Neutrino oscillation studies with IceCube-DeepCore

AbstractIceCube, a gigaton-scale neutrino detector located at the South Pole, was primarily designed to search for astrophysical neutrinos with energies of PeV and higher. This goal has been achieved with the detection of the highest energy neutrinos to date. At the other end of the energy spectrum, the DeepCore extension lowers the energy threshold of the detector to approximately 10 GeV and opens the door for oscillation studies using atmospheric neutrinos. An analysis of the disappearance of these neutrinos has been completed, with the results produced being complementary with dedicated oscillation experiments. Following a review of the detector principle and performance, the method used to make these calculations, as well as the results, is detailed. Finally, the future prospects of IceCube-DeepCore and the next generation of neutrino experiments at the South Pole (IceCube-Gen2, specifically the PINGU sub-detector) are briefly discussed

Optical and Mechanical Design of C-Mod Motional Stark Effect Diagnostic

A Motional Stark Effect (MSE) instrument is being installed on the Alcator C-Mod tokamak at MIT. This MSE diagnostic will provide measurements of the spatial profile of the internal poloidal magnetic field. The MSE has its primary collection optics inside the vacuum vessel. The light collected by the internal optics passes through a vacuum window and is relayed to a fiber optic array. The MSE optics are shared by a Beam Emission Spectroscopy (BES) diagnostic which measures electron density fluctuations and their spatial correlations. This optical system requires high throughput and spatial resolution of less than 1 cm at the focal plane in the plasma. The design requirements for the internal optics also include the effects associated with plasma impingement, plasma disruptions, and thermal excursions. The parameters that affect polarization measurement include the location and orientation of optical elements, the choice of substrates and optical materials. These unique design requirements led to a number of interesting optical and mechanical design features which are presented here

Transport and stability studies on TFTR

During the 1987 run, TFTR reached record values of Q/sub DD/, neutron source strength, and T/sub i/(0). Good confinement together with intense auxiliary heating has resulted in a plasma pressure greater than 3 x 10/sub 5/ Pascals on axis, which is at the ballooning stability boundary. At the same time improved diagnostics, especially ion temperature profile measurements, have led to increased understanding of tokamak confinement physics. Ion temperature profiles are found to be much more peaked than previously thought, implying that ion thermal diffusivity, even in high ion temperature supershot plasmas, is greater than electron thermal diffusivity. Based on studies of the effect of beam orientation on plasma performance, one of the four neutral beamlines has been re-oriented from injecting co-parallel to counter parallel, which will increase the available balanced neutral injection power from 14 MW to 27 MW. With this increase in balanced beam power, and the addition of 7 MW of ICRF power it is planned to increase the present equivalent Q/sub DT/ of 0.25 to close to break-even conditions in the coming run. 26 refs., 10 figs

Impurity and particle transport and control in TFTR

Degassing of the TFTR graphite limiter by low density deuterium or helium discharges enables the limiter to pump deuterium, thereby reducing recycling and improving energy confinement in neutral-beam-heated discharges. During a helium degassing sequence the hydrogen influx decreased by a factor of 20. As a consequence of degassing sequences the low density limit in 0.8 mA deuterium discharges decreased from 1 x 10/sup 19/ m/sup -3/ to 0.5 x 10/sup 19/ m/sup -3/, the density-decay time dropped from greater than 10 s to 0.15 s, and the recycling coefficient dropped from nearly 1 to less than 0.4. Z/sub eff/ values in 2.2 MA L-mode discharges on the toroidal limiter with neutral-beam-heating power up to 15 MW are between 2 and 3 if the pre-beam plasma has low Z/sub eff/ (high density), but can be as high as 4.5 if the pre-beam target has high Z/sub eff/ (low density). Z/sub eff/ values in enhanced confinement shots drop from 7 during the ohmic phase to 3 with neutral beam heating. The radiated power drops from 60 to 70% of total heating power to 30 to 35% for beam powers from 10 to 20 MW

TFTR plasma regimes

Significant extensions in the TFTR plasma operating regimes have been achieved with additional heating-system capability, installation of a multishot pellet injector, and the development of an enhanced confinement regime. In ohmically heated pellet-fueled discharges characterized by highly peaked density profiles, enhancements in tau/sub E/ have resulted in n/sub e/(0)tau/sub E/(a)-values of 1.5 x 10/sup 20/ m/sup -3/s. In neutral-beam-heated discharges, an operating regime has been developed in which substantial improvements in energy confinement time and neutron source strength are observed. Ion temperatures of approx.20 keV and n/sub e/(0)tau/sub E/(a)T/sub i/(0)-values of 2 x 10/sup 20/ m/sup -3/s keV have been achieved. This enhanced confinement regime is characterized by high values of ..beta../sub p/ and low values of collisionality. The observed surface voltage, which is negative during beam injection, is compared with models including beam-driven and bootstrap currents

