Jon C. Hillesheim

Jon C. Hillesheim
Culham Centre for Fusion Energy | CCFE · Tokamak Science Department

Ph.D.

About

76
Publications
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1,552
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Publications

Publications (76)
Article
Full-text available
High spatial resolution Doppler backscattering measurements in JET have enabled new insights into the development of the edge Er. We observe fine-scale spatial structures in the edge Er well with a wave number krρi≈0.4–0.8, consistent with stationary zonal flows, the characteristics of which vary with density. The zonal flow amplitude and wavelengt...
Article
Full-text available
A critical gradient threshold has been observed for the first time in a systematic, controlled experiment for a locally measured turbulent quantity in the core of a confined high-temperature plasma. In an experiment in the DIII-D tokamak where LTe-1=|∇Te|/Te and toroidal rotation were varied, long wavelength (kθρs≲0.4) electron temperature fluctuat...
Article
Full-text available
Tokamak plasmas rotate even without any external injection of momentum. A Doppler backscattering system installed at MAST has allowed this intrinsic rotation to be studied in Ohmic L-mode and H-mode plasmas, including the first observation of intrinsic rotation reversals in a spherical tokamak. Experimental results are compared to a novel 1D model,...
Article
Measurements of local density and magnetic field fluctuations near the pedestal top, conditionally averaged over the edge localized mode (ELM) cycle, have been made in Mega Amp Spherical Tokamak (MAST). A Doppler backscattering (DBS) system installed at MAST was used to measure intermediate-k density fluctuations at the top of the pedestal. A novel...
Preprint
Full-text available
Nonlinear multiscale gyrokinetic simulations of a Joint European Torus edge pedestal are used to show that electron-temperature-gradient (ETG) turbulence has a rich three-dimensional structure, varying strongly according to the local magnetic-field configuration. In the plane normal to the magnetic field, the steep pedestal electron temperature gra...
Preprint
Full-text available
A linear response, local model for the DBS amplitude applied to gyrokinetic simulations shows that radial correlation Doppler reflectometry measurements (RCDR, Schirmer et al., Plasma Phys. Control. Fusion 49 1019 (2007)) are not sensitive to the average turbulence radial correlation length, but to a correlation length that depends on the binormal...
Article
Full-text available
Remote control of the diagnostic systems is the basic requirement for the high performance plasma operation in a fusion device. This work presents the development of the remote control system for the multichannel Doppler backward scattering (DBS) reflectometers. It includes a remote controlled quasi-optical system and a remote intermediate frequenc...
Preprint
Full-text available
We report on a detailed study of magnetic fluctuations in the JET pedestal, employing basic theoretical considerations, gyrokinetic simulations, and experimental fluctuation data, to establish the physical basis for their origin, role, and distinctive characteristics. We demonstrate quantitative agreement between gyrokinetic simulations of microtea...
Preprint
Full-text available
Fusion performance in tokamaks hinges critically on the efficacy of the Edge Transport Barrier (ETB) at suppressing energy losses. The new concept of fingerprints is introduced to identify the instabilities that cause the transport losses in the ETB of many of today's experiments, from widely posited candidates. Analysis of the Gyrokinetic-Maxwell...
Preprint
This paper compares the gyrokinetic instabilities and transport in two representative JET pedestals, one (pulse 78697) from the JET configuration with a carbon wall (C) and another (pulse 92432) from after the installation of JET's ITER-like Wall (ILW). The discharges were selected for a comparison of JET-ILW and JET-C discharges with good confinem...
Article
The dependence of plasma transport and confinement on the main hydrogenic ion isotope mass is of fundamental importance for understanding turbulent transport and, therefore, for accurate extrapolations of confinement from present tokamak experiments, which typically use a single hydrogen isotope, to burning plasmas such as ITER, which will operate...
Article
L to H transition studies at JET have revealed an n = 0, m = 1 magnetic oscillation starting immediately at the L to H transition (called M-mode for brevity). While the magnetic oscillation is present a weak ELM-less H-mode regime is obtained, with a clear increase of density and a weak electron temperature pedestal. It is an intermediate state bet...
