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Title: Spherical tokamak with a plasma center column

Abstract

Low aspect ratio toroidal pinches such as the standard (q>1) and the ultralow q (q<1) spherical tori or tokamaks (ST), would have a far more robust reactor engineering design if a plasma center column (PCC) can be used in place of a material center post. Biased electrodes across the plasma center column would drive a plasma current to produce the toroidal magnetic field in lieu of the toroidal field (TF) coils. The operation of such a device is naturally divided into two distinct phases: formation by driven relaxation under magnetic helicity injection and sustainment by auxiliary current drive and heating such as rf and neutral beam injection (NBI). The initial design constraints of a ST-PCC experiment are primarily motivated by the formation rather than the sustainment physics. With a Taylor-relaxed plasma as the baseline case, it is shown that three essential factors guide the design. First, the flux amplification factor determines the aspect ratio of the ST-PCC. Second, the plasma shaping in general and plasma elongation in particular gives the most freedom in shaping the q profile of the relaxed plasma. Two examples are the standard spherical tokamak with q>1 throughout the plasma and the ultralow q (ULQ) spherical tokamakmore » with q much less than unity for the bulk of the plasma. Third, the vacuum bias magnetic flux plays the second most important role in modifying the q profile. As an example, it is shown how the bias flux can be designed to delineate a standard spheromak experiment from that of an ULQ ST-PCC. These physics understandings help define the design space of the ST-PCC experiments and directions for optimization.« less

Authors:
;  [1];  [2]
  1. Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)
  2. (United States)
Publication Date:
OSTI Identifier:
20782750
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 13; Journal Issue: 4; Other Information: DOI: 10.1063/1.2195381; (c) 2006 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; ASPECT RATIO; BEAM INJECTION HEATING; DESIGN; ELECTRIC CURRENTS; ELONGATION; HELICITY; MAGNETIC FIELDS; MAGNETIC FLUX; OPTIMIZATION; PINCH EFFECT; PLASMA; PLASMA BEAM INJECTION; PLASMA CONFINEMENT; RADIATION TRANSPORT; RELAXATION; RF SYSTEMS; SPHEROMAK DEVICES; THERMONUCLEAR REACTORS

Citation Formats

Tang, X.Z., Boozer, A.H., and Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027. Spherical tokamak with a plasma center column. United States: N. p., 2006. Web. doi:10.1063/1.2195381.
Tang, X.Z., Boozer, A.H., & Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027. Spherical tokamak with a plasma center column. United States. doi:10.1063/1.2195381.
Tang, X.Z., Boozer, A.H., and Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027. Sat . "Spherical tokamak with a plasma center column". United States. doi:10.1063/1.2195381.
@article{osti_20782750,
title = {Spherical tokamak with a plasma center column},
author = {Tang, X.Z. and Boozer, A.H. and Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027},
abstractNote = {Low aspect ratio toroidal pinches such as the standard (q>1) and the ultralow q (q<1) spherical tori or tokamaks (ST), would have a far more robust reactor engineering design if a plasma center column (PCC) can be used in place of a material center post. Biased electrodes across the plasma center column would drive a plasma current to produce the toroidal magnetic field in lieu of the toroidal field (TF) coils. The operation of such a device is naturally divided into two distinct phases: formation by driven relaxation under magnetic helicity injection and sustainment by auxiliary current drive and heating such as rf and neutral beam injection (NBI). The initial design constraints of a ST-PCC experiment are primarily motivated by the formation rather than the sustainment physics. With a Taylor-relaxed plasma as the baseline case, it is shown that three essential factors guide the design. First, the flux amplification factor determines the aspect ratio of the ST-PCC. Second, the plasma shaping in general and plasma elongation in particular gives the most freedom in shaping the q profile of the relaxed plasma. Two examples are the standard spherical tokamak with q>1 throughout the plasma and the ultralow q (ULQ) spherical tokamak with q much less than unity for the bulk of the plasma. Third, the vacuum bias magnetic flux plays the second most important role in modifying the q profile. As an example, it is shown how the bias flux can be designed to delineate a standard spheromak experiment from that of an ULQ ST-PCC. These physics understandings help define the design space of the ST-PCC experiments and directions for optimization.},
doi = {10.1063/1.2195381},
journal = {Physics of Plasmas},
number = 4,
volume = 13,
place = {United States},
year = {Sat Apr 15 00:00:00 EDT 2006},
month = {Sat Apr 15 00:00:00 EDT 2006}
}
  • Results are presented from experiments on building up the plasma density in Tuman-3 using automatic control of gas puffing. A microwave phasometer was used to monitor the density, with a device for defining an evaluating function which is independent of the displacement of the plasma column. Data on the evolution of the density profile are used to determine the conditions under which this sensor operates effectively. Results are also presented on a device for detecting the displacement of the column using the microwave phasometer with an evaluating function which is independent of the density. Results are presented from the experimentalmore » comparison of the efficacy of microwave and electromagnetic sensors for the displacement. It is shown that the optimum choice is automatic control of gas puffing with subsequent use first of electromagnetic and then of microwave sensors. This arrangement is more sensitive to slow shifts of the plasma column in the quasistationary stage of the discharge than automatic control using electromagnetic sensors. 4 refs., 4 figs.« less
  • Recently, there have been several proposals to build low-aspect-ratio or spherical tokamaks with plasma currents in the range of 1 MA. These low-aspect-ratio tokamaks employ conventional engineering, except in the central core, which contains the central toroidal field conductors and an ohmic heating solenoid (if present). To achieve low aspect ratios, these components must be engineered to the limits of stress and thermal properties. Solutions are found for the steady-state cooling of the toroidal field conductors. The solenoid, which must be high performance to produce the flux swing required for a 1-MA plasma current, cannot be cooled steady state. Themore » mathematics and procedures necessary to study these issues are given. 26 refs., 18 figs., 4 tabs.« less
  • In the LATE device, Spherical Tokamak (ST) plasmas are formed by electron cyclotron heating (ECH) without center solenoid. Two types of experiments are described: (1) slow formation of ST plasmas and (2) spontaneous formation of ST plasmas with rapid-current-rise. A ST formation scenario is discussed including the current-drive mechanism, particle confinement and MHD equilibrium.
  • Two-dimensional lithium beam imaging technique has been applied in the spherical tokamak CPD (compact plasma wall interaction experimental device) to study the effects of magnetic field configurations on rf plasma boundary in the absence of any plasma current, and also for the measurement of a two-dimensional edge electron density profile. With the present working condition of the diagnostics, the minimum measured electron density can be {approx}1.0x10{sup 16} m{sup -3}; this is considered to be the definition for the plasma boundary. The performance of the lithium sheet beam is absolutely calibrated using a quartz crystal monitor. Experimental results reveal that magneticmore » field configuration, either mirror or so-called null, critically affects the rf plasma boundary. A sharp lower boundary is found to exist in magnetic null configuration, which is quite different from that in the weak mirror configuration. Theoretical calculations of particle drift orbit and magnetic connection length (wall-to-wall) suggest that only mirror trapped particles are confined within a region where the magnetic connection length is {approx}4.0 m or more. A two-dimensional edge electron density profile is obtained from the observed Li I intensity profile. Overdense plasma formation is discussed from the viewpoint of mode conversion of rf wave into electron Bernstein wave and its dependence on the electron density profile.« less