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MSE measurements reveal hybrid-like flux-pumping associated with 2/1 NTM-ELM coupling. Analysis of MSE signals using digital lock-in amplifiers shows the strength of the flux-pumping is more than twice that of typical hybrid discharges. This flux-pumping maintains the minimum safety factor above unity, thereby avoiding sawteeth. The strength of the flux-pumping and ELM-NTM coupling have a clear upward dependence on normalized beta and NTM-pedestal proximity. The size of the island does not appear to effect flux-pumping, except that the mode must be present, suggesting the island chain serves as a radial pivot surface around which poloidal flux is pumped from the core to the edge. This result implies that higher normalized fusion performance (lower q95 and higher beta) may be achieved in hybrid discharges that contain a partially suppressed 2/1 NTM. ELM-NTM coupling consists of an Alfvénic timescale drop in the island width followed by a resistive recovery. The recovery phase is successfully modeled using the modified Rutherford equation. The depth of the drop in island width increases as the size of the ELM increases. To aid in the design of a highly resolved MSE pedestal measurement, full spectral analysis was preformed on existing edge channels. This analysis has revealed that coherent core MHD oscillations cause interference with present dual PEM polarimeters. Avoiding this interference requires a dedicated pedestal polarimeter with second harmonic frequencies greater than those of MHD fluctuations.
A new diagnostic, B-Stark, has been developed at the DIII-D tokamak for measurements of the magnitude and direction of the internal magnetic field. The B-Stark system is a version of a motional Stark effect (MSE) diagnostic based on the Stark split D/[alpha] emission from injected neutral beams. This diagnostic uses the spacing of the Stark lines to measure the magnitude of the magnetic field, and the intensities of the [pi]3 and [sigma]1 lines to measure the magnetic pitch angle. These lines originate from the same upper level, and are therefore not dependent on the n=3 level populations. The measurement of the magnetic pitch angle requires a specific viewing geometry with respect to the neutral beams, which is provided by the B-Stark diagnostic installation. The B-Stark technique may have advantages over MSE polarimetry diagnostics in future devices with high densities and temperatures, such as ITER. Under these conditions coatings on the plasma facing mirrors are expected, which can cause changes in the polarization state of the reflected light. The B-Stark technique is insensitive to the polarization direction, and can calibrate for polarization dependent transmission by using an in-situ beam-into-gas calibration. This dissertation describes the development and characterization of the B-Stark diagnostic. The hardware design and spectral fitting techniques are discussed in detail. Calibration procedures are described including the in-situ determination of the beam emission line profiles, viewing geometry and properties of the collection optics. The performance of the system is evaluated over the range of plasma conditions accessible at DIII-D. Measurements of the magnetic field have been made with toroidal fields in the range 1.2 - 2.1Tesla, plasma currents in the range 0.5 - 2.0MA, densities between 1.7 - 9.0 x 1019m−3, and neutral beam voltages between 50 - 81keV. These results are compared to values found from plasma equilibrium reconstructions (EFIT) and the MSE polarimetry system on DIII-D. The B-Stark system has been shown to provide measurements with a random errors as low as 0.2-0.3° in the magnetic pitch angle and 0.001-0.002T in [B]. Finally, proposed future improvements for the B-Stark diagnostic are presented.
A repositioning of a heating neutral beam on the DIII-D tokamak provides an opportunity to expand and improve the Motional Stark Effect diagnostic (MSE) used to constrain the current profile. D[alpha] emission from the neutral beam is split into components parallel ([pi]) and perpendicular ([sigma]) to the total electric field ETotal = vxB + Eplasma. The MSE diagnostic measures the polarization of the [sigma] component to determine the local magnetic field pitch angle Bz/B[phi] and the local radial plasma electric field ER. This is typically done using the EFIT current profile reconstruction code. Two independent measurements of the pitch angle [gamma] at each radius are required to differentiate the contributions from the Stark and plasma electric fields. Presently, three MSE diagnostics provide multiple views of a single neutral beam. Our ability to accurately differentiate Bz and ER is limited because these views do not overlap with sufficient radial resolution in some locations, and this limits the accuracy of the current profile reconstructions. The beam rearrangement allows us to build a fourth MSE view of a second beam injected counter to the plasma current. The combination of the new view with the old will improve radial resolution about a factor of 3, reduce ER uncertainty by a factor of 2 in the core and 5-6 in the edge, and reduce Bz uncertainty by 20-30%. The design of the new system is presented in this paper, focusing on the mechanical and optical details at the tokamak port on which it will be installed.