Article
New results from MAST are presented that focus on validating models in order to extrapolate to future devices. Measurements during start-up experiments have shown how the bulk ion temperature rise scales with the square of the reconnecting field. During the current ramp up models are not able to correctly predict the current diffusion. Experiments...
Article
Geodesic acoustic modes (GAMs) have been investigated in JET ohmic discharges using mainly Doppler backscattering. Characteristics and scaling properties of the GAM are studied. Time and spatial resolved measurements of the perpendicular velocity indicate that GAMs are located in a narrow layer at the edge density gradient region with amplitude cor...
Article
Full-text available
Quasi-coherent (QC) modes have been reported for more than 10 years in reflectometry fluctuations spectra in the core region of fusion plasmas. They have characteristics in-between coherent and broadband fluctuations as they oscillate at a marked frequency but have a wide spectrum. This work presents further evidences of the link recently establish...
Article
Full-text available
The Mega Ampère Spherical Tokamak (MAST) programme is strongly focused on addressing key physics issues in preparation for operation of ITER as well as providing solutions for DEMO design choices. In this regard, MAST has provided key results in understanding and optimizing H-mode confinement, operating with smaller edge localized modes (ELMs), pre...
Article
The high-k (7 ≲ k⊥ρi ≲ 11) wavenumber spectrum of density fluctuations has been measured for the first time in MAST (Lloyd et al 2003 Nucl. Fusion 43 1665). This was accomplished with the first implementation of Doppler backscattering (DBS) for core measurements in a spherical tokamak. DBS has become a well-established and versatile diagnostic tech...
Article
This paper reports the progress made at JET-ILW on integrating the requirements of the reference ITER baseline scenario with normalized confinement factor of 1, at a normalized pressure of 1.8 together with partially detached divertor whilst maintaining these conditions over many energy confinement times. The 2.5 MA high triangularity ELMy H-modes...
Article
Full-text available
A sixteen channel millimeter-wave diagnostic system, covering the frequency range 30-75 GHz, has been installed on MAST [B. Lloyd et al., Nucl. Fusion 43, 1665 (2003)] and has been successfully used for both Doppler backscattering (DBS) and conventional (normal-incidence) fluctuation reflectometry. DBS has become a well-established and versatile di...
Article
Full-text available
We present direct evidence of low frequency, radially sheared, turbulence-driven flows (zonal flows (ZFs)) triggering edge transport barrier formation preceding the L- to H-mode transition via periodic turbulence suppression in limit-cycle oscillations (LCOs), consistent with predator–prey dynamics. The final transition to edge-localized mode-free...
Article
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The type I ELMy H-mode is the baseline operating scenario for ITER. While it is known that the type I ELM ultimately results from the peeling-ballooning instability, there is growing experimental evidence that a mode grows up before the ELM crash that may modify the edge plasma, which then leads to the ELM event due to the peeling-ballooning mode....
Article
The geodesic acoustic mode (GAM), a coherent form of the zonal flow, plays a critical role in turbulence regulation and cross-magnetic-field transport. In the DIII-D tokamak, unique information on multi-field characteristics and radial structure of eigenmode GAMs has been measured. Two simultaneous and distinct, radially overlapping eigenmode GAMs...
Article
A series of carefully designed validation experiments conducted on DIII-D to rigorously test gyrofluid and gyrokinetic predictions of transport and turbulence stiffness in both the ion and electron channels have provided an improved assessment of the experimental fidelity of those models over a range of plasma parameters. The first set of experimen...
Article
A critical gradient for long wavelength (kθρs≲0.4) electron temperature fluctuations has been observed in an experiment in the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)], where below a threshold value of LTe−1 = |∇Te|/Te electron temperature fluctuations are constant and above they steadily increase. Above the critical gradient, the...
Article
Full-text available
Energetic ion transport due to microturbulence is investigated in magnetohydrodynamic-quiescent plasmas by way of neutral beam injection in the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)]. A range of on-axis and off-axis beam injection scenarios are employed to vary relevant parameters such as the character of the background microturb...
Article
A critical gradient threshold has been observed for the first time in a systematic, controlled experiment for a locally measured turbulent quantity in the core of a confined high-temperature plasma. In an experiment in the DIII-D tokamak where L_{T_{e}}^{-1}=|∇T_{e}|/T_{e} and toroidal rotation were varied, long wavelength (k_{θ}ρ_{s}≲0.4) electron...
