Y.
    Ortolani, S.
    Paccagnella, R.
    First results from intelligent shell experiments with partial coil coverage in the EXTRAP T2R reversed field pinch2004Inngår i: Europhysics Conf. Abstracts Vol 29C, 2004, s. P-5.190-Konferansepaper (Fagfellevurdert)
  • 4.
    Cecconello, Marco
    et al.
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Malmberg, Jenny A.
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Spizzo, G.
    Chapman, B. E.
    Gravestjin, Robert M.
    KTH, Tidigare Institutioner, Fysik.
    Franz, P.
    Piovesan, P.
    Martin, P.
    Drake, James R.
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Current profile modification experiments in EXTRAP T2R2004Inngår i: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 46, nr 1, s. 145-161Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Pulsed poloidal current drive (PPCD) experiments have been conducted in the resistive shell EXTRAP T2R reversed-field pinch experiment. During the current profile modification phase, the fluctuation level of the m = 1 internally resonant tearing modes decreases, and the velocity of these modes increases. The m = 0 modes are not affected during PPCD, although termination occurs with a burst in the m = 0 amplitude. The PPCD phase is characterized by an increase in the central electron temperature (up to 380 eV) and in the soft x-ray signal. Spectroscopic observations confirm an increase in the central electron temperature. During PPCD, the plasma poloidal beta increases to 14%, and the estimated energy confinement time doubles, reaching 380 mus. The reduction in the fluctuation level and the corresponding increase in the energy confinement time are qualitatively consistent with a reduction in parallel transport along stochastic magnetic field lines.

  • 5.
    Cecconello, Marco
    et al.
    KTH, Tidigare Institutioner                               , Alfvénlaboratoriet.
    Malmberg, Jenny-Ann
    KTH, Tidigare Institutioner                               , Alfvénlaboratoriet.
    Nielsen, P
    Pasqualotto, R
    Drake, James Robert
    KTH, Tidigare Institutioner                               , Alfvénlaboratoriet.
    Study of the confinement properties in a reversed-field pinch with mode rotation and gas fuelling2002Inngår i: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 44, nr 8, s. 1625-1638Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    An extensive investigation of the global confinement properties in different operating scenarios in the rebuilt EXTRAP T2R reversed-field pinch (RFP) experiment is reported here. In particular, the role of a fast gas puff valve system, used to control plasma density, on confinement is studied. Without gas puffing, the electron density decays below 0.5 x 10(19) M-3. The poloidal beta varies between 5% and 15%, decreasing at large I/N. The energy confinement time ranges from 70 to 225 mus. With gas puffing, the density is sustained at n(e) approximate to 1.5 x 10(19) m(-3). However, a general slight deterioration of the plasma performances is observed for the same values of I/N: the plasma becomes cooler and more radiative. The poloidal beta is comparable to that in the scenarios without puff but the energy confinement time drops ranging from 60 to 130 mus. The fluctuation level and the energy confinement time have been found to scale with the Lundquist number as S-0.05+/-0.07 and S0.5+/-0.1, respectively. Mode rotation is typical for all the discharges and rotation velocity is observed to increase with increasing electron diamagnetic velocity.

  • 6.
    Cecconello, Marco
    et al.
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Malmberg, Jenny-Ann
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Sallander, Eva
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Drake, Drake James
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Self-organisation and intermittent coherent oscillations in the EXTRAP T2 reversed field pinch2002Inngår i: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 65, nr 1, s. 69-75Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Many reversed-field pinch (RFP) experiments exhibit a coherent oscillatory behaviour that is characteristic of discrete dynamo events and is associated with intermittent current profile self-organisation phenomena. However, in the vast majority of the discharges in the resistive shell RFP experiment EXTRAP T2, the dynamo activity does not show global, coherent oscillatory behaviour, The internally resonant tearing modes are phase-aligned and wall-locked resulting in a large localised magnetic perturbation. Equilibrium and plasma parameters have a level of high frequency fluctuations but the average values are quasi-steady. For some discharges, however, the equilibrium parameters exhibit the oscillatory behaviour characteristic of the discrete dynamo events. For these discharges, the trend observed in the tearing mode spectra, associated with the onset of the discrete relaxation event behaviour, is a relative higher amplitude of m = 0 mode activity and relative lower amplitude of the m = 1 mode activity compared with their average values. Global plasma parameters and model profile calculations for sample discharges representing the two types of relaxation dynamics are presented.

  • 7.
    Cecconello, Marco
    et al.
    KTH, Tidigare Institutioner                               , Alfvénlaboratoriet.
    Malmberg, Jenny-Ann
    KTH, Tidigare Institutioner                               , Alfvénlaboratoriet.
    Spizzo, G
    Gravestijn, Robert
    KTH, Tidigare Institutioner                               , Fysik.
    Franz, P
    Martin, P
    Chapman, B
    Drake, James Robert
    KTH, Tidigare Institutioner                               , Alfvénlaboratoriet.
    Current profile control experiments in EXTRAP T2RManuskript (preprint) (Annet vitenskapelig)
  • 8.
    Drake, James R.
    et al.
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Malmberg, Jenny A.
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Experimental studies of MHD dynamics in a RFP magnetically confined plasma2004Inngår i: Contributions to Plasma Physics, ISSN 0863-1042, E-ISSN 1521-3986, Vol. 44, nr 06-maj, s. 503-507Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Studies of non-linear dynamics of MHD tearing modes in the EXTRAP reversed-field pinch experiments [1] have demonstrated such phenomena as mode rotation, phase-locking and wall locking.

