Alternative solutions to caesium in negative-ion sources - a study of negative-ion surface production on diamond in H2/D2 plasmas

At a Glance

Metadata	Details
Publication Date	2017-02-23
Journal	New Journal of Physics
Authors	Gilles Cartry, D. Kogut, K. Achkasov, Jean-Marc Layet, Thomas A. Farley
Institutions	Physique des interactions ioniques et moléculaires, University of Liverpool
Citations	43
Analysis	Full AI Review Included

Technical Documentation and Collateral: Diamond for High-Yield Negative-Ion Sources

6CCVD Reference Analysis: Cartry et al., Alternative solutions to caesium in negative-ion sources: a study of negative-ion surface production on diamond in H2/D2 plasmas

Executive Summary

This study successfully validates high-purity MPCVD diamond layers as a viable, cesium-free alternative material for enhancing negative hydrogen/deuterium (H^-/D^-) ion surface production, critical for fusion (ITER/DEMO) and particle accelerator applications.

Core Advantage: Diamond’s favorable electronic properties, particularly its Negative Electron Affinity (NEA), are confirmed as highly conducive to efficient negative-ion formation, contrasting with traditional low-work-function metals requiring Cs injection.
Performance Metrics: Diamond (Micro-Crystalline Boron-Doped Diamond, MCBDD) exhibited relative negative-ion yields double that of reference graphite (HOPG) under continuous bias.
Temperature Optimization: A critical finding identifies an optimal surface temperature range of 400-500 °C for all tested diamond layers, maximizing yield by promoting surface reconstruction and recovery of pristine electronic properties.
Defect Sensitivity: Positive ion bombardment creates surface defects (sp² hybridization) which reduce yield. Methods to mitigate defects (heating, pulsed-DC bias) significantly increase negative-ion flux.
Material Versatility: Both intrinsic and highly Boron-Doped (BDD) single-crystal and polycrystalline diamond layers demonstrate similar favorable behavior, although BDD doping is required for sufficient conductivity at low temperature plasma exposure.
Future Relevance: Diamond offers potential solutions to key operational bottlenecks associated with cesium use (consumption, maintenance, pollution) in high-current density negative-ion sources (targeting 200 A/m² for ITER).

Technical Specifications

The following parameters were extracted from the plasma source experiments comparing various diamond materials (MCD, MCBDD, NCD, SCBDD) against HOPG.

Parameter	Value	Unit	Context
Optimal Temperature (Diamond)	400 - 500	°C	Maximum negative-ion yield observed for all diamond types (DC bias).
Negative-Ion Yield (Relative)	> 2X	N/A	Diamond vs. HOPG (Graphite) under equivalent D₂ plasma conditions.
Bias Mode Performance	2X to 5X Increase	Factor	Pulsed-DC bias yield increase over continuous DC bias (on MCD).
Boron Doping Level (MCBDD)	1.5 × 10²¹	cm^-3	High p-doping level, primarily used to ensure surface conductivity.
Single Crystal Diamond (SCBDD) Area	9	mm²	Sample size used for (100) SCBDD testing.
SCBDD Thickness	20	µm	Single Crystal Boron-Doped Diamond thickness.
RF Plasma Pressure (D₂)	2.0	Pa	Standard plasma condition for yield comparison experiments (20 W).
DC Bias Voltage (V_s)	-130	V	High ion energy condition, resulting in ~45 eV/nucleon H₃⁺ impact.
Low DC Bias Voltage (V_s)	-20	V	Low ion energy condition, resulting in ~12 eV/nucleon H₃⁺ impact.
RF Ion Flux to Sample	10¹⁷	m^-2s^-1	Order of magnitude of positive ion flux during 20 W RF plasma.
Electron Density (ECR Plasma)	2.5 × 10¹⁵	m^-3	High-density plasma condition (1 Pa, 60 W ECR).

Key Methodologies

The experimental setup used a low-pressure H₂/D₂ plasma diffusion chamber, designed to simplify ion extraction physics and focus on the surface production mechanism.