Article
Doppler backscattering (DBS) is a plasma diagnostic used in tokamaks and other magnetic confinement devices to measure the fluctuation level of intermediate wavenumber (k(θ)ρ(s) ∼ 1) density fluctuations and the lab frame propagation velocity of turbulence. Here, a synthetic DBS diagnostic is described, which has been used for comparisons between m...
Article
A critical gradient threshold for electron temperature fluctuations, directly correlated with an abrupt increase in the electron heat flux and reproduced in gyrofluid and gyrokinetic predictions, has been directly measured for the first time in the core of a confined high-temperature plasma. In an experiment in the DIII-D tokamak where 1/LTe=-∇Te/T...
Article
Experiments on DIII-D have measured the stiffness of electron heat transport using a new method that combines heat pulse (HP) propagation and power balance (PB) analysis. Using a single modulated gyrotron, in addition to 5 cw gyrotrons, the radial profiles of Te oscillations from the fundamental to the 9^th harmonic are fit to determine the diffusi...
Article
In DIII-D, localized electron cyclotron heating (ECH) is used to probe the critical gradient of, and onset of stiffness in, the electron temperature Te profile. While keeping the total injected ECH power constant, the deposition profile was varied to investigate the relationship between the Te gradient and the electron power balance heat flux. A cr...
Article
Low frequency Zonal Flows (ZFs) have been observed to trigger the L-H transition near the power threshold, by either an extended predator-prey limit cycle oscillation (LCO [1]) or a short (˜0.5-1.5 ms) ZF burst executing only part of one limit cycle. Localized turbulence suppression (kθρs˜0.5) is initiated as the ZF shearing rate approaches the tur...
Article
Geodesic Acoustic Modes (GAMs) are coherent flows induced by plasma turbulence that in turn affect the turbulence and turbulent transport. Recently, in a neutral beam and electron cyclotron heated L-mode plasma in the DIII-D tokamak, strong GAM oscillations have been observed in electron temperature fluctuations Te in addition to the often-observed...
Article
Electron profile stiffness was studied in DIII-D L-mode discharges by systematically varying the heat flux in a narrow region with electron cyclotron heating and measuring the local change produced in {nabla}T{sub e}. Electron stiffness was found to slowly increase with toroidal rotation velocity. A critical inverse temperature gradient scale lengt...
Article
An extensive set of tests comparing gyrokinetic predictions of temperature-gradient driven electron turbulence to power balance transport analyses and fluctuation measurements are presented. These tests use data from an L-mode validation study on the DIII-D tokamak (Luxon 2002 Nucl. Fusion 42 614) in which the local value of is varied by modulated...
Article
Direct evidence of zonal flow (ZF) predator-prey oscillations and the synergistic roles of ZF- and equilibrium E×B flow shear in triggering the low- to high-confinement (L- to H-mode) transition in the DIII-D tokamak is presented. Periodic turbulence suppression is first observed in a narrow layer at and just inside the separatrix when the shearing...
Article
Geodesic acoustic modes (GAMs) and zonal flows are nonlinearly driven, axisymmetric (m = 0andn = 0) E×B flows, which are thought to play an important role in establishing the saturated level of turbulence in tokamaks. Results are presented showing the GAM’s observed spatial scales, temporal scales, and nonlinear interaction characteristics, which...
Article
Full-text available
The first systematic investigation of core electron thermal transport and the role of local ion temperature gradient/trapped electron mode/electron temperature gradient (ITG/TEM/ETG)-scale core turbulence is performed in high temperature, low collisionality H-mode plasmas in the DIII-D tokamak. Wavenumber spectra of L-mode and H-mode density turbul...
Article
High-power electron cyclotron (EC) waves are used to increase performance in several Advanced Tokamak (AT) regimes on DIII-D where there is a simultaneous need for high noninductive current and high beta. In the Quiescent High-confinement mode (QH-mode), a direct measurement of the electron cyclotron current drive (ECCD) profile is made using modul...
Article
As part of the FY2011 DOE Joint Research Target on Pedestal Physics, tests are being performed on DIII-D for gyrokinetic modes that have been proposed as physics mechanisms for controlling the H-mode pedestal structure. These modes include kinetic ballooning modes (KBM), candidates for limiting the total pedestal pressure gradient, ion temperature...
