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Title: An investigation of alpha-like particles in the DIII-D tokamak using fusion-product diagnostics

Abstract

Two important issues associated with alpha physics, the single-particle behavior and the effect of collective fast-ion-driven instabilities on fast-ion confinement, have been studied in the DIII-D tokamak using fusion product diagnostics. The single-particle behavior of alpha-like tritons and [sup 3]He ions produced in deuterium-deuterium fusion reactions is studied using the d(t,n)[alpha] and d([sup 3]he,p)[alpha] fusion reactions. Fusion-produced MeV ions exhibit classical behavior in high field (B[sub T] [ge] 1.0 T) DIII-D discharges, including the new class of [open quotes]very high[close quotes] confinement plasmas. However, discharges with strong sawtooth, fishbone, or TAE activity exhibit anomalous fusion product losses. For the high field discharges without strong MHD activity, the data imply an effective diffusion coefficient smaller than [approximately]0.1 m[sup 2]/s, but in the presence of strong MHD activity, the effective diffusion of the MeV ions exceeds 1.0 m[sup 2]/s. Collective alpha particle processes are simulated by means of neutral beam injected energetic ion minorities. Toroidicity-induced Alfven Eigenmodes (TAE) are observed in DIII-D when energetic beam ions ([approximately]75 keV) are used to destabilize the mode. Measurements of the neutron emission indicate that up to 70% of the injected power is lost during strong TAE activity. Measurements of the poloidal distribution of fast-ion lossesmore » suggest that the losses are greatest near the vessel midplane. Fast-ion losses in discharges with combined fishbones and TAE bursts are 1.5 to 2 times greater than losses in fishbone discharges without TAE activity. The scaling of fast ion losses with MHD mode amplitude exhibits no threshold in the mode amplitude, suggesting that mode-particle pumping is the dominant loss mechanism.« less

Authors:
Publication Date:
Research Org.:
California Univ., Irvine, CA (United States)
OSTI Identifier:
7041595
Resource Type:
Miscellaneous
Resource Relation:
Other Information: Thesis (Ph.D.)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; HELIUM 3; CHARGED PARTICLE DETECTION; TOKAMAK DEVICES; DEUTERON REACTIONS; PLASMA INSTABILITY; TRITONS; HEAVY ION FUSION REACTIONS; PLASMA CONFINEMENT; CHARGED PARTICLES; CHARGED-PARTICLE REACTIONS; CLOSED PLASMA DEVICES; CONFINEMENT; DETECTION; EVEN-ODD NUCLEI; HELIUM ISOTOPES; INSTABILITY; ISOTOPES; LIGHT NUCLEI; NUCLEAR REACTIONS; NUCLEI; RADIATION DETECTION; STABLE ISOTOPES; THERMONUCLEAR DEVICES 700360* -- Fusion Reactions-- (1992-)

Citation Formats

Duong, H.H. An investigation of alpha-like particles in the DIII-D tokamak using fusion-product diagnostics. United States: N. p., 1993. Web.
Duong, H.H. An investigation of alpha-like particles in the DIII-D tokamak using fusion-product diagnostics. United States.
Duong, H.H. 1993. "An investigation of alpha-like particles in the DIII-D tokamak using fusion-product diagnostics". United States. doi:.
@article{osti_7041595,
title = {An investigation of alpha-like particles in the DIII-D tokamak using fusion-product diagnostics},
author = {Duong, H.H.},
abstractNote = {Two important issues associated with alpha physics, the single-particle behavior and the effect of collective fast-ion-driven instabilities on fast-ion confinement, have been studied in the DIII-D tokamak using fusion product diagnostics. The single-particle behavior of alpha-like tritons and [sup 3]He ions produced in deuterium-deuterium fusion reactions is studied using the d(t,n)[alpha] and d([sup 3]he,p)[alpha] fusion reactions. Fusion-produced MeV ions exhibit classical behavior in high field (B[sub T] [ge] 1.0 T) DIII-D discharges, including the new class of [open quotes]very high[close quotes] confinement plasmas. However, discharges with strong sawtooth, fishbone, or TAE activity exhibit anomalous fusion product losses. For the high field discharges without strong MHD activity, the data imply an effective diffusion coefficient smaller than [approximately]0.1 m[sup 2]/s, but in the presence of strong MHD activity, the effective diffusion of the MeV ions exceeds 1.0 m[sup 2]/s. Collective alpha particle processes are simulated by means of neutral beam injected energetic ion minorities. Toroidicity-induced Alfven Eigenmodes (TAE) are observed in DIII-D when energetic beam ions ([approximately]75 keV) are used to destabilize the mode. Measurements of the neutron emission indicate that up to 70% of the injected power is lost during strong TAE activity. Measurements of the poloidal distribution of fast-ion losses suggest that the losses are greatest near the vessel midplane. Fast-ion losses in discharges with combined fishbones and TAE bursts are 1.5 to 2 times greater than losses in fishbone discharges without TAE activity. The scaling of fast ion losses with MHD mode amplitude exhibits no threshold in the mode amplitude, suggesting that mode-particle pumping is the dominant loss mechanism.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = 1993,
month = 1
}

