An HIBP for NSTX Why and How

1. An HIBP for NSTXWhy and How Paul Schoch, Associate Professor Diane Demers, Research Assistant Professor Kenneth Connor, Professor Electrical, Computer, and Systems Engineering Department Rensselaer Polytechnic Institute Troy, NY

2. Why-The Heavy Ion Beam Probe - HIBP • Probe the plasma with energetic, high mass ions • Not confined by magnetic field • Electron impact ionization will produce higher charge state ions – secondaries • The change of energy of secondary compared to the primary is a measure of the potential at the point of ionization • Steer the sample location across the plasma and profiles are obtained.

3. Why - Data from 500kV HIBP on TEXT • Time variation of the secondary ion energy is a measure of the fluctuating potential in the plasma, fig (b). • Time variation of the total signal is a measure of the fluctuation of the electron density, fig (a) • These signal can be cross correlated to yield the coherence and the phase relation Power spectra of density fluctuations, potential fluctuations, coherence and phase shift

4. Why - Two point TEXT data Power spectra from density fluctuations using two detectors • TEXT system had 3 detector sets • Alignment of sample volumes is along primary ion beam, so typically are displaced both radially and poloidally

5. Scaled MHD Frequency GAM Frequency Why - TEXT data The GAM part of the Zonal Flow as measured in TEXT. It appears as a fluctuation of potential with m=0 mode structure, with little or on corresponding density fluctuation.

6. . Figure 5 Cross bicoherence. Data is plotted over a limited range to highlight nonlinear coupling of the 40kHz zonal flow potential fluctuations with broadband density fluctuations. The left axis is frequency of the potential fluctuations. The bottom axis is the frequency of the density fluctuations. Why - TEXT data Higher order spectral analysis showed three wave coupling. The narrow band potential fluctuation, GAM, is coupled to the broad band density fluctuations.

7. Proposed HIBP for NSTX • 500kV accelerator from TEXT – in storage • Maximum operating voltage is 545kV • Sodium as ion of choice, 545keV very useful, 900keV offers greater plasma coverage • Lithium is too light, (requires too high an energy.) • Potassium allows greater coverage or even possible use of a 300kV accelerator, but will be strongly attenuated for moderate density plasma • Sodium is best option • ~0.4% resolution for density fluctuations (0-500kHz) • Limited by predicted signal level and detector electronic noise • Electronic noise is broadband resistor noise • ~3Vrms resolution for potential fluctuations (0-500kHz) • Also limited by signal level and electronic noise • Sensitive to the low wavenumbers, k<3cm-1

8. Sample volume (injection point[1.8 0 2.375]. detection point[2.2 0.8 –0.2]) How - HIBP for NSTX Red is 580kV Na or 345kV for K, BT=0.45T, Blue is 930kV Na, 540kV K

9. 2.50 PF1a PF2 PF3 PF4 PF5 Z(m) Overlay of EFIT by F. Paoletti Shot 105094, t=241ms βt = 19.5% BT = 0.35T Circled stars represent: 350kV Na for BT = 0.35T 575kV Na for BT = 0.45T 345kV K for BT = 0.45T Blue stars represent: 550kV Na for BT = 0.35T 910kV Na for BT = 0.45T Primary sweep at 1.8, 0, 2.375 Detector at 2.2, 0.8, -0.2

10. Fig.29-32 top view, side view of the energy scan for high  field and the puncture plots ( injection point [1.8 0 2.375], detection point [2.2 0.8 –0.2]) Same sample locations as previous slide, shows trajectories at ports Zhang, Schoch, Connor Rev. Sci. Instrum., Vol. 74, No. 3, March 2003

11. HIBP for NSTX • Attenuation • 1.6m is an the mean free path for 500keV Na+ for a plasma with ne = 3x1019m-3, and Te=1keV. • This gives a nice balance between sufficient Na+2 production and not too much. • The mfp for K+ for the same energy and plasma is about 0.53m • Still useful but not optimal • Allows for greater plasma coverage but reduced sensitivity. • Frequency range (0-500kHz) and resolution • Frequency range is limited by detector electronics • Noise is dominated by resistor noise in transimpedence amplifiers

12. Fig. 34 The primary beam and the geometry of the sample volumes ( Eb=350keV, =223º ) Fig. 36 The primary beam and the geometry of the sample volumes ( Eb=350keV, =225º) HIBP for NSTX • Wavenumber sensitivity • Sample volumes are disk shaped with the size determined by the ion beam size, the detector slit size and the magnetic geometry. • Predict sample size of about 2cm in the longest direction • Spacing between samples is determined by the detector entrance slit geometry and the magnetic field. • Sensitive to the low wave numbers, k<3cm-1

13. How -HIBP for NSTX • 500kV accelerator from TEXT • Maximum operating voltage is 545kV • Sodium as ion choice • Potassium would allow greater coverage or even possible use of a 300kV accelerator, but will be strongly attenuated for moderate density plasma • ~0.4% resolution for density fluctuations (0-500kHz) • Limited by predicted signal level and detector electronic noise • Electronic noise is broadband resistor noise • ~3Vrms resolution for potential fluctuations (0-500kHz) • Also limited by signal level and electronic noise • Sensitive to the low wavenumbers, k<3cm-1