SUSTAINED STABILIZATION OF THE RESISTIVE WALL MODE BY PLASMA ROTATION IN THE DIII-D TOKAMAK. PDF Download
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Author: Publisher: ISBN: Category : Languages : en Pages : 13
Book Description
OAK-B135 A path to sustained stable operation, at plasma pressure up to twice the ideal magnetohydrodynamic (MHD) n = 1 free-boundary pressure limit, has been discovered in the DIII-D tokamak. Tuning the correction of the intrinsic magnetic field asymmetries so as to minimize plasma rotation decay during the high beta phase and increasing the angular momentum injection, have allowed maintaining the plasma rotation above that needed for stabilization of the resistive wall mode (RWM). A new method to determine the improved magnetic field correction uses feedback to sense and minimize the resonant plasma response to the non-axisymmetric field. At twice the free-boundary pressure limit, a disruption precursor is observed, which is consistent with having reached the ''ideal wall'' pressure limit predicted by stability calculations.
Author: Publisher: ISBN: Category : Languages : en Pages : 13
Book Description
OAK-B135 A path to sustained stable operation, at plasma pressure up to twice the ideal magnetohydrodynamic (MHD) n = 1 free-boundary pressure limit, has been discovered in the DIII-D tokamak. Tuning the correction of the intrinsic magnetic field asymmetries so as to minimize plasma rotation decay during the high beta phase and increasing the angular momentum injection, have allowed maintaining the plasma rotation above that needed for stabilization of the resistive wall mode (RWM). A new method to determine the improved magnetic field correction uses feedback to sense and minimize the resonant plasma response to the non-axisymmetric field. At twice the free-boundary pressure limit, a disruption precursor is observed, which is consistent with having reached the ''ideal wall'' pressure limit predicted by stability calculations.
Author: Publisher: ISBN: Category : Languages : en Pages : 11
Book Description
OAK A271 RESISTIVE WALL STABILIZATION OF HIGH BETA PLASMAS IN DIII-D. Recent DIII-D experiments show that ideal kink modes can be stabilized at high beta by a resistive wall, with sufficient plasma rotation. However, the resonant response by a marginally stable resistive wall mode to static magnetic field asymmetries can lead to strong damping of the rotation. Careful reduction of such asymmetries has allowed plasmas with beta well above the ideal MHD no-wall limit, and approaching the ideal-wall limit, to be sustained for durations exceeding one second. Feedback control can improve plasma stability by direct stabilization of the resistive wall mode or by reducing magnetic field asymmetry. Assisted by plasma rotation, direct feedback control of resistive wall modes with growth rates more than 5 times faster than the characteristic wall time has been observed. These results open a new regime of tokamak operation above the free-boundary stability limit, accessible by a combination of plasma rotation and feedback control.
Author: Publisher: ISBN: Category : Languages : en Pages : 18
Book Description
One promising approach to maintaining stability of high beta tokamak plasmas is the use of a conducting wall near the plasma to stabilize low-n ideal MHD instabilities. However, with a resistive wall, either plasma rotation or active feedback control is required to stabilize the more slowly growing resistive wall modes (RWMs). Experiments in the DIII-D, PBHX-M, and HBT-EP tokamaks have demonstrated that plasmas with a nearby conducting wall can remain stable to the n = 1 ideal external kink above the beta limit predicted with the wall at infinity, with durations in DIII-D up to 30 times [tau]{sub w}, the resistive wall time constant. More recently, detailed, reproducible observation of the n = 1 RWM has been possible in DIII-D plasmas above the no-wall beta limit. The DIII-D measurements confirm characteristics common to several RWM theories. The mode is destabilized as the plasma rotation at the q = 3 surface decreases below a critical frequency of 1 to 7 kHz. The measured mode growth times of 2 to 8 ms agree with measurements and numerical calculations of the dominant DIII-D vessel eigenmode time constants, [tau]{sub w}. From its onset, the RWM has little or no toroidal rotation and rapidly reduces the plasma rotation to zero. Both DIII-D and HBT-EP have adopted the smart shell concept as an initial approach to control of these slowly growing RWMs; external coils are controlled by a feedback loop designed to make the resistive wall appear perfectly conducting by maintaining a net zero radial field at the wall. Initial experiment results from DIII-D have yielded encouraging results.
Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
Resistive wall mode (RWM) instabilities are found to be a limiting factor in advanced tokamak (AT) regimes with low internal inductance. Even small amplitude modes can affect the rotation profile and the performance of these ELMing H-mode discharges. Although complete stabilization of the RWM by plasma rotation has not yet been observed, several discharges with increased beam momentum and power injection sustained good steady-state performance for record time extents. The first investigation of active feedback control of the RWM has shown promising results: the leakage of the radial magnetic flux through the resistive wall can be successfully controlled, and the duration of the high beta phase can be prolonged. The results provide a comparative test of several approaches to active feedback control, and are being used to benchmark the analysis and computational models of active control.