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  • A multichannel reflectometer diagnostic has been used to investigate density profiles and density fluctuations in the DIII-D tokamak. Dramatic decreases in high frequency microturbulence are observed at the L to H-mode transition. The spatial resolution of the diagnostic has allowed, for the first time, determination of the radial extent of these modifications. Fluctuation changes are observed to occur in a region from {approx}1 cm outside to {approx}10 cm inside the magnetic separatrix. These changes occur on a very rapid timescale, typically {le} 50 {mu}s and they occur simultaneously with the decrease in H{sub {alpha}} emission observed at the L tomore » H transition. The density fluctuation spectra during an ELM event are similar to those in L-mode. However, the fluctuation changes precede the rise in H{sub {alpha}} emission at the ELM, often by as much as 100 {mu}s. In H-mode, the width of the frequency spectra is observed to scale inversely with plasma current, but has little dependence on toroidal field. In L-mode the situation is reversed and the spectral width varies inversely with B{sub {phi}} and shows little variation with I{sub p}. Edge density gradient modifications are observed to occur within 5-10 ms of the L to H transition, whereas the average density continues to rise for more than 100 ms. This is consistent with predictions of an edge transport barrier formation in H-mode. In addition two distinct profile effects associated with ELMs have been identified. These are the density gradient collapse and the density pulse ELMS. Coherent density oscillations associated with low m/n number MHD activity have been observed. Using the multichannel reflectometer it is shown that these fluctuations persist in a region located within 1 to 2 cm of the magnetic separatrix. ELMs and sawteeth are seen to affect the coupling between the electromagnetic and electrostatic components of these oscillations.« less
  • Products of fusion reactions have been used to diagnose fusion plasmas in TFTR and PLT. In TFTR, pellet injection was used to determine time evolution of the triton tail present after neutral beam injection into low-density plasmas. Results show that for some plasmas, after the beam-injected deuterium ions have slowed down, the neutron emission can be dominated by the burnup of fusion-produced tritons. Neutron emission transients seen at the onset of the q-mode were studied and determined to be caused by an increase in the deuterium density at the transition. The subsequent decrease in electron temperature then led to amore » reduced neutron emission. A study of neutron sawteeth during neutral beam heating on TFTR was done to determine the effect of sawteeth on fast ion confinement. The neutron emission was observed to drop on a time scale of a few tens to a few hundred microseconds. The magnitude of the drop scaled with the sawtooth period and depended on qc. A method for measuring the birth profile of alpha particles in a tokamak using solid state nuclear track detectors was developed and used to make the first measurements of escaping alpha particles from a tokamak. The feasibility of measuring the products of fusion reactions in coincidence was evaluated. Initial tests of the method were performed using reactions between beam deuterium ions and the background gas in the neutralizer region of a TFTR neutral beam. An experiment to measure the diffusion of electron-mass particles by depositing positrons in a plasma by ionization of a positronium beam was simulated taking into account the relevant atomic physics of positronium.« less
  • Although it has long been suggested that microturbulence is responsible for anomalous transport, surprisingly little is known about the turbulence drive and suppression mechanisms which determine the observed levels of fluctuations. In DIII-D H-mode discharges, microturbulence has been observed to change in two regions. Collective FIR scattering has confirmed a rapid suppression in edge turbulence at the transition, as well as locally increased edge electric field shear, strongly supporting theoretical models of shear suppression of edge turbulence. During the ELM-free H-mode phase, interior turbulence is observed to reduce on a slower timescale, coincident with increased electric field