72 Matching Results

Search Results

Advanced search parameters have been applied.

Transient plasma estimation: a noise cancelling/identification approach

Description: The application of a noise cancelling technique to extract energy storage information from sensors occurring during fusion reactor experiments on the Tandem Mirror Experiment-Upgrade (TMX-U) at the Lawrence Livermore National Laboratory (LLNL) is examined. We show how this technique can be used to decrease the uncertainty in the corresponding sensor measurements used for diagnostics in both real-time and post-experimental environments. We analyze the performance of algorithm on the sensor data and discuss the various tradeoffs. The algorithm suggested is designed using SIG, an interactive signal processing package developed at LLNL.
Date: March 1, 1985
Creator: Candy, J.V.; Casper, T. & Kane, R.
Partner: UNT Libraries Government Documents Department

Microstability of the TMX tandem mirror experiments

Description: In the tandem mirror device, an efficient source of warm ions, the central cell, is available for stabilization of ion loss-cone instabilities. These instabilities previously limited ion confinement in single-cell mirror experiments. In the simple tandem mirror device, TMX, the drift cyclotron loss-cone (DCLC) mode was stabilized by plasma flow from the central cell into the end cell. However, to enhance the central-cell confinement and provide MHD stability, neutral beams were injected perpendicular to the magnetic field, which resulted in the excitation in the end cell of the Alfven ion-cyclotron (AIC) instability driven by plasma pressure and velocity distribution anisotropy. In the thermal-barrier experiment, TMX-U, the end-cell beams were injected at a 45/sup 0/ angle to the magnetic field to produce a sloshing-ion distribution, which is required to form the thermal barrier and the plugging potential. Ion distributions created by oblique injection were stable to the AIC mode and to the midplane (minimum magnetic field location) DCLC mode. However, an ion loss-cone instability remained at an axial location just outside the outboard peak of the sloshing-ion axial density profile, which is the density peak closest to the end wall. This mode can enhance the sloshing-ion loss rate, particularly at the lower levels of electron-cyclotron resonance heating (ECRH) used to form the thermal barrier. The stability to ion-cyclotron modes is critical to the performance of tandem mirrors and to designs for a mirror-based, high-fluence neutron source.
Date: August 1, 1987
Creator: Casper, T.A. & Berzins, L.V.
Partner: UNT Libraries Government Documents Department

Signal processing methods for MFE plasma diagnostics

Description: The application of various signal processing methods to extract energy storage information from plasma diamagnetism sensors occurring during physics experiments on the Tandom Mirror Experiment-Upgrade (TMX-U) is discussed. We show how these processing techniques can be used to decrease the uncertainty in the corresponding sensor measurements. The algorithms suggested are implemented using SIG, an interactive signal processing package developed at LLNL.
Date: February 1, 1985
Creator: Candy, J.V.; Casper, T. & Kane, R.
Partner: UNT Libraries Government Documents Department

Current profile modeling to extend the duration of high performance advanced Tokamak modes in DIII-D

Description: We use a model for negative central shear (NCS) heat transport which has a parametric dependence on the plasma conditions with a transport barrier dependence on the minimum of the safety factor profile, 4, qualitatively consistant with experimental observations. Our intention is not to do a detailed investigation of transport models but rather to provide a reasonable model of heat conductivity to be able to simulate effects of electron cyclotron heating (ECH) and current drive (ECCD) on confinement in NCS configurations. We adjust free parameters (c, cl and c2) in the model to obtain a reasonable representation of the temporal evolution of electron and ion temperature profiles consistent with those measured in selected DIII-D shots. In all cases, we use the measured density profiles rather than self- consistently solve for particle sources and particle transport at this time In these results, we employ a simple model for the ECH power deposition by providing an externally supplied heat source for the electrons. The heating deposition location and profile are specified as a function of the toroidal flux coordinate to allow us to independently vary the heating dynamics For the results shown here, we assume a Gaussian profile, typically using a width of {delta}{rho}=O.O5 ({delta}r-3 cm in minor radius), with {rho} defined as the square root of toroidal flux All powers are interpreted as that absorbed by the plasma Similarly, the current drive location and profile are specified with the total current (integrated over the assumed profile) modeled as I{sub EccD}={Gamma}PEcH/neR with {Gamma}=0 005T{sub e}, providing current drive efficiency consistent with earlier experiments [6] with a dependence on T{sub e}, but fixed Z{sub eff} and trapped particle effects in these simulations Future work will integtate the existing TORCH [7] code into this ECH modeling effort.
Date: June 25, 1998
Creator: Casper, T. A.
Partner: UNT Libraries Government Documents Department

