Photodetachment diagnostics of plasma with a high n−/n
 e ratio

Balghiti-Sube, F. El; Baksht, F. G.; Bacal, M.

doi:10.1063/1.1147040

Cited by 52 publications

(47 citation statements)

References 6 publications

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“…The positive effect of biasing the plasma grid in negative ion sources is known since the 1980s and is used in many different ion sources [9][10][11][12][13] ; the general effect on the source performance has been reported also for the BATMAN (Bavarian Test Machine for Negative Ions) test facility in earlier publications 6,14 . Variation of the bias voltage changes the potential profile in the sheath in front of the plasma grid from an electron repelling case at low bias towards an electron attracting sheath at high bias.…”

Section: Introductionmentioning

confidence: 99%

Extraction of negative charges from an ion source: Transition from an electron repelling to an electron attracting plasma close to the extraction surface

Wimmer

Fantz

NNBI-Team³

2016

Journal of Applied Physics

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Large-scale sources for negative hydrogen ions, capable of delivering an extracted ion current of several ten amperes, are a key component of the neutral beam injection system of the upcoming ITER fusion device. Since the created heat load of the inevitably co-extracted electrons after magnetic separation from the extracted beam limits their tolerable amount, special care must be taken for the reduction of coextracted electrons -in particular in deuterium operation, where the larger amount of co-extracted electrons often limits the source performance. By biasing the plasma grid (PG, first grid of the extraction system) positively with respect to the source body the plasma sheath in front of the PG can be changed from an electron repelling towards an electron attracting sheath. In this way, the flux of charged particles onto the PG can be varied, thus changing the bias current and inverse to it the amount of co-extracted electrons. The PG bias affects also the flux of surface-produced H − towards the plasma volume as well as the plasma symmetry in front of the plasma grid, strongly influenced by an ⃗ E × ⃗ B drift. The influence of varying PG sheath potential profile on the plasma drift, the negative hydrogen ion density and the source performance at the prototype H − source is presented, comparing hydrogen and deuterium operation. The transition in the PG sheath profile takes place in both isotopes, with a minimum of co-extracted electrons formed in case of the electron attracting PG sheath. The co-extracted electron density in deuterium operation is higher than in hydrogen operation, which is accompanied by an increased plasma density in deuterium.

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Section: Introductionmentioning

confidence: 99%

Extraction of negative charges from an ion source: Transition from an electron repelling to an electron attracting plasma close to the extraction surface

Wimmer

Fantz

NNBI-Team³

2016

Journal of Applied Physics

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“…However, according to Ref. [65], a sharp peak may be observed at the beginning of the probe current pulse for large negative ion fraction. In this case the authors in Ref.…”

Section: Negative Ion Fraction Measurementsmentioning

confidence: 99%

“…In this case the authors in Ref. [65] recommend to use the peak value (∆I e peak correction) in equation 5. Our signals show an overshoot at the beginning of the pulse and large high frequency oscillations are also observed.…”

Section: Negative Ion Fraction Measurementsmentioning

confidence: 99%

Experimental investigation of double layers in expanding plasmas

2007

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Double layers (DLs) have been observed in a plasma reactor composed of a source chamber attached to a larger expanding chamber. Positive ion beams generated across the DL were characterized in the low plasma potential region using retarding field energy analyzers. In electropositive gases, DLs were formed at very low pressures (between 0.1 and 1 mTorr) with the plasma expansion forced by a strongly diverging magnetic field. The DL remains static, robust to changes in boundary conditions, and its position is related to the magnetic field lines. The voltage drop across the DL increases with decreasing pressure, i.e. with increasing electron temperature (around 20 V at 0.17 mTorr). DLs were also observed in electronegative gases without a magnetic field over a greater range of pressure (0.5 to 10 mTorr). The actual profile of the electronegative DL is very sensitive to external parameters and intrusive elements, and they propagate at high negative ion fraction. Electrostatic probes measurements and laser induced photodetachment show discontinuities in all plasma parameters (electron density, electron temperature, negative ion fraction) at the DL position. The voltage drop across the electronegative DL is about 8 V, is independent of the gas pressure and therefore of the electron temperature.

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“…This type of negative ion rich plasma localized near the PE of a volume hydrogen negative ion source has been first reported by El Balghiti-Sube et al 15 In the experiment of Ref. 15 the high ratio N -/N e = 12 was found at optimum PE bias with a stainless steel PE covered with a tungsten film.…”

Section: Responsible Mechanismsmentioning

confidence: 84%