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Alcator C-Mod

Diagnostic Needs (Updated as of June 2010)

Profiles

Electron density profiles:

We desire a density profile diagnostic with much finer time resolution than Thomson scattering. The measurement should be such that there is minimal ambiguity of interpretation. It should not require spatial inversion. Target capabilities would be 100 microsecond time resolution and 1 cm spatial resolution. Measurements with ~1 mm resolution in the edge plasma would aid studies of the H-mode pedestal.

Contact: M. Greenwald

Transport

Multiple measurements of plasma profiles during disruptions:

We desire measurement of the evolutions of the electron temperature and density profiles during the thermal quench of a disruption. Such a diagnostic would measure profiles during both disruptions mitigated by a massive gas puff and "unmitigated" disruptions. The disruption time scale is a few msec, so measurements on a ~100 microsec time scale are desired.

Contact: R. Granetz

MHD

Edge and SOL ion temperature profiles:

Ion temperature profiles in the plasma edge and Scrape-Off-Layer (SOL) are desired. Ideally, these would have time resolution of 5-10 msec, but more importantly spatial resolution of ~ 2mm is desired.

Contact: J. Rice

Transport

q or magnetic shear profile:

The ability to measure these profiles across the entire cross section with good spatial resolution (~ 2cm) and good time resolution 20-50 msec are essential to computing micro (and macro) stability. High resolution measurements in the plasma edge (~ 1 mm) would aid studies of pedestal stability.

Contact: M. Greenwald

Transport

 

Radial electric field profiles:

Measurements of Er across the profile are desired with high time and spatial resolution to aid understanding of the formation of edge and core barriers. Ideally, we desire ms time resolution and spatial resolution ~ 5mm in the core and ~ 1mm in the edge.

Contact: A. Hubbard

Transport

 

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Fluctuations

Localized density fluctuations:

We desire a measurement of density fluctuations localized to 1-2 cm with a kperp range from kperp rhoi ~ 0.1 to kperp rhoe ~ 0.5

Contact: M. Greenwald

Transport

 

Core fluctuations in other quantities:

We desire measurement of core fluctuations in any of the quantities Te, Ti, potential, Vtor, or Vpol or B in wavenumber and frequency ranges relevant to turbulent transport.

Contact: M. Greenwald

Transport

 

Turbulence imaging:

High resolution imaging of any fluctuating field would aid in the study of turbulent transport. Time dependent measurements are of even greater value. Spatial and temporal resolution must be sufficient to resolve some part of the transport-relevant turbulence fields.

Contact: M. Greenwald

Transport

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Power Balance

RF power deposition:

A "first principles" measurement of ICRF or LH power deposition would aid all confinement studies.

Contact: M. Greenwald

Transport

Minority species concentrations:

The relative concentrations of H, D and He3 in the plasma are critical to understanding and optimizing ICRF heating. We desire a routine measurement of the H and/or He3 concetration in the core plasma; coarse time and spatial resolution would be acceptable as these are not likely to change rapidly.

Contact: S. Wukitch

ICRF

 

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Plasma-Material Interactions

 

Boron coatings:

We desire a diagnostic that would measure the thickness of B films/coatings on the C-Mod molybdenum tile surfaces. The diagnostic should have shot-to-shot time resolution to inform plasma operations. In particular, we want to monitor the longevity of boron coating on outboard limiters and other PFC surfaces that are believed to be areas of critical importance as sources for molybdenum core contamination which can limit C-Mod H-mode performance.

Contact: B. Lipschultz

Plasma Boundary

 

Hydrogen and deuterium fuel retention in the wall:

We desire a diagnostic that would measure the absolute and relative hydrogen and deuterium fuel content in the wall. The H/D ratio is critical for efficient ICRH heating and must be reduced after a vent to ~2%, typically requiring significant run-time. Therefore, a desired time resolution is shot-to-shot to monitor the wall condition. Additionally, the quantification of the D content in the wall is desired for extrapolating C-Mod experience with Mo tiles to tritium retention in the metal walls of D-T burning plasmas.

Contact: B. Lipschultz

Plasma Boundary

 

Diagnostics for dust:

We desire diagnostics that provide measurements of composition, spatial-distribution and size-distribution of intrinsic dust particles existing in C-Mod. In particular, the accounting of dust in shadowed or poorly-accessed regions is desired. These measurements need not be "real-time".

Contact: R. Granetz

MHD

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General

Poloidal/radial distribution of the Edge Ionization Source:

We desire a measurement of the ionization source (ionization of deuterium atoms) over the entire poloidal cross-section of C-Mod's "main-chamber" plasma. Probably this would be accomplished optically, using either Balmer alpha or Lyman alpha line emission. Resolution in the radial (poloidal) dimension of a few mm (cm) is desired.

Contact: J. Terry

Plasma Boundary

 

In-vessel illumination:

We desire a simple in-vessel illumination system that would light up the inside of C-Mod on a between-shot basis. The safest system would bring the light into the vessel via fibers, but in principle light-producing filaments inside the vessel are not ruled out, as long as they are non-contaminating and robust.  

Contact: J. Terry

Plasma Boundary

 

Long optical periscopes:

We desire a number of optical periscopes that would be used bring views of the plasma and various in-vessel components to regions outside C-Mod superstructure. Once outside of the superstructure the periscopes would couple the views to CCD cameras. The periscopes must be less than 1 cm in the transverse dimension, but must transfer the image over a length of ~ 1m. They must be mechanically robust and stable.

Contact: J. Terry

Plasma Boundary

 

Sensor(s) for disruptions and disruption mitigation:

We desire a non-magnetics-based diagnostic that senses a future major disruption in time to activate the existing disruption-mitigation-by-massive-gas-injection system. Typically at least 3 ms is needed after receipt of a sensor's "fire" signal for the injected gas to reach and affect the about-to-disrupt plasma. Thus detection must take place at least 3 ms before the disruption. There are a number of different kinds of disruptions (e.g. Vertical Displacement Events, High-Beta, Locked-Mode, Density-Limit, Impurity-Radiation-Induced). To date we have used magnetics-based plasma-position sensors to successfully sense VDE disruptions. We desire a non-magnetics-based sensor for redundancy and for possible relevance to disruption mitigation on ITER. Ideally this sensor should sense reliably any of these types of disruptions.   (For more details see http://www-internal.psfc.mit.edu/research/alcator/pubs/APS/APS2006/Granetz-poster.pdf)

Contact: R. Granetz

MHD

 

Diagnostic for lost or confined fast ions:

ICRF generated fast ions are lost from the plasma due to interactions with MHD instabilities such as Toroidal Alfvén Eignemodes. An energy and pitch angle resolved diagnostic of the lost fast ions with 1 MHz time resolution would provide very useful measurements of the interaction of the fast ions with these MHD modes to better understand the particle-wave interactions. Additionally, information about the disruption functions of the confined fast ions is desired.

Contact: R. Granetz

MHD

Transport

 

2D flow measurements in the SOL and Divertor:

We desire a measurement of the 2D pattern of flows in the SOL and divertor volume. Existing probes at 3 different poloidal locations already measure a rich phenomenolgy of flows in the SOL. Additional understanding of the SOL flows and the 2D flows patterns in the divertor is desired. These measurements would also be used to evaluate the convective terms in the heat-flux analyses. This diagnostic should be compatible with the high-power ICRF auxillary heating and the Lower-Hybrid current-drive scenarios in place on C-Mod.

Contact: B. LaBombard

Plasma Boundary

 

 

 

 

 

 

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