Remote Experimental Site: A command and analysis center for Big Physics'' experimentation

Description: The next generation of tokamaks, ITER or BPX, will be characterized by an even greater emphasis on joint operation and experimentation. With anticipation of an increased number and diversity of collaborations, we are preparing for such shared facilities by developing a systematic approach to remote, joint physics operation involving experimental teams at several locations. The local area network of computers used for control and data acquisition on present and future experiments can be extended over a wide area network to provide a mechanism for remote operation of subsystems required for physics experiments. The technology required for high bandwidth ({ge}45Mbps) connections between multiple sites either exists or will be available over the next few years. With the rapid development of high performance workstations, network interfaces, distributed computing, and video conferencing, we can proceed with the development of a system of control and analysis sites to provide for consistent, efficient, and continuing collaborations. Early establishment of such sites could also enhance existing joint design and development efforts. 2 refs., 3 figs.
Date: September 1, 1991
Creator: Casper, T.A. & Lennon, W.J.
Partner: UNT Libraries Government Documents Department

Nature and effects of ion-cyclotron fluctuations in TMX

Description: In the tandem mirror experiment (TMX), coherent oscillations have been identified as resulting from the Alfven ion-cyclotron instability. Although the drive for this instability is localized in the end cell, the waves generated propagate out of the unstable region and interact with the central-cell ions. This interaction leads to an experimentally observed scaling of the stored end-cell energy with axial ion end-loss current.
Date: February 19, 1982
Creator: Casper, T.A.; Poulsen, P. & Smith, G.R.
Partner: UNT Libraries Government Documents Department

Effects of end cell ion cyclotron fluctuations on central-cell ion confinement in the tandem mirror experiment (TMX)

Description: The tandem mirror device (TMX) exhibits gross stability to both MHD and microinstability modes. The end-cell plasmas provide the tandem with average minimum-B stability, while the efflux of plasma from the central cell maintains the end cells (plugs) at marginal stability to loss cone modes. For some operating conditions, a residual level of plug ion cyclotron fluctuations is detected. These oscillations dominate the fluctuation frequency spectra in both the plugs and the central cell. The presence of plug ion cyclotron fluctuations in the central cell leads to resonance heating of some of the central cell ions. This heating degrades the confinement of the central cell ions; thereby increasing the amount of warm plasma stream flowing through the plugs.
Date: April 7, 1980
Creator: Grubb, D.P.; Casper, T.A. & Clauser, J.F.
Partner: UNT Libraries Government Documents Department

Ambipolar potential formation in TMX

Description: TMX experimental data on ambipolar potential control and on the accompanying electrostatic confinement are reported. New results on the radial dependence of the central-cell confining potential are given. Radial and axial particle losses as well as scaling of the central-cell axial confinement are discussed.
Date: May 5, 1981
Creator: Correl, D.L.; Allen, S.L. & Casper, T.A.
Partner: UNT Libraries Government Documents Department

Ambipolar potential formation in TMX

Description: TMX experimental data on ambipolar potential control and on the accompanying electrostatic confinement are reported. New results on the radial dependence of the central-cell confining potential are given. Radial and axial particle losses as well as scaling of the central-cell axial confinement are discussed.
Date: May 5, 1981
Creator: Correll, D.L.; Allen, S.L. & Casper, T.A.
Partner: UNT Libraries Government Documents Department