Article
The trigger mechanism of the L- to H-mode transition is not currently fully understood. Empirical scaling studies of the L-H transition power threshold have discovered global plasma parameter dependences, including a strong density dependence. The current work investigates the potential role of edge turbulence and flows in this density dependence b...
Article
Resonant magnetic perturbations (RMPs) have been shown to suppress ELM activity during H-mode and so is a very attractive (but not well understood) technology for ITER and future burning plasmas. In a series of experiments conducted on DIII-D, RMPs are found to significantly affect fluctuations, flow, transport, and resulting profiles on the DIII-D...
Article
Several heat flux scans have been performed in an L-mode discharge in DIII-D with the goal of investigating the stiffness and critical gradient in the electron channel at ρ=0.6, 0.4 and 0.3. The heat flux scans employed 6 gyrotrons operating for 3.5 s with a shot-by-shot variation in heat flux achieved by moving 1 gyrotron/shot from just outside to...
Article
Turbulence model validation experiments have generally shown more disagreement towards the edge, r/a>0.75, than in the core; this motivates more detailed comparisons of measured turbulence and zonal flow characteristics to models and simulations. Measurements have been made in DIII-D with two multichannel Doppler backscattering systems at toroidal...
Article
Core electron/ion thermal transport and its dependence on ITG/TEM/ETG-scale turbulence are examined in high temperature, strongly rotating QH-mode plasmas, at ITER-relevant collisionality (νe^* ˜0.05). To simulate central electron heating by α-particles, ECH has been used to achieve 0.6 <=Te/Ti<=1.1. ITG/TEM-scale density fluctuations remain virtua...
Article
The multi-field/multi-scale core (ρ ∼ 0.5–0.8) turbulence response to electron cyclotron heating (ECH) of DIII-D Ohmic plasmas is reported for the first time. Long wavelength (low-k) electron temperature (e/Te) and high-k density turbulence levels (e/ne) are observed to strongly increase during ECH. In contrast, low-k and intermediate-k e/ne showe...
Article
Full-text available
A series of carefully designed experiments on DIII-D have taken advantage of a broad set of turbulence and profile diagnostics to rigorously test gyrokinetic turbulence simulations. In this paper the goals, tools and experiments performed in these validation studies are reviewed and specific examples presented. It is found that predictions of trans...
Article
Full-text available
Robust validation of predictive turbulent transport models requires quantitative comparisons to experimental measurements at multiple levels, over a range of physically relevant conditions. Toward this end, a series of carefully designed validation experiments has been performed on the DIII-D tokamak [ J. L. Luxon, Nucl. Fusion 42, 614 (2002) ] to...
Article
Full-text available
This paper describes measurements of long wavelength, turbulent electron temperature fluctuations in the core plasma of the DIII-D tokamak made with a correlation electron cyclotron emission (CECE) radiometer-based diagnostic. Experimental and simulation results indicate that long wavelength electron temperature fluctuations (1) are similar in ampl...
Article
The progress in rigorously testing gyrokinetic turbulence simulations through a series of carefully designed experiments is described. A unique array of multi-field, multi-scale turbulence diagnostics is utilized, including new measurements of TEM-scale n, turbulence flows, ne- Te crossphase, as well as previously available ITG and ETG scale n and...
Article
Electron thermal transport and the role of local ITG/TEM/ETG-scale core turbulence are investigated in high temperature DIII-D H-mode/QH-mode plasmas at ITER-relevant electron to ion temperature ratio (0.5
Article
Understanding plasma turbulence in existing fusion devices is essential to establishing a predictive capability of turbulence and transport model for future devices like ITER. Historically, such effort has tended to focus on long wavelength density turbulence with little attention given to electron temperature fluctuations and/or short-scale densit...
Article
Geodesic acoustic modes (GAMs) are nonlinearly driven, axisymmetric (m=0, n=0) ExB flows, which may play an important role in establishing the saturated level of turbulence in tokamaks. Doppler backscattering (DBS) measures the flow of turbulent structures and the level of intermediate-k (krhos˜1-4) density fluctuations. Measurements have been made...