Author: John Wesson Publisher: Oxford University Press ISBN: 0199592233 Category : Science Languages : en Pages : 828
Book Description
The tokamak is the principal tool in controlled fusion research. This book acts as an introduction to the subject and a basic reference for theory, definitions, equations, and experimental results. The fourth edition has been completely revised, describing their development of tokamaks to the point of producing significant fusion power.
Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
A271 RESISTIVE WALL MODES AND PLASMA ROTATION IN DIII-D. The stabilization of the resistive wall mode (RWM) by toroidal plasma rotation has been demonstrated in neutral beam heated DIII-D discharges for values of[beta] up to 70% above the no-wall stability limit. The stabilizing effect of plasma rotation is explained by assuming some dissipation, which is caused by the rapid plasma flow through a perturbed magnetic field. Sufficient plasma rotation is predicted to extend the operating regime of tokamaks from the conventional no-wall[beta] limit up to the ideal wall[beta] limit. While plasma rotation has a stabilizing effect on the RWM, a finite amplitude RWM also increases the drag on the plasma, which leads to a non-linear interaction between the RWM and the plasma rotation. A good understanding of the underlying dissipation mechanism is crucial for reliable predictions of the plasma rotation which will be required for wall-stabilization in a burning-plasma experiment. In order to measure the stabilizing effect of plasma rotation on the RWM the technique of active MHD spectroscopy, which was previously applied to MHD modes at frequencies above 10 kHz, is extended to frequencies of a few Hz.
Author: Publisher: ISBN: Category : Languages : en Pages : 29
Book Description
Detailed analysis of recent high beta discharges in the DIII-D tokamak demonstrates that the resistive vacuum vessel can provide stabilization of low n magnetohydrodynamic (MHD) modes. The experimental beta values reaching up to [beta]{sub T} = 12.6% are more than 30% larger than the maximum stable beta calculated with no wall stabilization. Plasma rotation is essential for stabilization. When the plasma rotation slows sufficiently, unstable modes with the characteristics of the predicted {open_quotes}resistive wall{close_quotes} mode are observed. Through slowing of the plasma rotation between the q = 2 and q = 3 surfaces with the application of a non-axisymmetric field, the authors have determined that the rotation at the outer rational surfaces is most important, and that the critical rotation frequency is of the order of [Omega]/2[pi] = 1 kHz.
Author: Publisher: ISBN: Category : Languages : en Pages : 10
Book Description
One of the main goals for the DIII-D research program is to establish an advanced tokamak plasma with high bootstrap current fraction that can be sustained in-principle steady-state. Substantial progress has been made in several areas during the last year. The resistive wall mode stabilization has been done with spinning plasmas in which the plasma pressure has been extended well above the no-wall beta limit. The 3/2 neoclassical tearing mode has been stabilized by the injection of ECH into the magnetic islands, which drives current to substitute the missing bootstrap current. In these experiments either the plasma was moved or the toroidal field was changed to overlap the ECCD resonance with the location of the NTMs. Effective disruption mitigation has been obtained by massive noble gas injection into shots where disruptions were deliberately triggered. The massive gas puff causes a fast and clean current quench with essentially all the plasma energy radiated fairly uniformly to the vessel walls. The run-away electrons that are normally seen accompanying disruptions are suppressed by the large density of electrons still bound on the impurity nuclei. Major elements required to establish integrated, long-pulse, advanced tokamak operations have been achieved in DIII-D: [beta]{sub T} = 4.2%, [beta]{sub p} = 2, f{sub BS} = 65%, and [beta]{sub N}H9 = 10 for 600 ms (H"4[tau]{sub E}). The next challenge is to integrate the different elements, which will be the goal for the next five years when additional control will be available. Twelve resistive wall mode coils are scheduled to be installed in DIII-D during the summer of 2003. The future plans include upgrading the tokamak pulse length capability and increasing the ECH power, to control the current profile evolution.
Author: Frankie Murray & Publisher: Scientific e-Resources ISBN: 183947324X Category : Languages : en Pages : 320
Book Description
Regarding the matter of differential equations a considerable number of rudimentary books have been composed. This book overcomes any issues between rudimentary courses and the examination writing. The essential ideas important to contemplate differential equations - basic focuses and balance, occasional arrangements, invariant sets and invariant manifolds - are examined. Security hypothesis is created beginning with linearisation methods backpedaling to Lyapunov and Poincare. The global direct method is then examined. To acquire more quantitative data the Poincare-Lindstedt method is acquainted with estimated occasional arrangements while in the meantime demonstrating presence by the certain capacity hypothesis. The method of averaging is presented as a general estimation standardization method. The last four sections acquaint the peruser with unwinding motions, bifurcation hypothesis, focus manifolds, disarray in mappings and differential equations, Hamiltonian frameworks (repeat, invariant tori, intermittent arrangements). The book displays the subject material from both the subjective and the quantitative perspective. There are numerous cases to delineate the hypothesis and the peruser ought to have the capacity to begin doing research in the wake of concentrate this book.
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