TMX-Upgrade (TMX-U) operation in the sloshing-ion mode

Description: This report summarizes initial results from TMX-U carried out from June through August 1982. In these successful experiments we operated TMX-U in the sloshing-ion mode. We generated sloshing ions, measured improved energy confinement, and observed improved microstability compared to TMX. The experiments operated about as we expected and we are pleased with the results. During this period many additional achievements were also recorded. The magnetically confined sloshing ions constitute one of the two ingredients needed to build a thermal barrier. The second ingredient consists of magnetically confined electrons, which will be studied in the next series of TMX-U experiments using microwave heating of the electrons. Later, the hot ions and electrons will be combined to form thermal barriers.
Date: September 24, 1982
Creator: Simonen, T.C.; Allen, S.L. & Casper, T.A.
Partner: UNT Libraries Government Documents Department

Applications of digital processing for noise removal from plasma diagnostics

Description: The use of digital signal techniques for removal of noise components present in plasma diagnostic signals is discussed, particularly with reference to diamagnetic loop signals. These signals contain noise due to power supply ripple in addition to plasma characteristics. The application of noise canceling techniques, such as adaptive noise canceling and model-based estimation, will be discussed. The use of computer codes such as SIG is described. 19 refs., 5 figs.
Date: November 11, 1985
Creator: Kane, R.J.; Candy, J.V. & Casper, T.A.
Partner: UNT Libraries Government Documents Department

Throttle coil operation of TMX-U

Description: A tandem-mirror configuration with an axisymmetric central cell, similar to the geometry of MARS (Mirror Advanced Reactor Study) or the Kelley-TDF mode of MFTB-B, can be generated by inserting a 6-tesla, throttle coil in each end of the TMX-U central cell. The throttle coil geometry of TMX-U will test the physics issues associated with axisymmetric tandem-mirror reactors, such issues as: (1) increased radial confinement time for central-cell ions confined by axisymmetric mirror cells and electrostatic potentials; (2) theoretical limits set by the trapped particle instability for the required passing density between the central cell and the end-cell anchor; and (3) pumping of trapped particles within the thermal barrier and transition regions with methods other than neutral beams. The central-cell plasma parameters for the throttle coil geometry are evaluated for two operating points. The first requires heating hardware (neutral beams and ECRH) and vacuum performance at the TMX-U proposal level, yielding plasma parameters, central-cell betas, and plasma confinement exceeding those of the original TMX-U proposal. The second operating point, requiring approximately half the ECRH end-cell performance of the first, is predicted to equal the beta and to exceed the plasma pressure and confinement time of the central cell in the standard TMX-U geometry.
Date: January 1, 1983
Creator: Correll, D.L.; Byers, J.A. & Casper, T.A.
Partner: UNT Libraries Government Documents Department

Plasma confinement in the TMX tandem mirror

Description: Plasma confinement in the Tandem Mirror Experiment (TMX) is described. Axially confining potentials are shown to exist throughout the central 20-cm core of TMX. Axial electron-confinement time is up to 100 times that of single-cell mirror machines. Radial transport of ions is smaller than axial transport near the axis. It has two parts at large radii: nonambipolar, in rough agreement with predictions from resonant-neoclassical transport theory, and ambipolar, observed near the plasma edge under certain conditions, accompanied by a low-frequency, m = 1 instability or strong turbulence.
Date: April 29, 1981
Creator: Hooper, E.B. Jr.; Allen, S.L. & Casper, T.A.
Partner: UNT Libraries Government Documents Department

Plasma confinement experiments in the TMX tandem mirror. Paper IAEA-CN-38/F-1

Description: Results from the new Tandem Mirror Experiment (TMX) are described. Tandem-mirror density and potential profiles are produced using end-plug neutral-beam injection and central-cell gas-fueling. TMX parameters are near those predicted theoretically. The end-plug electron temperature is higher than in the comparably sized single-mirror 2XIIB. Axial confinement of the finite-beta central-cell plasma is improved by the end plugs by as much as a factor of 9. In TMX, end-plug microinstability limits central-cell confinement in agreement with theory.
Date: May 22, 1980
Creator: Simonen, T.C.; Anderson, C.A. & Casper, T.A.
Partner: UNT Libraries Government Documents Department