  • 9.
    Drake, James Robert
    et al.
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Brunsell, Per R
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Yadikin, Dmitriy
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Cecconello, Marco
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Malmberg, Jenny-A.
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Gregoratto, D
    Paccagnella, R
    Bolzonella, T
    Manduchi, G
    Marrelli, L
    Ortolani, S
    Spizzo, G
    Zanca, P
    Bondeson, A
    Liu, Y Q
    Experimental and theoretical studies of active control of resistive wall mode growth in the EXTRAP T2R reversed-field pinch2004Inngår i: IAEA-CN-116, 2004, s. Paper EX/P2-20-Konferansepaper (Fagfellevurdert)
  • 10.
    Drake, James Robert
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Brunsell, Per
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Yadikin, Dimitry
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Cecconello, Marco
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Malmberg, Jenny
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Liu, Y.
    Experimental and theoretical studies of active control of resistive wall mode growth in the EXTRAP T2R reversed-field pinch2005Inngår i: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 45, nr 7, s. 557-564Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 11.
    Malmberg, Jenny A.
    et al.
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Brzozowski, Jerzy H.
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Brunsell, Per R.
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Cecconello, Marco
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Drake, James R.
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Mode- and plasma rotation in a resistive shell reversed-field pinch2004Inngår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 11, nr 2, s. 647-658Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Mode rotation studies in a resistive shell reversed-field pinch, EXTRAP T2R [P. R. Brunsell , Plasma Phys. Control. Fusion 43, 1 (2001)] are presented. The phase relations and nonlinear coupling of the resonant modes are characterized and compared with that expected from modeling based on the hypothesis that mode dynamics can be described by a quasi stationary force balance including electromagnetic and viscous forces. Both m=0 and m=1 resonant modes are studied. The m=1 modes have rotation velocities corresponding to the plasma flow velocity (20-60 km/s) in the core region. The rotation velocity decreases towards the end of the discharge, although the plasma flow velocity does not decrease. A rotating phase locked m=1 structure is observed with a velocity of about 60 km/s. The m=0 modes accelerate throughout the discharges and reach velocities as high as 150-250 km/s. The observed m=0 phase locking is consistent with theory for certain conditions, but there are several conditions when the dynamics are not described. This is not unexpected because the assumption of quasi stationarity for the mode spectra is not fulfilled for many conditions. Localized m=0 perturbations are formed in correlation with highly transient discrete dynamo events. These perturbations form at the location of the m=1 phase locked structure, but rotate with a different velocity as they spread out in the toroidal direction. (C) 2004 American Institute of Physics.

  • 12.
    Malmberg, Jenny-Ann
    et al.
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Brunsell, Per
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Resistive wall instabilities and tearing mode dynamics in the EXTRAP T2R thin shell reversed-field pinch2002Inngår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 9, nr 1, s. 212-221Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Observations of resistive wall instabilities and tearing mode dynamics in the EXTRAP T2R thin shell (tau (w)=6 ms) reversed field pinch are described. A nonresonant mode (m=1,n=-10) with the same handedness as the internal field grows nearly exponentially with an average growth time of about 2.6 ms (less than 1/2 of the shell time) consistent with linear stability theory. The externally nonresonant unstable modes (m=1,n >0), predicted by linear stability theory, are observed to have only low amplitudes (in the normal low-Theta operation mode of the device). The radial field of the dominant internally resonant tearing modes (m=1,n=-15 to n=-12) remain low due to spontaneous fast mode rotation, corresponding to angular phase velocities up to 280 krad/s. Phase aligned mode structures are observed to rotate toroidally with an average angular velocity of 40 krad/s, in the opposite direction of the plasma current. Toward the end of the discharge, the radial field of the internally resonant modes grows as the modes slow down and become wall-locked, in agreement with nonlinear computations. Fast rotation of the internally resonant modes has been observed only recently and is attributed to a change of the front-end system (vacuum vessel, shell, and TF coil) of the device.

  • 13.
    Yadikin, Dimitry
    et al.
    KTH, Tidigare Institutioner                               , Alfvénlaboratoriet.
    Brunsell, Per. R.
    KTH, Tidigare Institutioner                               , Alfvénlaboratoriet.
    Malmberg, Jenny-Ann
    KTH, Tidigare Institutioner                               , Alfvénlaboratoriet.
    Martin, P.
    Milani, F.
    Marrelli, L.
    Drake, James Robert
    KTH, Tidigare Institutioner                               , Alfvénlaboratoriet.
    Marchiori, G.
    Feedback controller for saddle coils for suppression of resistive wall modes in EXTRAP T2R2004Inngår i: 31st European Physical Society Conference on Controlled Fusion and Plasma Physics, 2004Konferansepaper (Fagfellevurdert)
1 - 13 of 13
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