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

Columbia University

X-Ray Measurements

• f the Levitated Dipole Experiment
• J. L. Ellsworth, J. Kesner

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

• n 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, Te ≈ 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

Top Top View View

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.

22.5°

A

Solid circles = design angles Open circles = actual angles

Section View A Section View A

Shot 40917-19 t=3.22 s

Design Collimation Angles

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 µs.

PHA Collimator

Top View Back 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 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.

Photograph of the DXP-2X

Digital Pulse Filtering

∗ Digital pulse filtering is similar to analog pulse shaping. ∗ DXP uses two trapezoidal filters (similar to shaping amplifiers).

• Slow filter improves energy

resolution and reduces pulse pile-up.

• Fast filter determines pulse

arrival time and pulse height.

Gs Ls Lf Gf

Input pulse Fast Filter SlowFilter

Calibration Data

∗ Detectors are calibrated using an iterated Gaussian fit to the 59.5 keV

• f 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.

100 200 300 400 500 600 700 800 10 20 30 40 50 60 70 80 X-Ray Energy [keV] Counts

Am-241

Accuracy and Error

0.E+00 1.E+04 2.E+04 3.E+04 4.E+04 5.E+04 6.E+04 7.E+04

• 10
• 5

5 10 Energy [keV] Baseline counts CZT B1241 CZT B1242 CZT B1243 CZT B1244

Serial # Serial # mean [keV] mean [keV] variance [keV] variance [keV] B1241 0.56 2.05 B1242 0.69 1.75 B1243 0.53 1.85 B1244 0.6 1.75

 Baseline measurement: Voltage is sampled when there are no x-ray events to process.  Mean position of the baseline provides a zero location.  The width of the spectrum is a measure of the energy resolution of the detector.  Measurements taken using 8192 bins at 10 eV per bin.

Data Correction

∗ Raw data must be corrected for losses in Be vacuum window, air, and aluminum detector window as well as for the efficiency of the CZT crystal. ∗ We assumed each loss was independent and calculated the transmission probability and that all x-rays were incident perpendicular to the detector, then inverted it to get efficiency.

10 20 30 40 50 60 70 80 90 100 10 100 1000 Energy [keV] % efficiency 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. Energy range: 10 keV - 2 MeV

Functional schematic of preamplifier and amplifier.

Estimated Parameters and Bremsstrahlung Power

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.50 1.00 1.50 2.00 2.50 R [m] P,n,T [normalized units] 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 B[T] pressure density temperature B 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 0.50 1.00 1.50 2.00 2.50 R [m]

• Est. Bremsstrahlung Power

[W-cm^-3] nmax=10^16 nmax=10^17 nmax=10^18

• Estimated pressure and magnetic

field were determined from equilibrium simulation, assuming a peak beta of 0.562.

• Density and temperature were

determined by assuming η = 2/3 where, .

• Bremsstrahlung power is

estimated from where we have assumed Zeff = 1 and all of the pressure is carried by the hot electrons.

X-Ray Intensity Measurement

Typical Spectrum from First LDX run

• 6.4 GHz Heating
• 4 s of heating time
• 3 kW power
• Spectrum is x-ray

emission integrated over 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

Chan 0 -- black Chan 1 -- green Chan 2 -- yellow Chan 3 -- blue Energy [keV]

Typical Spectrum from Second LDX Run

• 6.4 GHz Heating
• 4 s of heating

time

• 3 kW power
• And 2.45 GHz

Heating

• 3 kW power
• 300 A/turn in

Floating Coil. Chan 0 -- black Chan 1 -- green Chan 2 -- yellow Energy [keV]

Gas Scan

0.E+00 5.E+04 1.E+05 2.E+05 2.E+05 3.E+05 3.E+05 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] Total x-ray counts chan 1 chan 0

8/13 run

10 20 30 40 50 60 70 80 1.00E+19 1.00E+20 1.00E+21 1.00E+22 Number of particles puffed into vessel Tail Temperature [keV]

ch:0 ch:1 ch:2 ch:3

Temperature Response to Vessel Pressure

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

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. Shot 18 (2 ms puff) -- blue Shot 23 (4 ms puff) -- green Shot 34 (10 ms puff) -- black

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.