Improved fusion performance in low-q, low triangularity plasmas with negative central magnetic shear

Description: Fusion performance in DIII-D low-q single-null divertor discharges has doubled as a result of improved confinement and stability, achieved through modification of pressure and current density profiles. These discharges extend the regime of neoclassical core confinement associated with negative or weak central magnetic shear to plasmas with the low safety factor (q{sub 95}{approximately}3) and triangularity ({delta}{approximately}0.3) anticipated in future tokamaks such as ITER. Energy confinement times exceed the ITER-89P L- mode scaling law by up to a factor of 4, and are almost twice as large as in previous single-null cases with 3{le}q{sub 95}{le}4. The normalized beta [{beta}(aB/I)] reaches values as high as 4, comparable to the best previous single-null discharges. Although high triangularity allows a larger plasma current, the fusion gain in these low triangularity plasmas is similar to that of high triangularity double-null plasmas at the same plasma current. These results are encouraging for advanced performance operation in ITER and for D-T experiments in JET.
Date: July 1, 1996
Creator: Strait, E.J.; Casper, T.N. & Chu, M.S.
Partner: UNT Libraries Government Documents Department

Collaboratory for support of scientific research

Description: Collaboration is an increasingly important aspect of magnetic fusion energy research. With the increased size and cost of experiments needed to approach reactor conditions, the numbers being constructed has become limited. In order to satisfy the desire for many groups to conduct research on these facilities, we have come to rely more heavily on collaborations. Fortunately, at the same time, development of high performance computers and fast and reliable wide area networks has provided technological solutions necessary to support the increasingly distributed work force without the need for relocation of entire research staffs. Development of collaboratories, collaborative or virtual laboratories, is intended to provide the capability needed to interact from afar with colleagues at multiple sites. These technologies are useful to groups interacting remotely during experimental operations as well as to those involved in the development of analysis codes and large scale simulations The term ``collaboratory`` refers to a center without walls in which researchers can perform their studies without regard to geographical location - interacting with colleagues, accessing instrumentation, sharing data and computational resources, and accessing information from digital libraries [1],[2]. While it is widely recognized that remote collaboration is not a universal replacement for personal contact, it does afford a means for extending that contact in a manner that minimizes the need for relocation and for travel while more efficiently utilizmg resources and staff that are geographically distant from the central facility location, be it an experiment or design center While the idea of providing a remote environment that is ``as good as being there`` is admirable, it is also important to recognize and capitalize on any differences unique to being remote [3] Magnetic fusion energy research is not unique in its increased dependence on and need to improve methods for collaborative research Many research disciplines find themselves ...
Date: June 25, 1998
Creator: Casper, T. A.; Meyer, W. H. & Moller, J. M.
Partner: UNT Libraries Government Documents Department

Rotational and magnetic shear stabilization of magnetohydrodynamic modes and turbulence in DIII-D high performance discharges

Description: The confinement and the stability properties of the DIII-D tokamak high performance discharges are evaluated in terms of rotational and magnetic shear with emphasis on the recent experimental results obtained from the negative central magnetic shear (NCS) experiments. In NCS discharges, a core transport barrier is often observed to form inside the NCS region accompanied by a reduction in core fluctuation amplitudes. Increasing negative magnetic shear contributes to the formation of this core transport barrier, but by itself is not sufficient to fully stabilize the toroidal drift mode (trapped- electron-{eta}{sub i}mode) to explain this formation. Comparison of the Doppler shift shear rate to the growth rate of the {eta}{sub i} mode suggests that the large core {bold E x B} flow shear can stabilize this mode and broaden the region of reduced core transport . Ideal and resistive stability analysis indicates the performance of NCS discharges with strongly peaked pressure profiles is limited by the resistive interchange mode to low {Beta}{sub N} {lt} 2.3. This mode is insensitive to the details of the rotational and the magnetic shear profiles. A new class of discharges which has a broad region of weak or slightly negative magnetic shear (WNS) is described. The WNS discharges have broader pressure profiles and higher values than the NCS discharges together with high confinement and high fusion reactivity.
Date: August 1, 1996
Creator: Lao, L.L.; Burrell, K.H. & Casper, T.S.
Partner: UNT Libraries Government Documents Department

