Core Internal Transport Barriers on Alcator C-Mod PDF Download

Author: Catherine L. Fiore
Publisher:
ISBN:
Category :
Languages : en
Pages : 26

View

Book Description
The formation of internal transport barriers (ITB) has been observed in the core region of Alcator C-Mod under a variety of conditions. The improvement in core confinement following pellet injection (pellet enhanced performance or PEP mode) has been well documented on Alcator C-Mod in the past. Recently three new ITB phenomena have been observed which require no externally applied particle or momentum input. Short lived ITBs form spontaneously following the high confinement (H) to low confinement (L) mode transition and are characterized by a large increase in the global neutron production (enhanced neutron or EN modes.) Experiments with ICRF (ion cyclotron range of frequencies) power injection to the plasma off-axis on the high field side results in the central density rising abruptly and becoming peaked. The ITB formed at this time lasts for 10 energy confinement times. The central toroidal rotation velocity decreases and changes sign as the density rises. Similar spontaneous ITBs have been observed in ohmically heated H-mode plasmas. All of these ITB events have strongly peaked density profiles with a minimum in the density scale length occurring near r/a = 0.5 and have improved confinement parameters in the core region of the plasma. Keywords: Alcator C-Mod; confinement; tokamaks; transport phenomena; neutrons.

Author: Catherine L. Fiore
Publisher:
ISBN:
Category :
Languages : en
Pages : 90

View

Book Description
Internal transport barriers, marked by steep density and pressure profiles and reduction of core transport are obtained in Alcator C-Mod. Transient single barriers are observed at the back transition from H- to L- mode, and also when pellet injection is accompanied by ICRF power. Double barriers are induced with injection of off-axis ICRF power deposition. These also arise spontaneously in Ohmic H-mode plasmas when the H-mode lasts for several energy confinement times. C-Mod provides a unique platform for studying such discharges: the ions and electrons are tightly coupled by collisions with Ti/Te=1, and the plasma has no internal particle or momentum sources. ITB plasmas with average pressure greater than 1 atmosphere have been obtained. To form an ITB, particle and thermal flux are reduced in the barrier region, allowing the neoclassical pinch to peak the density while maintaining the central temperature. Gyrokinetic simulation suggests that long wavelength drift wave turbulence in the core is marginally stable at the ITB onset, but steepening of the density profile destabilizes trapped electron (TEM) modes inside the barrier. The TEM ultimately drives sufficient outgoing particle flux to balance the inward pinch and halt further density rise, which allows control of particle and impurity peaking.

Author: Catherine L. Fiore
Publisher:
ISBN:
Category :
Languages : en
Pages : 58

View

Book Description
Recent studies of internal transport and double transport barrier regimes in Alcator C-Mod [I.H. Hutchinson, et al., Phys. Plasmas 1, 1511 (1994)] have explored the limits for forming, maintaining, and controlling these plasmas. C-Mod provides a unique platform for studying such discharges: the ions and electrons are tightly coupled by collisions and the plasma has no internal particle or momentum sources. The double-barrier mode comprised of an edge barrier with an internal transport barrier (ITB) can be induced at will using off-axis ion cyclotron range of frequency (ICRF) injection on either the low or high field side of the plasma with either of the available ICRF frequencies (70 or 80 MHz). When enhanced D [alpha] high confinement mode (EDA H-mode) is accessed in Ohmic plasmas, the double barrier ITB forms spontaneously if the Hmode is sustained for [approx.] high confinement times. The ITBs formed in both Ohmic and ICRF heated plasmas are quite similar regardless of the trigger method. They are characterized by strong central peaking of the electron density, and reduction of the core particle and energy ...

Author: Catherine L. Fiore
Publisher:
ISBN:
Category :
Languages : en
Pages : 21

View

Book Description
(cont.) Examination of the gyrokinetic stability indicates that the density and temperature profiles of the plasma core are inherently stable to long- wavelength drift mode driven turbulence at the onset time of the ITB, but that the increasing density gradients cause the trapped electron mode (TEM) to play a role in providing a control mechanism to ultimately limit the steepness of the density gradient in the ITB region.

Author: Kirill Zhurovich
Publisher:
ISBN:
Category :
Languages : en
Pages : 185

View

Book Description
Internal transport barriers (ITBs) in tokamak plasmas are characterized by the reduction of transport in one or more of the particle, momentum, or energy channels in the core plasma region. On Alcator C-Mod, significant contributions to ITB studies were made possible with the core Thomson scattering (TS) diagnostic, which measures profiles of electron temperature (0.03

Author: Stephen J. Wukitch
Publisher:
ISBN:
Category :
Languages : en
Pages : 42

View

Book Description
Double transport barrier modes (simultaneous core and edge transport barrier) have been observed with off-axis ion cyclotron range of frequencies (ICRF) heating in the Alcator C-Mod tokamak [I.H. Hutchinson et al., Phys. Plasmas 1, 1511(1994)]. An internal transport barrier (ITB) is routinely produced in enhanced D[alpha] H-mode (EDA) discharges where the minority ion cyclotron resonance layer is at r/a (0.5) during the current flat top phase of the discharge. The density profile becomes peaked without the presence of a particle source in the plasma core and continues to peak until the increased core impurity radiation arrests the improved energy confinement, ultimately leading to a barrier collapse. With the addition of moderate (0.6 MW) central ICRF heating, the double barrier mode was maintained for as long as the ICRF power was applied and modeling shows that the internal thermal barrier was maintained throughout the discharge. The presence of sawteeth throughout most of the ITB discharge allows sawtooth induced heat pulse analysis to be performed. This analysis indicates that there is an abrupt radial discontinuity in the heat pulse time to peak profile when an ITB is present. Furthermore, this discontinuity appears to move into the core plasma from the edge region in about 0.2 sec, several confinement times. The deduced thermal diffusivity, Xhp indicates a barrier exists in the electron thermal transport, the barrier is limited to a narrow radial region, and the transport is unaffected outside this narrow radial extent.

