X-Ray Measurements of the Levitated Dipole Experiment J. L. - PowerPoint PPT Presentation

X-Ray Measurements of the Levitated Dipole Experiment J. L. Ellsworth, J. Kesner Columbia University MIT Plasma Science and Fusion Center D.T. Garnier, A.K. Hansen, M.E. Mauel Columbia University Presented at The APS Division of Plasma Physics Annual Meeting 2004 Savannah, Georgia 15 November 2004

Abstract Initial plasma experiments in the Levitated Dipole Experiment focus on producing hot electron, high beta plasmas using a supported dipole configuration. Plasmas are created using multi-frequency ECRH; it is therefore expected that most of the plasma energy will be stored in the fast electrons, T e ≈ 100 keV. As a consequence, x-ray flux from bremsstrahlung emission is expected to be easily detectable. The energy spectrum of the x-ray emission below 740 keV is measured by a four channel pulse height analyzer using cadmium zinc telluride detectors. In addition, a single sodium iodide detector which views energies up to 3 MeV will measure the intensity of x-ray emission from the plasma. The electron temperature may be inferred from the x-ray energy. These x-ray spectral measurements can then be combined with the reconstructed plasma equilibria and line-integrated density measurements to give an estimate of the hot electron pressure profile. X-ray measurements will be essential in diagnosing the effectiveness of various ECRH configurations. Initial measurements will be discussed. ∗ This work supported by USDOE OFES.

X-Ray Intensity Detector and X-Ray Pulse Height Analyzer Views 22.5 ° Design Collimation Angles Solid circles = design angles Open circles = actual angles A Section View A Section View A Top Top View View Shot 40917-19 t=3.22 s The PHA design views have identical collimation angles, and are symmetric about the 22.5 ° axis. For August and September runs, the pinhole was not used, so viewing angles(typically 26 ° ) are larger than design angles (8 ° ) and views for adjacent channels overlap.

X-Ray Detection ∗ 4 Cadmium Zinc Telluride (CZT) detectors from eV products with built in preamplifier units. ∗ Energy range: 10 keV - 670 keV. ∗ Energy resolution: 4% FWHM at 122 keV. ∗ Nominal sensitivity: 0.11 mV/keV. ∗ Rise time at the source: 35 ns. ∗ RC decay time: 750 µ s.

PHA Collimator Back View Top View Pinhole Viewing angles can be adjusted by sliding the detectors further into the lead tubes.

Digital X-Ray Processor  X-ray Instrumentation Associates DXP-4c2x Photograph of the DXP-2X CAMAC module.  The DXP is a multi-element digital x-ray processor which includes a shaping amplifier and multi channel analyzer.  4 independent channels  Count rates up to 500 kcps.  Programmable peaking times: 125ns-80ms  40 dB gain adjustment  External gate and sync inputs allow for timing control.

Digital Pulse Filtering ∗ Digital pulse filtering is similar to analog pulse Input pulse shaping. ∗ DXP uses two trapezoidal filters (similar to shaping amplifiers). Fast Filter • Slow filter improves energy resolution and reduces G f L f pulse pile-up. SlowFilter • Fast filter determines pulse arrival time and pulse height. G s L s

Calibration Data 800 700 Am-241 600 500 Counts 400 300 200 100 0 0 10 20 30 40 50 60 70 80 X-Ray Energy [keV] Detectors are calibrated using an iterated Gaussian fit to the 59.5 keV ∗ of an Am-241 source. Calibration routine is built in to the software that accompanies the ∗ DXP. Zero is determined from baseline measurement. ∗ Representative spectrum measured with the CZT Spear detectors is ∗ shown above.

Accuracy and Error 7.E+04  Baseline measurement: 6.E+04 Voltage is sampled when 5.E+04 Baseline counts there are no x-ray events to CZT B1241 4.E+04 CZT B1242 process. CZT B1243 3.E+04  Mean position of the CZT B1244 baseline provides a zero 2.E+04 location. 1.E+04  The width of the 0.E+00 spectrum is a measure of -10 -5 0 5 10 Energy [keV] the energy resolution of the detector. Serial # mean [keV] Serial # mean [keV] variance [keV] variance [keV]  Measurements taken B1241 0.56 2.05 using 8192 bins at 10 eV per B1242 0.69 1.75 bin. B1243 0.53 1.85 B1244 0.6 1.75

Data Correction ∗ Raw data must be 100 corrected for losses in Be 90 vacuum window, air, and aluminum detector 80 window as well as for the 70 efficiency of the CZT % efficiency 60 crystal. 50 ∗ We assumed each loss was independent and 40 calculated the 30 transmission probability and that all x-rays were 20 incident perpendicular to 10 the detector, then 0 inverted it to get 10 100 1000 efficiency. Energy [keV] Be Air Al CZT total

X-Ray Intensity Time Resolved Measurement The output of a single sodium- iodide PMT tube measures the total x-ray intensity from the plasma. Functional schematic of preamplifier Energy range: 10 keV - 2 MeV and amplifier.