Model development for transport studies in negative shear modes

Description: This study develops a simple transport model which can be used predictively for tokamak negative central shear (NCS) modes, with the assistance of Lawrence Livermore National Laboratory`s core plasma simulation code, CORSICA. The results show that the the Rebut-Lallia-Watkins Critical Electron Temperature Gradient Model, coupled with an NCS transport model and TRANSP data, renders a reasonably close match to experimental temperature profiles. Additionally, this research offers the first benchmark calculation indicating that the CORSICA code itself, when given transport coefficients from the analysis of experimental data, replicates the experimental profiles, indicating that both TRANSP and CORSICA together are consistent in their analysis of the plasma evolution. This means CORSICA is working properly and has no known major internal flaws. 14 refs., 12 figs.
Date: May 1, 1997
Creator: Spang, M.C.; Casper, T.B. & Thomassen, K.I.
Partner: UNT Libraries Government Documents Department

Milestone report: Status report on time-dependent modeling for current profile feedback control

Description: During the past year, LLNL efforts in the DIII-D experimental program have expanded to include time-dependent modeling of advanced tokamak (AT) operating modes. Consistent with our involvement in experimental operations, we have directed our initial efforts at modeling the negative central shear (NCS) configuration, an important and attractive mode of operation for reducing the size and cost of future tokamak experiments without sacrificing performance. In this endeavor, we have brought into use the Corsica modeling code as a tool for investigating the time-dependent evolution and control of various operating modes. In our current efforts, we are contributing to the analysis of the NCS experimental data using analysis tools such as the EFIT equilibrium code and the ONETWO and TRANSP transport codes. Results of these analyses are being used for comparisons with the Corsica modeling. Future directions include the modeling of startup and sustaining of NCS (and other AT) configurations, the understanding of current drive effects, the development of current drive scenarios and control algorithms, and the design of experiments and prediction of experimental results. We are currently in the early stages of applying this powerful modeling tool to the DIII-D experimental program.
Date: September 29, 1995
Creator: Casper, T.A.; Crotinger, J. & Haney, S.
Partner: UNT Libraries Government Documents Department

Remote experimental site concept development, LDRD final report

Description: Scientific research is now often conducted on large and expensive experiments that utilize collaborative efforts on a national or international scale to explore physics and engineering issues. This is particularly true for the current US magnetic fusion energy program where collaboration on existing facilities has increased in importance and will form the basis for future efforts. As fusion energy research approaches reactor conditions, the trend is towards fewer large and expensive experimental facilities, leaving many major institutions without local experiments. Since the expertise of various groups is a valuable resource, it is important to integrate these teams into an overall scientific program. To sustain continued involvement in experiments, scientists are now often required to travel frequently, or to move their families, to the new large facilities. This problem is common to many other different fields of scientific research. The next-generation tokamaks, such as the Tokamak Physics Experiment (TPX) or the International Thermonuclear Experimental Reactor (ITER), will operate in steady-state or long pulse mode and produce fluxes of fusion reaction products sufficient to activate the surrounding structures. As a direct consequence, remote operation requiring robotics and video monitoring will become necessary, with only brief and limited access to the vessel area allowed. Even the on-site control room, data acquisition facilities, and work areas will be remotely located from the experiment, isolated by large biological barriers, and connected with fiber-optics. Current planning for the ITER experiment includes a network of control room facilities to be located in the countries of the four major international partners; USA, Russian Federation, Japan, and the European Community.
Date: January 20, 1995
Creator: Casper, T. A.; Meyer, W. & Butner, D.
Partner: UNT Libraries Government Documents Department