Studies of impurity behavior in TFTR

Central medium- and low-Z impurity concentrations and Z/sub eff/ have been measured by x-ray spectrometry in Tokamak Fusion Test Reactor discharges during three periods of operation. These were the (1) start-up period, (2) ohmic heating, and (3) ohmic heating portion of the two neutral beam periods, distinguished mainly by different vacuum vessel internal hardware and increasing plasma current and toroidal field capability. Plasma parameters spanned minor radius a = 0.41 - 0.83 m, major radius R = 2.1 - 3.1 m, current I/sub p = 0.25 - 2.0 MA, line-averaged electron density n-bar/sub e/ = 0.9 - 4.0 x 10/sup 19/ m/sup -3/, and toroidal magnetic field B/sub T/ = 1.8 - 4.0 T. The metal impurities came mostly from the limiter. At low densities titanium or nickel approached 1% of n/sub e/ during operation on a TiC-coated graphite or Inconel limiter, respectively. Lower levels of Cr, Fe, and Ni (less than or equal to0.1%) were observed with a graphite limiter at similarly low densities; these elements were removed mainly from stainless steel or Inconel hardware within the vacuum vessel during pulse discharge cleaning or plasma operation on an Inconel limiter and then deposited on the graphite limiter. Hardware closest to the graphite limiter contributed most to the deposits

High poloidal beta equilibria in TFTR limited by a natural inboard poloidal field null

Recent operation of the Tokamak Fusion Test Reactor TFTR, has produced plasma equilibria with values of {Lambda} {triple bond} {beta}{sub p eq} + l{sub i}/2 as large as 7, {epsilon}{beta}{sub p dia} {triple bond} 2{mu}{sub 0}{epsilon}<p{perpendicular}>/{much lt}B{sub p}{much gt}{sup 2} as large as 1.6, and Troyon normalized diamagnetic beta, {beta}{sub N dia} {triple bond} 10{sup 8}<{beta}{sub t}{perpendicular}>aB{sub 0}/I{sub p} as large as 4.7. When {epsilon}{beta}{sub p dia} {approx gt} 1.25, a separatrix entered the vacuum chamber, producing a naturally diverted discharge which was sustained for many energy confinement times, {tau}{sub E}. The largest values of {epsilon}{beta}{sub p} and plasma stored energy were obtained when the plasma current was ramped down prior to neutral beam injection. The measured peak ion and electron temperatures were as large as 24 keV and 8.5 keV, respectively. Plasma stored energy in excess of 2.5 MJ and {tau}{sub E} greater than 130 msec were obtained. Confinement times of greater than 3 times that expected from L-mode predictions have been achieved. The fusion power gain. Q{sub DD}, reached a values of 1.3 {times} 10{sup {minus}3} in a discharge with I{sub p} = 1 MA and {epsilon}{beta}{sub p dia} = 0.85. A large, sustained negative loop voltage during the steady state portion of the discharge indicates that a substantial non-inductive component of I{sub p} exists in these plasmas. Transport code analysis indicates that the bootstrap current constitutes up to 65% of I{sup p}. Magnetohydrodynamic (MHD) ballooning stability analysis shows that while these plasmas are near, or at the {beta}{sub p} limit, the pressure gradient in the plasma core is in the first region of stability to high-n modes. 24 refs., 10 figs

Overview of TFTR transport studies

A review of TFTR plasma transport studies is presented. Parallel transport and the confinement of suprathermal ions are found to be relatively well described by theory. Cross-field transport of the thermal plasma, however, is anomalous with the momentum diffusivity being comparable to the ion thermal diffusivity and larger than the electron thermal diffusivity in neutral beam heated discharges. Perturbative experiments have studied non-linear dependencies in the transport coefficients and examined the role of possible non-local phenomena. The underlying turbulence has been studied using microwave scattering, beam emission spectroscopy and microwave reflectometry over a much broader range in k{perpendicular} than previously possible. Results indicate the existence of large-wavelength fluctuations correlated with enhanced transport. MHD instabilities set important operational constraints. However, by modifying the current profile using current ramp-down techniques, it has been possible to extend the operating regime to higher values of both {var epsilon}{beta}{sub p} and normalized {beta}{sub T}. In addition, the interaction of MHD fluctuations with fast ions, of potential relevance to {alpha}-particle confinement in D-T plasmas, has been investigated. The installation of carbon-carbon composite tiles and improvements in wall conditioning, in particular the use of Li pellet injection to reduce the carbon recycling, continue to be important in the improvement of plasma performance. 96 refs., 16 figs