Author: John E. Rice
Publisher:
ISBN:
Category :
Languages : en
Pages : 32

View

Book Description
Internal transport barrier (ITB) plasmas with peak pressures of 0.25 MPa and pressure gradients as large as 2.5 MPa/m have been produced in Alcator C-Mod using off-axis ICRF heating. The onset of the ITBs is apparent when the pressure gradient exceeds 1.0 MPa/m, which is similar to the JET criterion of [rho]* s/L [equal to or greater than] 0.014. Concomitant with the peaking of the core pressure as the ITB develops is a drop of the toroidal rotation velocity profile inside of the barrier foot; the maximum of the velocity gradient coincides with the peak in the pressure gradient. The quasi-coherent (QC) mode, associated with the enhanced D[alpha] (EDA) H-mode plasmas which evolve the ITBs, breaks up and disappears as the barriers develop, even though the measured edge pedestal parameters remain fixed. The position of the ITB foot has been moved over a range of 1/3 of the plasma minor radius by varying the toroidal magnetic field. The peak in the calculated bootstrap current density profile has correspondingly been regulated over a similar range in plasma minor radius. The location of the density profile foot is found to expand as the toroidal magnetic field is reduced and the ICRF frequency is lowered. The density foot radius is relatively independent of q95, however, in a scan of the plasma current at fixed BT and wave frequency.

Author: Martin Greenwald
Publisher:
ISBN:
Category :
Languages : en
Pages : 174

View

Book Description
(cont.) This link unified L-mode and H-mode and established a strong connection between local and global transport. Further work on the role of critical gradient lengths and marginal stability lent quantitative support to the ITG theories for ion transport and have helped elucidate nonlinear saturation mechanisms for the turbulence. Local transport studies demonstrated connections between transport channels, with energy, particle and momentum transport varying across regimes in similar ways. Experiments carried out in collaboration with the DIII-D, ASDEX-U and JET groups confirmed the dimensionless scaling approach over the widest available range in machine sizes. These studies suggest that plasma physics is the dominant influence on transport in the core and pedestal for standard L- and H-mode discharges. Dimensionless scaling experiments have shown a strong improvement in confinement with the normalized gyro-size (1/p*). Confinement was found to be Bohm-like in L-mode and gyro-Bohm-like in H-mode. These experiments also showed a strong degradation in confinement with collisionality. Other articles in this issue discuss impurity transport, momentum transport, H-mode pedestal and threshold physics and internal transport barrier regimes.

Author: John E. Rice
Publisher:
ISBN:
Category :
Languages : en
Pages : 38

View

Book Description
Double transport barrier plasmas comprised of an edge enhanced Da (EDA) H-mode pedestal and an internal transport barrier (ITB) have been observed in Alcator C-Mod. The ITB can be routinely produced in ICRF heated plasmas by locating the wave resonance off-axis near r/a [approx.] 0.5, provided the target plasma average density is above [approx.] 1.4 1020/m3, and can develop spontaneously in some Ohmic H-mode discharges. The formation of the barrier appears in conjunction with a decrease or reversal in the central (impurity) toroidal rotation velocity. The ITB foot is located near r/a = 0.5, regardless of how the barrier was produced. The ITBs can persist for [approx.] 15 energy confinement times (tE), but exhibit a continuous increase of the central electron density, up to values near 11021/m3 (in the absence of an internal particle source), followed by collapse of the barrier. This barrier is also evident in the ion temperature profiles, and a significant drop of the core thermal conductivity, ceff, when the barrier forms is confirmed by modeling. Application of additional on-axis ICRF heating arrests the density and impurity peaking, which occurs along with an increase (co-current) in the core rotation velocity. Steady state double barrier plasmas have been maintained for 10 tE or longer, with n/nGW [approx.] 0.75 and with a bootstrap fraction of 0.13 near the ITB foot. The trigger mechanism for the ITB formation is presently not understood.

Author:
Publisher:
ISBN:
Category :
Languages : en
Pages :

View

Book Description
H-mode experiments on Alcator C-Mod [I.H. Hutchinson, et al., Phys. Plasma 1 (1994) 1511] which exhibit an internal transport barrier (ITB), have been examined with gyrokinetic simulations, near the ITB onset time. Linear simulations support the picture of ion and electron temperature gradient (ITG, ETG) microturbulence driving high [chi][sub i] and [chi][sub e], respectively, and that stable ITG correlates with reduced particle transport and improved ion thermal confinement on C-Mod. In the barrier region ITG is weakly unstable, with a critical temperature gradient higher than expected from standard models. Nonlinear calculations and the role of E x B shear suppression of turbulence outside the plasma core are discussed in light of recent profile measurements for the toroidal velocity. The gyrokinetic model benchmarks successfully against experiment in the plasma core.