Microstability of TMX-U during initial thermal barrier operation

Description: During the initial thermal barrier experiments on the Tandem Mirror Experiment-Upgrade (TMX-U), we successfully demonstrated the principle of improved axial tandem mirror confinement achieved by establishment of both the thermal barrier and the ion confining potential peak. During this operation, we created both hot (100-keV) mirror-confined electron and hot (8-keV) mirror-confined ion populations in the end cells. In certain parameter ranges, we observed these species to be weakly unstable to various microinstabilities, but we did not observe clear evidence for an absolute limit to confinement.
Date: March 1, 1984
Creator: Casper, T.A.; Berzins, L.V.; Ellis, R.F.; James, R.A. & Lasnier, C.
Partner: UNT Libraries Government Documents Department

Perpendicular electron cyclotron emission from hot electrons in TMX-U

Description: Perpendicular electron cyclotron emission (PECE) from the electron cyclotron resonant heating of hot electrons in TMX-U is measured at 30 to 40 and 50 to 75 GHz. This emission is optically thin and is measured at the midplane, f/sub ce/ approx. = 14 GHz, in either end cell. In the west end cell, the emission can be measured at different axial positions thus yielding the temporal history of the hot electron axial profile. These profiles are in excellent agreement with the axial diamagnetic signals. In addition, the PECE signal level correlates well with the diamagnetic signal over a wide range of hot electron densities. Preliminary results from theoretical modeling and comparisons with other diagnostics are also presented.
Date: September 14, 1984
Creator: James, R.A.; Ellis, R.F.; Lasnier, C.J.; Casper, T.A. & Celata, C.M.
Partner: UNT Libraries Government Documents Department

Status report on Corsica modeling for current drive scenario development

Description: This milestone report covers the progress and status of Corsica modeling for DIII-D experiments over the past year, since our previous report in September, 1995. During this time, we have concentrated on improvements to the code in support of our ability to do self-consistent, predictive modeling of DIII-D discharges. Our interest is in obtaining a tool, benchmarked with experimental data, for developing advanced tokamak operations scenarios including simulation and analysis of high performance negative central shear (NCS) discharges and control of the current profile evolution. Our major focus has been on installing and improving the neutral beam current drive mode in Corsica; this element is critical to modeling the evolution of DIII-D discharges. The NFREYA neutral beam deposition code was installed (starting with a version consistent with GA`s ONETWO code) and the capability for following particle orbits, including the effects of drifts, was added for determining the current driven by neutral beam -injection. In addition, improved methods for more easily integrating experimental profile measurements into the code operation and for calculating Z{sub eff} either from models or from impurity density measurements have been added. We have recently begun to turn on various transport models in our simulation of discharge evolution. We have concentrated on the NCS configuration and have simulated the evolution of two different high neutron reactivity discharges; an NCS discharge with L-mode edge and a single- null, weak NCS discharge from the JET/ITER/DIII-D equivalent shape experiments. Corsica simulation results for these discharges were presented at the EPS meeting in Kiev, Ukraine in June, 1996.
Date: September 1, 1996
Creator: Casper, T. A.; Crotinger, J.; Moller, J.M. & Pearlstein, L.D.
Partner: UNT Libraries Government Documents Department

Assessment of hot-electron microstability in the initial TMX-U experiments

Description: During the initial TMX-U experiments, we investigated the sloshing-ion and hot-electron distributions. We require these components to ultimately construct a thermal barrier for improved tandem mirror confinement. The plasma parameters we achieved approach values required for thermal barrier operation but have been limited by the power available. This report is concerned with the stability of the hot electron distribution formed. Nonthermal microwave emissions near the electron-cyclotron frequency f/sub ce/ of the minimum end-cell magnetic field indicate the presence of electron microinstabilities, which we have tentatively identified by their frequencies. We observed the upper-hybrid loss-cone mode (f/f/sub ce/ approx. 1.1) during high density operation with a relatively small fraction of hot electrons. At lower density operation with a higher hot electron fraction, we observed emissions consistent with the whistler instability. During emission bursts at 12.5 GHz (f/f/sub ce/ approx. 0.9) we observed a rapid rise in the high frequency thermal emissions, indicating a spreading of the distribution. Some of the more violent bursts are correlated with enhanced end loss currents.
Date: April 1, 1983
Creator: Casper, T.A.; Chen, Y.J.; Ellis, R.; James, R. & Lasnier, C.
Partner: UNT Libraries Government Documents Department