The advanced tokamak research program at DIII-D relies critically on the measurement of the current density profile. This was made possible by the development of a Motional Stark Effect (MSE) polarimeter that was first installed in 1992. Three major upgrades have since occurred, and improvements in our understanding of critical performance issues and calibration techniques are ongoing. In parallel with these improvements, we have drawn on our DIII-D experience to begin studies and design work for MSE on burning plasmas and ITER. This paper first reviews how Motional Stark Effect polarimetry (MSE) is used to determine the tokamak current profile. It uses the DIII-D MSE system as an example, and shows results from the latest upgrade that incorporates an array of channels from a new counter-Ip injected neutral beam. The various calibration techniques presently used are reviewed. High-leverage or unresolved issues affecting MSE performance and reliability in ITER are discussed. Next, we show a four-mirror collection optics design for the two ITER MSE views. Finally, we discuss measurements of the polarization properties of a few candidate mirrors for the ITER MSE.
Tearing-type modes are observed in most high-confinement operation regimes in TFTR. Three different methods are used to measure the magnetic island widths: external magnetic coils, internal temperature fluctuation from the electron cyclotron emission (ECE) diagnostic, and an experiment where the plasma major radius is rapidly shifted ("Jog" experiments). A good agreement between the three methods is observed. Numerical and analytic calculations of delta prime (the tearing instability index) are compared with an experimental measurement of delta prime using the tearing-mode eigenfunction mapped from the jog data. The obtained negative delta prime indicates that the observed tearing modes cannot be explained by the classical current-gradient-driven tearing theory.
Tearing type modes are observed in most high-confinement operation regimes in TFTR. Three different methods are used to measure the magnetic island widths: external magnetic coils, internal temperature fluctuation from electron cyclotron emission (ECE) diagnostic and an experiment where the plasma major radius is rapidly shifted ('Jog' experiments). A good agreement between the three methods is observed. Numerical and analytic calculations of delta prime (the tearing instability index) are compared with an experimental measurement of delta prime using the tearing mode eigenfunction mapped from the Jog data. The obtained negative delta prime indicates that the observed tearing modes cannot be explained by the classical current-gradient-driven tearing theory.
OAK-B135 The local oscillating component of the poloidal magnetic field in plasma associated with MHD instabilities has been measured using the motional Stark effect (MSE) diagnostic on the DIII-D tokamak. The magnetic field perturbations associated with a resistive wall mode (RWM) rotated by internal coils at 20 Hz was measured using the conventional MSE operation mode. These first observations of perturbations due to a MHD mode were obtained on multiple MSE channels covering a significant portion of the plasma and the radial profile o the amplitude of the perturbed field oscillations was obtained. The measured profile is similar to the profile of the amplitude of the electron temperature oscillation measured by electron cyclotron emission (ECE) measurements. In a new mode of measurement, the amplitude of a tearing mode rotating at a high frequency ((almost equal to) 7 kHz) was observed using the spectral analysis of high frequency MSE data on one channel. The spectrum consists of the harmonics of the light modulation employed in the MSE diagnostics, their mutual beat frequencies and their beat frequencies with the rotation frequency of the tearing mode. The value and time variation of the frequency of the observed perturbations is in good agreement with that measured by Mirnov probes and ECE. The paper demonstrates that the MSE diagnostic can be used for observing low and high frequency phenomena such as MHD instabilities and electromagnetic turbulence.
Although the q profile plays a key role in theories of instabilities and plasma equilibrium, it has been quite difficult to measure until the recent development of the motional Stark effect (MSE) diagnostic. A multichannel motional Stark effect polarimeter system has recently been installed on the Tokamak Fusion Test Reactor (TFTR). The diagnostic can measure the magnetic field pitch angle (?{sub p} = tan−1 (B{sub T})/(B{sup p})) at ten radial locations. The doppler shifted D{sub alpha} radiation from a TFTR heating beam is viewed near tangential to the toroidal magnetic field via a re-entrant front surface reflecting mirror. The field of view covers from inboard of the magnetic axis to near the outboard edge of the plasma with a radial spatial resolution of 3--5 cm. A high throughput f/2 optics system results in an uncertainty for?{sub p} of -0.1°--0.2° with a time resolution of -5--10 ms. Initial pinch angle profiles from TFTR have been obtained. The MSE data is consistent with the estimated magnetic axis position from external magnetic measurements and the q=1 radius is in good agreement with the inversion radius from the electron cyclotron emission temperature measurements.
Tokamak discharges are frequently ended by disruptions, an instability leading to a very sudden termination of the plasma. Although numerous studies of disruptions have been made, critical data is still needed to develop theoretical models for fundamental aspects of the disruption. This paper describes simultaneous profile measurements of the current, temperature, and density during the disruptive phase of a beam heated single-null divertor discharge in the DIII-D tokamak. The stored energy is lost early in the first phase of the disruption (thermal quench) after which the central temperature is a few hundred eV, and following the start of the current decay (current quench) the temperature has fallen to 100 eV. The current profile measured using the Motional Stark Effect (MSE) diagnostic is observed to flatten rapidly (