Effects of Microchannels in IEC devices S. Krupakar Murali, * J. F. Santarius and G. L. Kulcinski University of Wisconsin, Madison * Lawrenceville Plasma Physics Work carried out at UW Madison

2 Outline Introduction –What is an IEC? And how does it work? Motivation –Why are we interested in Microchannels Poissor Structure How are microchannels studied ? –Eclipse disc experiments –Single loop experiments –Grid rotation experiments Previous experiments in support of Poissor structures –Gamma collimation results –Proton collimation results –Observations by Kiyoshi Yoshikawa Poissor structure and relation to microchannels What are its consequences and implications to IEC research? Conclusions and Future work

3 What is an IEC device ? How does it work ? IEC Inertial Electrostatic Confinement Device Laverent’yev (1963, USSR) and Philo Farnsworth (1966, USA) independently proposed inertial electrostatic confinement of fusion plasma

4 Scale: 30 cm Schematic of UW IEC Advanced Fusion Device up until August 2004 Proton Detector Fusion Reactions High Voltage Power Leads Neutron Detector Advanced Fuels Input RGA Pyrometer & Camcoder

5 IEC Operation

6 Eclipse diagnostic for fusion source regime characterization

7 Fusion Source Regimes Wall-Surface (Embedded) CX-neutral (Volume Source) Beam-Background (Volume Source) Beam-Target (Embedded) Beam-Beam (Converged Core)

8 Eclipsing Experimental Setup Annulus View through the detector port

9 Eclipsed Data Suggests Significant Converged Core D-D Reactions (error ~ 5%) D- 3 He (100 kV, 30 mA) No eclipse 24% 9% 1% 9% 14% 37% 78% 100% Small Offset Small MediumLarge Volume eclipsed

10 Relevant conclusions Mostly D-D reactions occur in the core of the device. A proton detector can see converged core reactions. In other words, if you put something infront of the cathode obstructing the view of the proton detector, it will pick this up.

11 Sequential grid construction

12 Single loop grid experiments Single loop acts like a line source and provides a higher proton rate than a spherical grid at some locations Easiest way to compare different materials for grid construction. Can be used to project performance of a spherical grid at higher input power 30 o 60 o 90 o 0o0o

13 Sequential grid construction for studying the influence of grid symmetry on the fusion rate S. Krupakar Murali, J. F. Santarius, and G. L. Kulcinski, Phys. Plasmas, 15, , (2008).

14 1.XWLoopRe-1 2.XWLoopRe-2 3.XWLoopRe-3 4.XWLoopRe-7 5.XWLoopRe-C

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16 Conclusions of single loop grid experiments A single loop grid generates a cylindrical line source of protons. The eclipse scan of the loop grids showed a positive fusion reaction gradient as we move from the edge to the center of the XWLoopRe-1 single loop grid. With improving symmetry added wires, the fusion rate increases and eventually saturates. The gradual transformation of a line source to a volume source is observed with the increasing symmetry obtained by the addition of more loops to the grid. The ionization source must be placed away from the cathode for efficient performance. The presence of the grid wires seems to affect the fusion rate more drastically than previously thought. This prompted the next set of experiments – Grid rotation experiments.

17 Grid Rotation Experiment

18 Grid rotation experiment

19 Results of Grid rotation experiment Consequences of this experiment - Grid has to be oriented properly every time an experiment is conducted. If not, almost 50% variation is to be expected. New calibration factor for the Si detector needs to be developed, accounting for the microchannels as the fusion source. Theory should accommodate this non- uniformity in ion flow. This new technique could be used to characterize the three modes of operation of an IEC device namely-star, halo and converged core modes.

20 Proton rate calibration assuming all volume source reactions occur within the microchannels Fusion rate has three contributors –Converged core (MCNP)* –Embedded source and –Volume source Microchannel source * M. A. Sawan, University of Wisconsin, Madison, Private Communications, (2002)

21 Microchannels form the volume source Fusion rate from a single microchannel is given by: (a) (b) Cathode 2r rcrc h d 2 r cy l   a c b Pole S. Krupakar Murali, J. F. Santarius, and G. L. Kulcinski, submitted to IEEE transactions on plasmas

22 Surface area (A) visible to the protons x h r b a    22 t1t1 t2t2 22 d t2t2 s l Si detector Entrance of the detector port Projection of the entrance of the detector port d O

23 General formula for calibration Using the earlier derived formulae, the general expression for the calibration factor for the UW IEC device is given by: Fusion source regime DDD- 3 He Converged core (x) 21 %6 % Embedded (y) 8 %94 % Volume (z) 71%Negligible Wall surfaceNegligible

24 Proton collimation results Could the grid wires interfere with the proton data measurements? Pivot distance is 1.8 mm, grid wire used in the construction of the cathode is comparable to this dimension. Gu Y. and G. H. Miley, IEEE transactions on Plasma Science, 2000.

25 Neutron and Gamma collimation experiments Results seem to show values contrary to one’s expectations. Hirsch, 1967, Journal of Appl. Phys.

26 LIF experimental results Unlike the proton and gamma collimation results the LIF technique seems to show only 200 V potential dip. These experiments must be adapted and repeated at higher cathode voltages and lower chamber pressures. Kiyoshi Yoshikawa et al.,

27 Visible light intensity measurement by Thorson If intensity of light were measured at discrete points it might also appear like potential structures

28 Conclusions Microchannel formation results in a spoke like fusion source that appears like a volume source to the neutron detector However, these microchannels appear non uniform to the proton detector measurements. Although more data is needed to support the following statement, the results from neutron/proton/x-ray collimation measurements seem to be influenced by the cathode grid wires. One possibility raised by the significant role of microchannels is that narrow or axially peaked microchannels converging near the origin of an IEC device might appear to be potential structures. Need a more robust technique to study the Poissor structure. LIF technique has to be adapted for higher voltage, lower pressure operation.

29 Conclusions New calibration factor for proton detector has been developed based on microchannel formation. Grid rotation experiment reveals the proper grid orientation is very important for repeatable proton measurements. Grid rotation experimental technique can be used to study the various modes of IEC operation (star mode, halo mode etc).

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