Estimated Parameters and Bremsstrahlung Power 1 0.2 •Estimated pressure and magnetic 0.9 pressure 0.18 P,n,T [normalized units] density 0.8 0.16 field were determined from temperature 0.7 0.14 B equilibrium simulation, assuming 0.6 0.12 B[T] 0.5 0.1 a peak beta of 0.562. 0.4 0.08 •Density and temperature were 0.3 0.06 0.2 0.04 determined by assuming η = 2/3 0.1 0.02 where, . 0 0 0.50 1.00 1.50 2.00 2.50 R [m] 1.E+01 •Bremsstrahlung power is nmax=10^16 nmax=10^17 Est. Bremsstrahlung Power 1.E+00 estimated from nmax=10^18 1.E-01 [W-cm^-3] 1.E-02 1.E-03 where we have assumed Z eff = 1 1.E-04 and all of the pressure is carried 1.E-05 by the hot electrons. 1.E-06 0.50 1.00 1.50 2.00 2.50 R [m]

X-Ray Intensity Measurement

Typical Spectrum from First LDX run • 6.4 GHz Heating • 4 s of heating time Chan 0 -- black • 3 kW power Chan 1 -- green • Spectrum is x-ray Chan 2 -- yellow emission integrated over Chan 3 -- blue entire shot time. • Peaks are present in the area of 65 keV and 75 keV in many shots during this campaign. -- Compton Scattering off lead collimator? • 250 A/turn in Floating Coil Energy [keV]

Typical Spectrum from Second LDX Run • 6.4 GHz Heating Chan 0 -- black • 4 s of heating Chan 1 -- green time Chan 2 -- yellow • 3 kW power • And 2.45 GHz Heating •3 kW power • 300 A/turn in Floating Coil. Energy [keV]

Gas Scan 3.E+05 3.E+05 Total x-ray counts 2.E+05 2.E+05 1.E+05 5.E+04 0.E+00 2.0E-05 2.5E-05 3.0E-05 3.5E-05 4.0E-05 4.5E-05 5.0E-05 vessel pressure at beginning of shot [Torr] chan 1 chan 0

Temperature Response to Vessel Pressure 8/13 run 80 70 60 Tail Temperature [keV] 50 ch:0 ch:1 40 ch:2 ch:3 30 20 10 0 1.00E+19 1.00E+20 1.00E+21 1.00E+22 Number of particles puffed into vessel Tail temperatures were determined by fitting an exponential to the tail of the measured bremsstrahlung spectra for shots with varied fill pressures/puff times. Each shot had a coil current of 250 A/turn and 4s of 6.45 GHz heating with 3kW of heating power.

X-Ray Spectra for Several Gas Pressures Shot 18 (2 ms puff) -- blue Shot 23 (4 ms puff) -- green Shot 34 (10 ms puff) -- black Comparison of three shots for 8/13/04 with different fill pressures. For each of the three shots, the gas was puffed in 1 second before the shot and the plasma was heated for 4 s by 3 kW of 6.45 heating.

Summary ∗ X-Rays were observed with energies ranging from 10 keV to 250 keV. ∗ Bursts of X-Rays were observed in a low density regime at the beginning of the shot. ∗ Big x-ray pulses in short period into afterglow indicate an instability during supported operation. • Hot electron interchange instability is followed by x-rays emitted when electrons strike the metal supports. • Or microinstability causes losses. • Largest emissions come during this time and may dominate the integrated spectrum.

Future Work Time Resolved Pulse Height Analysis TTL Gate Pulse Bremsstrahlung Spectra cartoon time ∗ Programmable spectra times. ∗ Continuous data collection. /Gate pulse trailing edge switches to collection of next spectrum only. ∗ Maximum 64 spectra with 8184 bins. ∗ Not implemented for August and September runs. Expected to be operational for December run.