Negative Ions in IEC Devices David R. Boris 2009 US-Japan IEC Workshop 12 th October, 2009 This work performed at The University of Wisconsin Fusion Technology Institute UW -IEC

Fusion Technology Institute 2 IEC devices operating at Pa are a good environment for negative ion formation Negative deuterium ion formation occurs through a variety of interactions between a deuterium plasma and comparatively high densities of neutral deuterium. –Low Energy (< 1 eV): Thermal electron attachment –High Energy(1 keV to 100 keV): Charge transfer reactions meta-stable lifetime τ = ~1 fs to ~1 ms (Cathode Region) (Inter-grid Region) BORIS et al., PHYSICAL REVIEW E, 80, (036408) 2009

UW -IEC Examples of charge transfer and thermal electron attachment within an IEC potential well. Thermal e- attachment In charge transfer events positive ions undergo charge transfer within the intergrid region and are accelerated out of the device with a fraction of the cathode energy The thermal electron population within the cathode, resultant from secondary emission, produces negative ions that attain the full cathode energy

UW -IEC The presence of negative ions can significantly impact particle flow in an IEC Negative ions add a divergent particle flux to the convergent ion flow of positive ions

UW -IEC Two methods were used to detect negative ions in the HOMER IEC device A magnetic deflection energy analyzer was used to measure the energy/nucleon of negative deuterium ions. A Faraday trap diagnostic was used to measure the particle flux of negative ions leaving the IEC.

UW -IEC Fusion Technology Institute 6 The magnetic deflection energy analyzer deflects negative ions into a detector according to q/m ratio A variable magnetic field deflects D - and D 2 - into a detector according to: Where θ d is the deflection angle, p is the ion momentum, and l is the spatial extent of the magnetic field

UW -IEC Scanning the magnetic field isolates small portions of the negative ion energy distribution Detector Trajectories generated used SIMION charged particle tracking software.

UW -IEC Fusion Technology Institute 8 Deuterium Anion Spectra show a multi-peaked structure Least squares fit to structure indicates a variety of processes: Charge transfer of ion species from source region explains 3 of the peaks. Ex. Thermal electron attachment to neutral gas in the cathode explains remaining peak. Ex. τ m = μs τ m = 10 fs D2-D2- D3+D-D3+D- D2+D-D2+D- D+D-D+D- D-D- D3+D-D3+D- D2+D-D2+D- D+D-D+D- D2-D2- D-D-

UW -IEC The location of negative ion formation within the potential well determines the ion’s energy Thermal e- attachment In charge transfer events positive ions undergo charge transfer within the intergrid region and are accelerated out of the device with a fraction of the cathode energy The thermal electron population within the cathode, resultant from secondary emission, produces negative ions that attain the full cathode energy

UW -IEC The energies of the various peaks scale linearly with cathode voltage. Anions from charge transfer attain kinetic energies of 1/2 to 2/3 the cathode voltage Anions from thermal electron attachment attain the full cathode energy D 2 - m=1/2

UW -IEC Background gas pressure affects which charge transfer reactions occur The relative sizes of the three charge transfer peaks reflect the changes in positive ion concentrations with varying background gas pressure. 90 kV, 30 mA 0.35 mTorr 0.7 mTorr 1.0 mTorr 1.5 mTorr 2.0 mTorr 2.6 mTorr 3.0 mTorr 3.75 mTorr D3+D3+ D2+D2+ D+D+

UW -IEC Faraday trap diagnostic confirmed the presence of negative ions Magnetic filter prevents collection of source plasma, and fast electrons. A negatively biased grid prevents secondary e- emission from the collector plate 8.5 µA/cm 2 of negative ion current was detected at 40 cm from the IEC cathode

UW -IEC Summary Using a magnetic deflection-energy analyzer, negative deuterium ions resultant from both charge-transfer and thermal electron attachment processes have been measured in the HOMER IEC device. Among these negative ions were long lived D 2 - ions with lifetimes of at least 0.5 µs. A Faraday trap diagnostic has confirmed the presence of these negative ions and indicates that they make up a significant portion of the particle flux within the HOMER IEC device.

UW -IEC Fusion Technology Institute 14 The UW-IEC Lab Questions?

UW -IEC Fusion Technology Institute 15 Deuterium Anion Spectra show a multi-peaked structure Least squares fit to structure indicates multiple species Charge transfer of ion species from source region explains 3 of the peaks. Ex. Thermal electron attachment to neutral gas in the cathode explains remaining peak. Ex. τ m = μs τ m = 10 fs D2-D2- D3+D-D3+D- D2+D-D2+D- D+D-D+D- D-D- D3+D-D3+D- D2+D-D2+D- D+D-D+D- D2-D2- D-D-

UW -IEC The negative ion energy distribution varies with pressure and cathode voltage VOLTAGE SCAN PRESSURE SCAN

UW -IEC Fusion proton energy spectra from FIDO diagnostic show significant contribution from negative ion fusion 0.25 mTorr1.5 mTorr2.5 mTorr Gaussian Fits D-D Proton Energy (keV) D-D Proton Energy (keV) counts/bin D-D Proton Energy (keV) counts/bin counts/bin counts/bin D-D- D-D- 100 kV 30 mA 100 kV 30 mA 100 kV 30 mA

UW -IEC Pressure Scan on Faraday Trap

UW -IEC Voltage Scan on Faraday Trap

UW -IEC Two diagnostics have been developed for negative ion detection within IEC devices Magnetic Deflection Energy Analyzer Faraday Trap