Plasma Generation: Plasma was generated either by:
- RF Coupling: 20 W, 13.56 MHz generator (used for low-flux studies, V_s = -130 V).
- ECR Coupling: 60 W, 2.45 GHz generator (used for high-flux studies, resulting in 7 × 10¹⁸ m^-2s^-1 ion flux).
Sample Biasing and Heating: The diamond sample was placed on a temperature-controlled holder with an embedded resistive heater.
- DC Bias: A negative DC bias (V_s, typically -130 V) was applied relative to the plasma potential to induce self-extraction of negative ions and positive ion bombardment.
- Pulsed-DC Bias: A specialized pulsed-DC scheme (15 µs pulse, 10 kHz frequency) was developed to study insulating materials (MCD) and minimize surface degradation.
Measurement and Analysis: Negative ions (H^-/D^-) were extracted and measured by a Hiden EQP 300 mass spectrometer equipped with an energy filter.
- Output Data: Negative-Ion Energy Distribution Function (NIEDF) and relative Negative-Ion Yield (total counts/second).
- Yield Comparison: Yields were measured as a function of surface temperature (300 K up to 800 °C) and sample tilt angle ($\alpha$) to determine the full Energy and Angle Distribution Function (NIEADF) on the surface.
Modeling and Verification: SRIM software was used to model the backscattering and sputtering contributions of ions impacting the surface, validating the experimental NIEDFs for hydrogenated carbon surfaces.

6CCVD Solutions & Capabilities

The findings confirm that high-quality, MPCVD-grown diamond is a critical enabling material for next-generation negative-ion sources. 6CCVD is uniquely positioned to supply the specific engineered diamond materials required to replicate, scale, and optimize this crucial research.

Applicable Materials

The study highlights the viability of both single crystal and polycrystalline diamond with specific doping and surface preparation. 6CCVD offers direct equivalents that meet or exceed the performance requirements:

Research Material Requirement	6CCVD Recommended Solution	Rationale and Capability Match
SCBDD (100) / MCBDD	Boron-Doped Single Crystal Diamond (BDD-SCD)	We provide highly-doped BDD-SCD (up to 10²¹ cm^-3 boron concentration, matching the study’s MCBDD doping) for maximum surface conductivity, eliminating the low-temperature insulation issues observed in MCD.
MCD / NCD (Nano/Micro-Crystalline)	Polycrystalline Diamond (PCD)	We offer high-purity PCD wafers, including specialized Nanocrystalline grades, ensuring the required mechanical and electronic properties demonstrated in the study for large-area applications.
Low-Defect, Pristine Surface	Polishing Service (Ra < 1 nm for SCD)	The study emphasizes that low defect density is essential for high yield. Our proprietary polishing achieves ultra-low roughness (Ra < 1 nm for SCD and < 5 nm for inch-size PCD), providing the optimal “pristine” surface state to maximize NEA effect.

Customization Potential for Negative-Ion Source Grids

The success of diamond depends on precise integration into the plasma grid. 6CCVD’s engineering capabilities directly address the dimensional and interface challenges:

Custom Dimensions: While the paper used small 9 mm² samples, ITER/DEMO require large-area production. 6CCVD supplies PCD plates and wafers up to 125mm diameter, ideal for scaling up source extraction grids.
Precision Fabrication: We offer custom laser cutting and micromachining to achieve the complex geometries (e.g., apertures and extraction grid patterns) required for ion source optics, ensuring perfect alignment and control as discussed in the paper.
Integrated Metalization and Biasing: The experiment relies on precise biasing (V_s) and high-temperature operation. We offer internal metalization services (Au, Pt, Pd, Ti, W, Cu) for creating robust electrical contacts and heater interfaces necessary for applying both continuous and pulsed-DC biases and maintaining the critical 400-500 °C operating temperature.

Engineering Support & Delivery

6CCVD’s in-house PhD team can assist with material selection for similar Fusion Plasma / Negative Ion Source projects, particularly focusing on optimizing doping levels, crystallographic orientation (e.g., (100) vs (111)), and surface termination to maximize Negative Electron Affinity (NEA) effects and minimize plasma-induced degradation.

We ensure global delivery of customized diamond materials, handling all logistics (DDU default, DDP available) to research facilities worldwide, including major fusion laboratories.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

This paper deals with a study of H-/D-negative ion surface production on\ndiamond in low pressure H2/D2 plasmas. A sample placed in the plasma is\nnegatively biased with respect to plasma potential. Upon positive ion impacts\non the sample, some negative ions are formed and detected according to their\nmass and energy by a mass spectrometer placed in front of the sample. The\nexperimental methods developed to study negative ion surface production and\nobtain negative ion energy and angle distribution functions are first\npresented. Different diamond materials ranging from nanocrystalline to single\ncrystal layers, either doped with boron or intrinsic, are then investigated and\ncompared with graphite. The negative ion yields obtained are presented as a\nfunction of different experimental parameters such as the exposure time, the\nsample bias which determines the positive ion impact energy and the sample\nsurface temperature. It is concluded from these experiments that the electronic\nproperties of diamond materials, among them the negative electron affinity,\nseem to be favourable for negative-ion surface production. However, the\nnegative ion yield decreases with the plasma induced defect density.\n

Tech Support

Original Source

DOI: https://doi.org/10.1088/1367-2630/aa5f1f