Article
Doppler backscattering has emerged in recent years as a powerful diagnostic tool in high temperature fusion plasmas. The technique is sensitive to plasma turbulence flow and has been utilized to determine radial electric field and to study geodesic acoustic modes, zonal flows, and intermediate scale density turbulence. The current manuscript descri...
Article
Full-text available
A novel multichannel, tunable Doppler backscattering (DBS)/reflectometry system has recently been developed and applied to a variety of DIII-D plasmas. Either DBS or reflectometry can be easily configured for use in a wide range of plasma conditions using a flexible quasi-optical antenna system. The multiple closely spaced channels, when combined w...
Article
Full-text available
The quasioptical design of a new integrated Doppler backscattering (DBS) and correlation electron cyclotron emission (CECE) system is presented. The design provides for simultaneous measurements of intermediate wavenumber density and long wavelength electron temperature turbulence behavior. The Doppler backscattering technique is sensitive to plasm...
Article
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This paper presents new measurements of the cross-phase angle, alpha{sub n{sub eT{sub e}}}, between long-wavelength (k{sub t}hetarho{sub s}<0.5) density, n-tilde{sub e}, and electron temperature, T-tilde{sub e}, fluctuations in the core of DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] tokamak plasmas. The coherency and cross-phase angle between...
Article
Full-text available
The local value of a/LTe, a turbulence drive term, was modulated with electron cyclotron heating in L-mode discharges on DIII-D [ J. L. Luxon, Nucl. Fusion 42, 614 (2002) ] and the density and electron temperature fluctuations in low, intermediate, and high-k regimes were measured and compared with nonlinear gyrokinetic turbulence simulations using...
Article
Full-text available
Comparisons of nonlinear GYRO simulations of DIII-D discharges with low and high elongation against experimental measurements are presented. Comparisons of measured low-k density spectra (from beam emission spectroscopy), intermediate-k density spectra (from Doppler backscattering), and low-k electron temperature spectra (from correlation electron...
Article
Geodesic acoustic modes (GAMs) are linearly stable, turbulence driven modes exhibiting oscillating axisymmetric (m=0, n=0) ExB flows. They potentially play an important role in establishing the saturated level of turbulence in fusion plasmas. Two Doppler backscattering (DBS) systems at locations separated toroidally by 180^o are aligned to make sim...
Article
Using a unique array of diagnostics the dependence of turbulence and transport on plasma shape (elongation, elongation shear, and triangularity) is studied. The diagnostic set includes FIR scattering, Doppler backscattering, correlation ECE, BES, reflectometry, and high-k backscattering. This covers a broad range in wavenumber (0
Article
First measurements of core low and intermediate-k correlation lengths as well as the dynamic turbulence amplitude behavior across near-balanced NBI-heated L- to H-mode transitions have been obtained on DIII-D. In these discharges, poloidal turbulence flow increases with little change in its shear as the L-H transition is approached. Leading up to t...
Article
Full-text available
The mission of the National Spherical Torus Experiment (NSTX) is the demonstration of the physics basis required to extrapolate to the next steps for the spherical torus (ST), such as a plasma facing component test facility (NHTX) or an ST based component test facility (ST-CTF), and to support ITER. Key issues for the ST are transport, and steady s...
Article
Full-text available
In this paper, we report observations of reduced low-k core electron temperature and intermediate-k density fluctuations in H-mode and quiescent (QH)-mode. L-mode electron temperature fluctuation levels (0.5% ≤ T e /T e ≤ 2% for k θ ρ s < 0.5 as measured by correlation ECE (CECE) radiometry) are observed to decrease by at least a factor of four in...
Article
A novel multichannel Doppler backscattering system has been designed and tested for application on the DIII-D [J. L. Luxon, Fusion Sci. Technol. 48, 828 (2005)] and National Spherical Torus Experiment (NSTX) [M. Ono et al., Nucl. Fusion 40, 557 (2000)] fusion plasma devices. Doppler backscattering measures localized intermediate wavenumber (k(perpe...
Article
Doppler backscattering (DBS) has been successfully used to measure the E x B flow velocity and local intermediate wavenumber density fluctuation levels in the DIII-D tokamak. Depending on the launch angle and the frequency of the probing beam, the signal backscattered from the plasma cut-off layer is sensitive to density fluctuations at a specific...