Ray Seraydarian

Beyond paradigm: Turbulence, transport, and the origin of the radial electric field in low to high confinement mode transitions in the DIII‐D tokamak

The paradigm of shear suppression of turbulence as the mechanism for the low to high confinement mode (L to H) transition is examined by quantitative comparison of the predictions of the paradigm with experimental results from the DIII‐D tokamak [Plasma Physics and Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1986), p. 159]. The L to H transition trigger is V×B rotation, not the main ion pressure gradient. The radial electric field Er shear increases before the fluctuation suppression, consistent with increasing Er shear as the cause of the turbulence suppression. The spatial dependence of the turbulence reduction is consistent with shear suppression for negative Er shear. For positive Er shear, the turbulence suppression is consistent with the effect of Er curvature for modes for which an Er well is destabilizing. Finally, the transport barrier depends on the phase angle between the density and potential fluctuations inside the Er well, an effect not included in existing L to H...

Recent liquid lithium limiter experiments in CDX-U

Recent experiments in the Current Drive Experiment-Upgrade (CDX-U) provide a first-ever test of large area liquid lithium surfaces as a tokamak first wall to gain engineering experience with a liquid metal first wall and to investigate whether very low recycling plasma regimes can be accessed with lithium walls. The CDX-U is a compact (R = 34 cm, a = 22 cm, Btoroidal = 2 kG, IP = 100 kA, Te(0) ∼ 100 eV, ne(0) ∼ 5 × 10 19 m −3 ) spherical torus at the Princeton Plasma Physics Laboratory. A toroidal liquid lithium pool limiter with an area of 2000 cm 2 (half the total plasma limiting surface) has been installed in CDX-U. Tokamak discharges which used the liquid lithium pool limiter required a fourfold lower loop voltage to sustain the plasma current, and a factor of 5–8 increase in gas fuelling to achieve a comparable density, indicating that recycling is strongly reduced. Modelling of the discharges demonstrated that the lithium limited discharges are consistent with Zeffective < 1.2 (compared with 2.4 for the pre-lithium discharges), a broadened current channel and a 25% increase in the core electron temperature. Spectroscopic measurements indicate that edge oxygen and carbon radiation are strongly reduced.

Spectroscopic determination of the singly ionized helium density in low electron temperature plasmas mixed with helium in a linear divertor plasma simulator

The spectroscopic method is developed to obtain the He+ ion density nHe+ in low electron temperature, Te=5–20eV, plasmas mixed with He. Plasmas were produced in the PISCES-B linear divertor plasma simulator [R. P. Doerner et al., Phys. Scr. T111, 75 (2004)] where the electron densities are ne=(1−15)×1018m−3 and the ionization degree is ∼1–10%. In the method, the He I line intensity IHeI at λ=447.1nm is used, instead of the He II line intensity in the conventional method. The radial confinement time of He+ ions is requisite, and is measured to be at a level of the Bohm confinement time. The He+ ion concentration, nHe+∕ne, is found to be proportional to IHeI, and to weakly depend on ne and Te. Because of the higher ionization energy of He than other species (D2, Ne, and Ar), the measured nHe+∕ne becomes systematically lower than the He gas pressure fraction, and agrees with data from an omegatron mass spectrometer. The omegatron measurement and estimates of the He+ ion loss rates indicate that the influence...

Particle balance measurements during detachment in a gas-target divertor simulator

Particle balance measurements have been performed in low-density [Ne(r=0)<1011 cm−3], low temperature [Te(r=0)<5 eV] hydrogen plasma discharges in a linear gas-target divertor simulator device. Axial density drops of up to a factor 50 were observed at high gas-target pressures (Ptarget≈30 mTorr). For these highly detached plasmas, the largest particle loss term was found to be radial transport to the side walls. These results show that cross-field transport can be important for plasma detachment, which is traditionally thought of as resulting from volume recombination.

