DNB Program W.L. Rowan, D. Beals, R.V. Bravenec, M.B. Sampsell, D.M. Patterson Fusion Research Center, University of Texas at Austin G. Schilling, G. Kramer, R. Feder Princeton Plasma Physics Laboratory H. Yuh, D.R. Terry, B. Lipschultz, J. Rice, J. Terry, E. Marmar R. Granetz MIT Plasma Science and Fusion Center Alcator C-Mod PAC Meeting 6-7 Feb 2002
! Appoint C-Mod DNB coordinator R. Granetz! CXRS highest priority (V and T i ) tried, but signal-to-ambient very low Measure beam re-ionization and attenuation in duct not a major problem Make definitive decision on DNB and diagnostics, and whether to consider deployment of alternate approach!!! Recommendations of last year s reviews PAC review DNB review get new beam; upgrade optics; new spectrometer/detector Condition source & reduce water by increasing source operation time did not help! Add diagnostics in duct (H α diodes, pressure gauges) to check for exponential re-ionization. Consider pumped duct. pumped duct rejected! Add shutters to CXRS to prevent coating worked well! Repair/upgrade MSE/BES optics repaired, but other components eventually failed
DNB full-energy component unimproved Despite extensive conditioning efforts, 1/3 1/2 1/18 1 Component fractions: 0.13/0.32/0.48/0.07 Not understood Note: no runaway re-ionization observed in duct
MSE Results Blue - Raw (shifted) MSE pitch angles. Green - EFIT calculated pitch angles Red - MSE data fitted to EFIT over several shots to obtain artificial calibration. Suffers from low S/N. 2 error still far larger than the 0.1 0.2 desired. Increase in beam full-energy component would greatly improve signals
CXRS and BES Data will be shown in Bill Rowan s talk CXRS Best result: visible B +4 7 6 line shows 2 enhancement with beam High plasma background line is symptomatic of high density Fine structure and Zeeman splitting greatly complicate analysis BES fluctuations Dominated by fluctuations in plasma background lines (carbon), and Harmonics from the MSE photo-elastic modulators (shares optics with MSE)
MSE/BES optics problems One of the in-vessel mirrors had loosened Overall transmission efficiency was found to be only 27%, presumably due to glass dust and loose optics
Synopsis of DNB diagnostic difficulties The difficulties obtaining useful physics from the DNB systems on C-Mod are basically due to the following factors: Low full-energy fraction in beam (MSE, also BES) Low overall intensity (CXRS, MSE, BES) Partly due to problems with optical transmission Large background plasma line emission (CXRS, BES) Characteristic of high-density plasmas. Averaging over longer beam pulse won t help
The RFX DNBI Beam parameters (cold cathode source): 50 kv 3 A equiv neutral current (~5.5 A ion current) 1/e diameter of 6 cm at 3.4 m (previous beam was 10 cm) 90+ % full-energy fraction 50 ms pulse length (possibly extendible to 100 ms) Built at Budker Institute in Novosibirsk, Russia Loaned to MIT until Spring 2004 (2 years) Also involves scientific collaboration with RFX
The RFX DNBI
Planned Installation of RFX beam on C-Mod
Installation of RFX beam on C-Mod Budker Institute sending a team of 11 staff to install their DNBI hardware (headed by A. Ivanov) Fee to Budker Institute is $75K for shipping, installation, and commissioning Schedule: 24 Jan 2002: injector left Novosibirsk 8 Feb: injector arrives at MIT 17 Feb: Budker team arrives and begins installation 2 nd week of March: commission DNBI Texas beam hardware has been dismantled, cannibalized Visit to Madison last Nov to observe similar installation process
Additional improvements in progress Re-design and re-build MSE/BES in-vessel optics Improve optical throughput 2-3 (new components) Make MSE polarizer removable (>2 throughput for BES) Reduce stray polarization characteristics (MSE background) Eliminate vignetting of innermost MSE views More robust mirror mounts Ease in-vessel installation New CXRS spectrometer and detector (improved throughput) Increase number of high-resolution edge poloidal views for CXRS (6 25) Re-direct CXRS toroidal views to concentrate on outer half of plasma
Projected Improvement in MSE Full energy fraction (0.90 / 0.13) 7 Beam diameter ( 10 cm/6 cm) 1.7 Optics throughput 2 3 (use 2) Noise reduction? (stray polarization, PMT noise, etc) Overall increase in S/N 24 0.1 resolution in core for n e 3 10 20 m -3 spatial resolution in core is an issue; smaller beam diameter helps 0.1 resolution at half-radius for n e 9 10 20 m -3
Expected improvements in CXRS and BES Improved beam helps, but not as much as for MSE: CXRS utilizes all beam energy components (Of course, the fullenergy component penetrates farther into the plasma.) BES can utilize any of the beam energy components, although the full-energy peak is not contaminated by carbon lines on C-Mod Repairs and improvements to optics will increase overall signal levels 5 for BES should detect 1% ñ/n for r/a > 0.8 2-3 for MSE New spectrometer and detector will increase overall signal levels, although signal-to-background line ratio will remain about unity. 25 for CXRS enable T i and V measurements
Summary of predicted improvement The combination of: Installing the RFX DNBI, and Improving the optical throughput by a factor of ~3 5, and Using a better CXRS spectrometer and detector will greatly improve the performance of all the DNB diagnostics. We are in the process of doing both of these upgrades in time for the beginning of the next run campaign. There is still an issue concerning the spatial resolution of the central MSE views
5-Year Plans Assuming the upgraded DNB systems prove to be successful, A long pulse DNB injector would be desired: 1-2 s pulse length High current density High full-energy component About the same energy (50 kv) Cost would be of order 10 6 $ A different view for central MSE (more to beam), to improve the spatial resolution Simultaneous MSE views from different directions to identify plasma electric fields