shear and reducedmore » [del]n[sub e]. The question then arises as to whether shear flow stabilization might also be responsible for the slower interior turbulence reduction. Experiments have been performed whereby the internal electric field is modified while maintaining similar pressure profiles. First, the long-term turbulence reduction observed in H-mode plasmas has been found to be much larger in boronized discharges when strong electric field shear is observed to penetrate deeper into the confinement region than in unboronized discharges. Second, comparison of co- and counter-injection discharges has revealed fluctuation levels increase in co- and decrease in counter-injection L-mode. Third, results have been obtained during experiments where the interior electric field was reduced through the use of [open quotes]magnetic braking[close quotes] of the toroidal rotation. As expected, the [rvec E] [times] [rvec B] Doppler shift of the scattered fluctuation spectrum decreased as the electric field was reduced. Additionally, increases in turbulence levels were observed as the internal electric field and associated shear was reduced. In summary, interior fluctuation measurements indicate large reductions correlated with changes in the density and radial electric field profiles.« less
  • This thesis analyzes impurity profiles in the DIII-D tokamak. Impurity emissions are studied in magnetically confined fusion research because they are an important component of the overall energy balance. Impurity studies are closely related to issues such as plasma resistivity and energy transport. Two novel, complementary methods were developed to determine the distribution and content of impurities in the DIII-D tokamak. The first method employed a spectrometer designed to record the simultaneous spatial profile history of many spectral lines. This spectrometer was used to study DIII-D plasmas from October 1986 through July 1988. Applications include the study of rapid redistributionmore » of impurities during discharges exhibiting enhanced energy confinement (H-mode). The spectrometer was also used characterize the general impurity characteristics of DIII-D plasma; the dominant impurities in the tokamak are carbon and nickel, which are primary constituents in vacuum vessel wall materials. The second method of impurity profile analysis combines measurements from several (non-spectroscopic) diagnostics to determine impurity concentration profiles. Application of this method to impurity studies of H-mode discharges of DIII-D has provided some surprising results. As opposed to the dramatic impurity accumulation in the plasma core that has been observed on other tokamaks, in high plasma current (2.0 MA) discharges on DIII-D, central impurity levels remain low (<0.02%) throughout the H-mode, and are lower than those seen in L-mode discharges.« less
  • With simultaneous spectral, temporal and chordal resolution, the STRS spectrometer was built to increase both the quality and quantity of spectroscopic impurity information available from tokamak discharges. The astigmatic properties of a grazing incidence spectrometer disperse spectra and provides angular resolution. STRS was installed at the DIII-D tokamak, and its data is compared to simulations to analyze impurity behavior. An unusual impurity behavior was discovered at DIII-D during H-mode with giant edge localized modes (ELMs): depending on the plasma current, impurities either accumulate in the plasma center, or are driven out from it. They also modulate at the ELM frequency.more » This behavior was found to depend on the electron density profile which oscillates between centrally peaked and hollow. Naturally occurring ELM phenomena produce electron density oscillations which cause impurity cycling and allow transport studies of intrinsic impurities. Particle flux is modeled with a constant anomalous diffusion coefficient and a convection coefficient which depends on electron density gradients. MIST impurity transport code simulation use time dependent electron density profiles, oscillating with giant ELMs, to show impurity concentrations follow the density peak as theoretically predicted. This reproduced the observations of intrinsic nickel spectra, and indicates that impurity transport in DIII-D h-mode is dominated by ion density gradients.« less