In the area of transport and turbulence, the initial CMPD research project seeks to deliver conclusive evidence linking, in one unbroken chain, all of the following elements:
experimentally measured profiles → a theoretically predicted instability → the theoretically predicted turbulent flux &rarr the simulated turbulent flux and fluctuations → experimentally measured turbulent fluctuations → experimentally inferred turbulent fluxes.

This will be carried out in focused campaigns on DIII-D, NSTX and C-MOD. Specific predictions of fluctuation spectra and steady-state radial energy flux will be generated and compared with observational data. An initial survey of high k turbulence in DIII-D has found evidence for fluctuations in the ETG range of frequencies, under plasma conditions appropriate for ETG turbulence. ETG turbulence is an excellent candidate for intensive study because its properties are likely to be less sensitive to profiles that are difficult to measure (such as impurity density gradients and radial electric fields). Although this project is not directly multiscale, it develops simulation-experimental collaborations and feeds into other CMPD projects.

X-mode backscattering of 100 GHz radiation in DIII-D has already produced fluctuation spectra in the ETG range of wavelengths and frequencies. Further diagnostic development in this area will soon increase the range of possibilities for experiment/theory interactions. Similar data from NSTX should be available in the first year or two of the Center. Even without dedicated new funding for diagnostic development or ETG simulations themselves, the proposed Center would enhance the coupling between the UCLA diagnostic effort and the Maryland simulation group, with improved chances for success.

CMPD scientists are also interacting strongly with the experimental programs in Alcator C-Mod and DIII-D using new Phase Contrast Imaging (PCI) diagnostics. PCI is capable of detecting very short wavelength fluctuations, up to and above the expected range of wavenumbers predicted from simulations. This diagnostic program is under the leadership of Miklos Porkolab. The diagnostics are currently being upgraded to look at frequency spectra up to 10 MHz, and will also allow localization vertically along the laser beam within 2-5 cm. The localization is made possible with the installation of new rotating phase plates. We expect diagnostic data would be available by next year on both machines.

Research Plan (experimental identification)

• Year 1: Test predictions of ETG simulations for core Ohmic plasmas against backscattering data from DIII-D immediately, and from NSTX as it becomes available.
• Year 2: Continue Ohmic turbulence studies. Begin simulation campaign for plasmas with PCI measurements.
• Year 3: Look for ETG turbulence signatures in ITB plasmas, with coordinated simulation/diagnostic campaign.
• Year 4: Look for ETG turbulence signatures around edge of neoclassical tearing mode, with coordinated simulation/diagnostic campaign.
• Year 5: Report on whether or not ETG fluctuations appear when predicted, with predicted characteristics.

Toroidal ETG instabilities saturate at levels large compared to mixing length estimates because streamers are excited. Since the expected electron energy transport from ETG modes is similar to the typical electron energy transport induced by long-wavelength instabilities (TEM, ITG, drift waves), it is possible that significant interactions between ETG instabilities and long-wavelength turbulence take place. In particular, the possibility that ETG turbulence could diffuse ion-scale perturbations and therefore fundamentally change the dynamics of the ion-scale instabilities must be considered.

The characteristic dynamical time and perpendicular space scales for these processes are separated by a factor of ~ 60. This presents a serious challenge to the direct numerical simulation approach. In the longer term, a projective integration package for simulating ETG/TEM turbulence will be developed and tested against the best direct numerical simulation results available. Finally, the CMPD will support support (with trained, dedicated personnel and dedicated simulations performed for diagnostic design and interpretation) ongoing experimental efforts to obtain simultaneous measurements of short and long-wavelength fluctuations in tokamak plasmas.

Research Plan (multiscale interactions)

• Year 1: A series of direct numerical simulations with artificially varying ion/electron mass ratios will be undertaken to ascertain the scaling of any interaction physics with the ratio of ion and electron gyroradii.
• Years 2-3: Develop patch-dynamics algorithm designed to advance ion-scale turbulence at ``coarse'' scales and electron-scale turbulence on the fine scale. Begin with ETG simulations in frozen-in-time bath of long wavelength ITG/TEM turbulence.
• Year 4: Test patch algorithm against best available direct numerical simulation results; refine algorithm. Study cascade dynamics for wavelengths shorter than the ion gyroradius but longer than the electron gyroradius.
• Year 5: Use multiscale algorithm to simulate interacting ITG/TEM/ETG turbulence in realistic conditions, including the physical mass ratio.

Peebles currently leads the backscattering diagnostic program, and will also lead the CMPD-based effort to relate backscattering diagnostic measurements to specific theoretical predictions. He will be assisted by Rhodes, Gilmore, Synakowski and Dorland. Porkolab currently leads the PCI diagnostic effort, and will also lead the Center-based effort to relate PCI results to theoretical predictions. Co-workers include Chris Rost plus an MIT graduate student on site at DIII-D, and two graduate students at Alcator C-Mod.

The CMPD funds the design, testing and deployment of the multiscale model, which should ultimately be more tractable. Key personnel in this area will be Dorland, Gear and Hammett.