Plasma deposition of boron films with high growth rate and efficiency using carborane

Abstract The injection of carborane (C2B10H12) on the PISCES-B linear plasma device has been used to produce boron containing films on various target species. Film growth rates achieved are extremely high (up to 30 nm/s) compared to those typically found for glow discharges (∼0.01 nm/s). For low-Z target materials (C and Al) the film production is highly efficient, with the boron film growth rate comparable to the incident ion flux and the injection rate of boron atoms. The boron to carbon ratio is 3.0–3.6 for these films. Similarly high growth rates (∼10 nm/s) are obtained with high-Z target (W), but with lower deposition efficiency and higher B/C film ratio. The high film growth rate/efficiency are apparently linked to the high degree of carborane ionization and dissociation caused by the ∼40 eV PISCES-B plasma, compared with T

Beryllium deposition on International Thermonuclear Experimental Reactor first mirrors: Layer morphology and influence on mirror reflectivity

Metallic mirrors will be essential components of the optical diagnostic systems in the International Thermonuclear Experimental Reactor (ITER). Reliability of these systems may be affected by mirror reflectivity changes induced by erosion and/or deposition of impurities (carbon, beryllium). The present study aims to assess the effect of beryllium (Be) deposition on the reflectivity of metallic mirrors and to collect data on the optical quality of these layers in terms of morphology, roughness, etc. Mirrors from molybdenum and copper were exposed in the PISCES-B linear plasma device to collect eroded material from graphite and beryllium targets exposed to beryllium-seeded deuterium plasma. After exposure, relative reflectivity of the mirrors was measured and different surface analysis techniques were used to investigate the properties of the deposited layers. Be layers formed in PISCES-B exhibit high levels of porosity which makes the reflectivity of the Be layers much lower than the reflectivity of pure B...

Plasma interaction with liquid lithium: Measurements of retention and erosion

Abstract This paper reports on recent studies of high flux deuterium and helium plasma interaction with liquid lithium in the Pisces–B edge plasma simulator facility. Deuterium retention is explored as a function of plasma ion fluence in the range 6×10 19 –4×10 22 atoms cm −2 and exposure temperatures of 523–673 K. The results are consistent with full uptake of the deuterium ions incident on the liquid metal surface, independent of the temperature of the liquid lithium. Full uptake continues until the sample is volumetrically converted to lithium deuteride. Helium retention is not observed for fluences up to 5×10 21 He atoms cm −2 . Measurements of the erosion of lithium are found to be consistent with physical sputtering for the lithium solid phase. However, a mechanism that provides an increased evaporative-like yield and is related to ion impact events on the surface, dominates during the liquid phase leading to an enhanced loss rate for liquid lithium that is greater than the expected loss rate due to evaporation at elevated temperatures. Further, the material loss rate is found to depend linearly on the incident ion flux, even at very high temperature.

CARBON IMPURITY CHARACTERIZATION ON A LINEAR PLASMA DEVICE USING VISIBLE EMISSION SPECTROSCOPY

Visible emission spectroscopy is used to quantify the carbon impurity concentration in the linear plasma divertor simulator of the Plasma Interaction Surface Component Experimental Station (PISCES-B). A technique has been developed to obtain noninvasively the absolute photometric calibration of a visible spectrometer in order to minimize exposure of equipment in the beryllium safety enclosure of PISCES-B. The principal intrinsic source of carbon appears to be chemical sputtering at the vessel walls, with a typical background concentration ∼0.2% with deuterium plasmas. Helium plasmas have a lower carbon contamination ∼0.01%. Methane gas injection is used to increase the carbon contamination in a controlled manner to better simulate tokamak edge plasma conditions. It is found that the spectroscopic method of determining the carbon fraction agrees well with the relative gaseous carbon contamination measured with a residual gas analyzer during plasma operations.

Transport properties of hydrogen isotopes in boron carbide structures

The transport of implanted hydrogen isotopes in the refractory semiconducting ceramic, boron carbide, is investigated using the TMAP4 code. A review of experimental results for the diffusivity and solubility of hydrogen isotopes in boron carbide is presented, which provide Arrhenius expressions for the kinetics of hydrogen isotope transport. These expressions are utilized in the TMAP4 model to provide predictions for the hydrogen isotope implantation and desorption for experiments now in progress at the UCSD PISCES laboratory for the National Ignition Facility (NIF).

Development of a pulsed-biasing system and temperature measurement techniques for transient heating experiments on plasma-material interactions

A power switching system has been developed to reverse the voltage polarity on the sample holder of PISCES-B from negative to positive potential in the microsecond time scale. Positive biasing draws electrons from the steady-state plasma through the sample, creating Ohmic heating on the surface. This pulsed biasing is used to replicate the transient heat loads that will be seen on plasma facing components during transient events, such as edge localized modes, in a device such as ITER. Surface temperatures are measured using two pyrometry techniques, a fast two-color system and a slower, more sensitive spectral system.