Author: Maxim V. UmanskyPublisher:
ISBN:
Category :
Languages : en
Pages : 12
Book Description
Comments on Particle and Energy Balance in the Edge Plasma of Alcator C-Mod
Author: Maxim V. UmanskyDensity Limit and Cross-field Edge Transport Scaling in Alcator C-Mod
Author: Brian LaBombardPublisher:
ISBN:
Category :
Languages : en
Pages : 16
Book Description
Recent experiments in Alcator C-Mod have uncovered a direct link between the character and scaling of cross-field particle transport in the edge plasma and the density limit, nG. As ne/nG is increased from low values to values approaching [approx.] 1, an ordered progression in the cross-field edge transport physics occurs: first benign cross-field heat convection, then cross-field heat convection impacting the scrape-off layer (SOL) power loss channels and reducing the separatrix electron temperature, and finally 1bursty2 transport (normally associated with the far SOL) invading into closed flux surface regions and carrying a convective power loss that impacts the power balance of the discharge. These observations suggest that SOL transport and its scaling with plasma conditions plays a key role in setting the empirically observed density limit scaling law.
Comparison of Particle Transport in the Scrapeoff Layer Plasmas of Alcator C-Mod and DIII-D
Author: B. LipschultzPublisher:
ISBN:
Category :
Languages : en
Pages : 50
Book Description
Scrapeoff Layer (SOL) data from DIII--D and C--Mod have been acquired and analyzed for radial particle transport based on a particle balance model. This has allowed a detailed comparison for L--mode plasmas. The inferred radial particle flux, ... , is parameterized in terms of diffusive [ ... ] and convective particle transport [ ... ]. The magnitude of the inferred Deff or veff increases across the SOL for both tokamaks. The inferred Deff or veff in the 'far' SOL (one density e-folding length from the separatrix and beyond) are essentially unchanged by changes in core density by factors of 2-3. This corresponds to changes in the far SOL density, collisionality (v*), and radial fluxes of a factor of 10 or more. Thus v* does not appear to be an important parameter in determining the radial particle transport in that region. The dimensionlessly-scaled SOL plasma profiles from the two tokamaks overlay for similar dimensionless plasma parameters. The SOL density profile near the separatrix is steeper than in the 'far' SOL. The scaled Deff and veff are slightly larger on DIII-D than C-Mod. This difference appears to be within experimental uncertainties. Neutral ionization in the SOL does not appear to affect radial transport but may be related to the observed flattening of the density profiles with increasing ... .
Scientific and Technical Aerospace Reports
Author: Perturbative Transport Experiments and Time-dependent Modeling in Alcator C-Mod and DIII-D
Author: Pablo RodrÃguez Fernández (Ph. D.)Publisher:
ISBN:
Category :
Languages : en
Pages : 191
Book Description
Perturbative transport experiments in magnetically confined plasmas have shown, for more than 20 years, that the injection of cold pulses at the plasma edge can trigger the fast increase of core temperature. Because no single standard local transport model tried to date has been able to reproduce satisfactorily all the observed temporal behavior in the experiments, these transient transport phenomena feature prominently as an open question in the community and as a challenge for predictive capabilities in tokamak burning plasmas, such as ITER and SPARC. For the first time after more than two decades of experimental evidence, this Thesis resolves this long-standing enigma in plasma transport, by modeling of experiments conducted on the Alcator C-Mod and DIII-D tokamaks. Predictive integrated simulations with the Trapped Gyro Landau Fluid (TGLF) quasilinear transport model demonstrate that the increase of core temperature in some regimes, and lack thereof in other regimes, can be explained by a change in dominant linear micro-instability in the plasma core. The effect of major radius, electron density and plasma current on the cold pulse are well captured by TGLF, including the relative change in position of the temperature flex point as current density changes. Linear stability analysis of simulated density and current scans in Alcator C-Mod reveals a competition between trapped electron and ion temperature gradient modes as the main driver of the core transient response. Measurements of electron density evolution during the cold-pulse propagation in DIII-D are enabled by a high time resolution density profile reflectometer. The density evolution reveals the quick propagation of a pulse from edge to core, which is the mechanism to transiently increase core temperature in low-collisionality plasmas. The work presented in this Thesis demonstrates that the existence of nonlocal heat transport phenomena is not necessary for explaining the behavior and time scales of cold-pulse experiments in tokamak plasmas.
Measurement of Particle Transport Coefficients on Alcator C-Mod
Author: Publisher:
ISBN:
Category :
Languages : en
Pages :
Book Description
The goal of this thesis was to study the behavior of the plasma transport during the divertor detachment in order to explain the central electron density rise. The measurement of particle transport coefficients requires sophisticated diagnostic tools. A two color interferometer system was developed and installed on Alcator C-Mod to measure the electron density with high spatial ([approx] 2 cm) and high temporal ([le] 1.0 ms) resolution. The system consists of 10 CO[sub 2] (10.6[mu]m) and 4 HeNe (.6328[mu]m) chords that are used to measure the line integrated density to within 0.08 CO[sub 2] degrees or 2.3[times] 10[sup 16]m[sup[minus]2] theoretically. Using the two color interferometer, a series of gas puffing experiments were conducted. The density was varied above and below the threshold density for detachment at a constant magnetic field and plasma current. Using a gas modulation technique, the particle diffusion, D, and the convective velocity, V, were determined. Profiles were inverted using a SVD inversion and the transport coefficients were extracted with a time regression analysis and a transport simulation analysis. Results from each analysis were in good agreement. Measured profiles of the coefficients increased with the radius and the values were consistent with measurements from other experiments. The values exceeded neoclassical predictions by a factor of 10. The profiles also exhibited an inverse dependence with plasma density. The scaling of both attached and detached plasmas agreed well with this inverse scaling. This result and the lack of change in the energy and impurity transport indicate that there was no change in the underlying transport processes after detachment.
Impurity Transport Experiments in the Edge Plasma of Alcator C-Mod Using Gas Injection Plumes
Author: Sanjay GangadharaControl of Internal Transport Barriers on Alcator C-Mod
Author: Catherine L. FiorePublisher:
ISBN:
Category :
Languages : en
Pages : 58
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 ...
Experiments and Simulations of ITER-like Plasmas in Alcator C-Mod
Author: Publisher:
ISBN:
Category :
Languages : en
Pages :
Book Description
Alcator C-Mod is performing ITER-like experiments to benchmark and verify projections to 15 MA ELMy H-mode Inductive ITER discharges. The main focus has been on the transient ramp phases. The plasma current in C-Mod is 1.3 MA and toroidal field is 5.4 T. Both Ohmic and ion cyclotron (ICRF) heated discharges are examined. Plasma current rampup experiments have demonstrated that (ICRF and LH) heating in the rise phase can save voltseconds (V-s), as was predicted for ITER by simulations, but showed that the ICRF had no effect on the current profile versus Ohmic discharges. Rampdown experiments show an overcurrent in the Ohmic coil (OH) at the H to L transition, which can be mitigated by remaining in H-mode into the rampdown. Experiments have shown that when the EDA H-mode is preserved well into the rampdown phase, the density and temperature pedestal heights decrease during the plasma current rampdown. Simulations of the full C-Mod discharges have been done with the Tokamak Simulation Code (TSC) and the Coppi-Tang energy transport model is used with modified settings to provide the best fit to the experimental electron temperature profile. Other transport models have been examined also. __________________________________________________
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