Joint experiment/theory/modelling research has led to increased confidence in predictions of the pedestal height in ITER. This work was performed as part of a US Department of Energy Joint Research Target in FY11 to identify physics processes that control the H-mode pedestal structure. The study included experiments on C-Mod, DIII-D and NSTX as well as interpretation of experimental data with theory-based modelling codes. This work provides increased confidence in the ability of models for peeling–ballooning stability, bootstrap current, pedestal width and pedestal height scaling to make correct predictions, with some areas needing further work also being identified. A model for pedestal pressure height has made good predictions in existing machines for a range in pressure of a factor of 20. This provides a solid basis for predicting the maximum pedestal pressure height in ITER, which is found to be an extrapolation of a factor of 3 beyond the existing data set. Models were studied for a number of processes that are proposed to play a role in the pedestal ne and Te profiles. These processes include neoclassical transport, paleoclassical transport, electron temperature gradient turbulence and neutral fuelling. All of these processes may be important, with the importance being dependent on the plasma regime. Studies with several electromagnetic gyrokinetic codes show that the gradients in and on top of the pedestal can drive a number of instabilities.

Validation of models of pedestal structure is an important part of predicting pedestal height and performance in future tokamaks. The Thomson scattering diagnostic at DIII-D has been upgraded in support of validating these models. Spatial and temporal resolution, as well as signal to noise ratio, have all been specifically enhanced in the pedestal region. This region is now diagnosed by 20 view-chords with a spacing of 6 mm and a scattering length of just under 5 mm sampled at a nominal rate of 250 Hz. When mapped to the outboard midplane, this corresponds to ~3 mm spacing. These measurements are being used to test critical gradient models, in which pedestal gradients increase in time until a threshold is reached. This paper will describe the specifications of the upgrade and present initial results of the system.

The DIII-D Thomson scattering system has been upgraded. A new data acquisition hardware was installed, adding the capacity for additional spatial channels and longer acquisition times for temperature and density measurements. Detector modules were replaced with faster transimpedance circuitry, increasing the signal-to-noise ratio by a factor of 2. This allows for future expansion to the edge system. A second phase upgrade scheduled for 2010-2011 includes the installation of four 1 J/pulse Nd:YAG lasers at 50 Hz repetition rate. This paper presents the first completed phase of the upgrade and performance comparison between the original system and the upgraded system. The plan for the second phase is also presented.

A redesign of the Thomson scattering data acquisition system located on the DIII-D tokamak is undergoing its trial phase. The redesign was made necessary by several factors, including the desire to improve the quality of the acquired data. In addition, the previous generation system was based on CAMAC technology, which has become difficult to maintain and is no longer supported. A big part of this improved redesign comes from the use of faster electronics allowing for greatly improved background light subtraction, the main source of noise. The past system utilized LeCroy FERA CAMAC gated integrator modules for integration and digitization. The redesigned system takes a much more distributed approach. The new system consists of a number of subsystems, including photodetectors, digitizers, distribution panels, and timing circuits. The most significant upgrade was performed on the photodetector electronic assembly. A key feature was to make the units much more self-contained in regard to the preamplifier, background subtraction, integration, and bias circuits. The redesigned photodetectors and preliminary test results of the trial phase will be presented.

A tangential viewing, 10.59 microm CO(2) laser polarimeter for electron density measurements based on plasma induced Faraday rotation has been installed on DIII-D. The system uses colinear right- and left-hand circularly polarized beams with a difference frequency of 40 MHz to generate the necessary signal for heterodyne phase detection. The high-resolution phase information required to adequately resolve degree level polarization rotation is obtained using an all-digital "real-time" phase demodulation scheme based on modern digital signal processing techniques. Initial application of the system to DIII-D disruption mitigation experiments utilizing "massive gas jet" injection exhibits reliable operation and excellent agreement with CO(2) interferometer measurements; interestingly, the obtained Faraday rotation angles are in the range of those expected in ITER plasmas.

A new system has been developed to protect the 18 field shaping coils (F-coils) of the DIII-D tokamak from excessive currents. The new system removes the limitations of the original system and improves reliability. Coil currents are sensed in the new system by Rogowski coils wrapped around conductors feeding the F-coils. The Rogowski coil signals are routed to a remote electronics rack where the signals are integrated to yield voltages proportional to the F-coil currents. Comparators monitor the F-coil current signals and command the coil power supplies to turn off in the event of excessive currents. The F-coil current signals are also sampled by a dedicated data acquisition system for comparison to signals acquired elsewhere. The integrating electronics are interfaced to the DIII-D Integrator Calibration System, which verifies proper integrator operation once daily (typically) or on demand. The new over-current system also features a "self-test" function, which ensures that the Rogowski coils and their signal paths have not open-circuited. The test is performed during the preparatory "get ready" procedure of each DIII-D experiment "shot". A test voltage is momentarily applied to each sensor circuit (Rogowski coil and signal cable) to produce a simulated current signal, which is measured and integrated. The resulting signal is checked by a window comparator circuit to verify that the sensor circuit has continuity, that the integrator has the proper time constant, and that the test signal disconnects when the test is complete. If all these conditions are met (for all 18 F-coils), the "get-ready" procedure is allowed to continue. Otherwise, the procedure is halted and warnings are generated. LED lamps on the system's modules indicate statuses and results of the tests for each of the 18 channels. Design details of this new system are presented.

—The DIII-D Tokamak relies on a facility wide timing network to synchronize machine operations. The first generation system was designed around cascaded CAMAC delay units feeding a custom timing network encoder. This system has become increasingly difficult to maintain and repair and the needs of DIII-D experiments are beginning to exceed its capabilities. To address these issues, a new second-generation system was designed with a modular architecture in a VME form factor that facilitates the future addition of features and output channels when required, while maintaining backwards compatibility with the original system. As part of the base design, modules for event triggers, multiple programmable sequences, first generation BiPhase serial outputs, fiber optic outputs, and event recording are provided. Each module is implemented with a form of programmable logic, either a CPLD or FPGA, which allows for future modification if needed. The system also has the capability of complete remote management, allowing for custom timing chains on a per-experiment basis. The feature set and design of this second-generation timing system is presented.

The DIII-D Tokamak relies on a facility wide timing network to synchronize machine operations. The first generation system was designed around cascaded CAMAC delay units feeding a custom timing network encoder. This system has become increasingly difficult to maintain and repair and the needs of DIII-D experiments are beginning to exceed its capabilities. To address these issues, a new second-generation system was designed with a modular architecture in a VME form factor that facilitates the future addition of features and output channels when required, while maintaining backwards compatibility with the original system. As part of the base design, modules for event triggers, multiple programmable sequences, first generation BiPhase serial outputs, fiber optic outputs, and event recording are provided. Each module is implemented with a form of programmable logic, either a CPLD or FPGA, which allows for future modification if needed. The system also has the capability of complete remote management, allowing for custom timing chains on a per-experiment basis. The feature set and design of this second-generation timing system is presented.