Bulletin of the American Physical Society
66th Annual Gaseous Electronics Conference
Volume 58, Number 8
Monday–Friday, September 30–October 4 2013; Princeton, New Jersey
Session DT4: Workshop on the Mysteries and Challenges of Negative Ion Sources |
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Chair: Robert Welton; Larry Grisham; Rod Boswell, Oak Ridge National Laboratory; Princeton Plasma Physics Laboratory; Australia National University Room: Fountain View |
Tuesday, October 1, 2013 10:00AM - 10:10AM |
DT4.00001: Introduction Rob Welton |
Tuesday, October 1, 2013 10:10AM - 10:50AM |
DT4.00002: The requirements and challenges of the next generation negative ion sources Olli Tarvainen Negative ion sources are often subsystems of particle accelerators and neutral beam injectors. Thus, the requirements of the next generation negative ion sources are largely driven by upgrades and new developments of these technologies. The performance of negative ion sources is typically defined by the beam current and emittance, long and short-term stability of the extracted beams, the ratio of desired ions to co-extracted electrons and the maintenance interval of the ion source. The requirements of the next generation negative ion sources in each of these categories are described and compared to the state-of-the-art technologies. The physics of negative ion production are discussed briefly to outline the fundamental and technical challenges of the future negative ion sources. An overview of their development effort will be given with the main focus being in H$^{-}$/D$^{-}$ ion sources, which are currently under active research and development in several laboratories. Some novel ideas and techniques to improve the performance of negative ion sources are described to facilitate discussion. [Preview Abstract] |
Tuesday, October 1, 2013 10:50AM - 11:30AM |
DT4.00003: Challenges of negative hydrogen ion sources for fusion Ursel Fantz The neutral beam injection systems for the international fusion experiment ITER (www.iter.org) are based on large negative hydrogen ion sources which have to operate at very ambitious parameters. They have to deliver an accelerated current of 40 A negative hydrogen ions extracted from 1280 apertures stable for one hour. The co-extracted electron current has to be kept below the extracted ion current to avoid damages of the grid system. At the source pressure of 0.3 Pa or below the negative ions have to be produced at a surface with low work function for which cesium is evaporated into the source. In order to fulfill all these requirements an R{\&}D program has been launched several years ago. The challenges, however, are enormous; among them the control of the cesium dynamics in the source which determines the reliability of the source performance, the amount of co-extracted electrons which limits the extractable negative ion current, and the size scaling of the source towards an area of 1.9 x 1 m$^{2}$. Among the open and less understood issues are the magnetic filter field (initially used to reduce the electron temperature and density) in combination with the biasing of the grid surface, the sensitivity of the co-extracted electrons on cesium, the high co-extracted electron current in deuterium, and the fortunately weak correlation of the plasma homogeneity with the beam homogeneity. The present status, the most critical issues and open and less understood issues will be addressed and may serve as a trigger for a stimulating discussion or even trigger experiments and modeling activities to these topics. [Preview Abstract] |
Tuesday, October 1, 2013 11:30AM - 1:00PM |
DT4.00004: LUNCH |
Tuesday, October 1, 2013 1:00PM - 1:30PM |
DT4.00005: DISCUSSION |
Tuesday, October 1, 2013 1:30PM - 2:10PM |
DT4.00006: Challenges/issues of NIS used in particle accelerator facilities Dan Faircloth High current, high duty cycle negative ion sources are an essential component of many high power particle accelerators. This talk gives an overview of the state-of-the-art sources used around the world. Volume, surface and charge exchange negative ion production processes are detailed. Cesiated magnetron and Penning surface plasma sources are discussed along with surface converter sources. Multicusp volume sources with filament and LaB6 cathodes are described before moving onto RF inductively coupled volume sources with internal and external antennas. The major challenges facing accelerator facilities are detailed. Beam current, source lifetime and reliability are the most pressing. The pros and cons of each source technology is discussed along with their development programs. The uncertainties and unknowns common to these sources are discussed. The dynamics of cesium surface coverage and the causes of source variability are still unknown. Minimizing beam emittance is essential to maximizing the transport of high current beams; space charge effects are very important. The basic physics of negative ion production is still not well understood, theoretical and experimental programs continue to improve this, but there are still many mysteries to be solved. [Preview Abstract] |
Tuesday, October 1, 2013 2:10PM - 2:50PM |
DT4.00007: Challenges in plasma and extraction modelling of negative ion sources Taneli Kalvas The physical processes taking place in negative ion source plasmas are modelled by state-of-the-art 3D particle-in-cell (PIC) codes. These codes are used to gain understanding and to find optimal solutions for negative ion beam production. The PIC codes can be made to match to the reality if all relevant processes were included. This is unfortunately limited by the availability of data about the processes and the huge amount of computational resources needed for the simulations. The optimization of the extraction system and beam transport ion optics is often made using computationally less intensive methods utilized in so-called gun codes. These codes use simplified plasma models to provide a starting point for the extracted beams being simulated. The relatively fast computation allows systematic studies, which are not practical with PIC codes. The gun codes often match well to reality, but they do have difficulties reproducing some effects, especially in negative ion extraction, due to the approximations made in the plasma model. Could the future solutions for beam production modelling couple the two types of simulations? [Preview Abstract] |
Tuesday, October 1, 2013 2:50PM - 3:30PM |
DT4.00008: BREAK
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Tuesday, October 1, 2013 3:30PM - 4:10PM |
DT4.00009: DISCUSSION |
Tuesday, October 1, 2013 4:10PM - 4:50PM |
DT4.00010: Unresolved problems in cesiation processes of negative hydrogen ion sources Motoi Wada Attempts are being made to optimize negative hydrogen (H$^-$) ion current by introducing Cs into an ion source, but there are some unanswered questions in properly handling Cs to realize stable extraction of H$^-$ ion beams. For example, Cs amount to optimize H$^-$ production often becomes much larger than the amount predicted to realize partial monolayer of Cs on the source wall. Additional charge of Cs into a source to recover reduced H$^-$ current by continuous operation does not necessarily realize the original value. Beam intensity of H$^-$ changes with the impurity content in the ion source. The purpose of the present paper is to list up these uncertainties and unknown factors in negative ion source performance operated with Cs. The paper tries to identify possible mechanisms causing these problems by running a simulation code ACAT (Atomic Collision in Amorphous Target). The code predicts that glancing injection of hydrogen ions doubles the numbers of both reflection coefficients and ion induced desorption yields from those for the normal incidence. It also indicates smaller hydrogen desorption yields for thick layer of adsorbed hydrogen on the surface. These results are compared with experimental data obtained in UHV conditions. [Preview Abstract] |
Tuesday, October 1, 2013 4:50PM - 5:30PM |
DT4.00011: DISCUSSION |
Tuesday, October 1, 2013 5:30PM - 6:00PM |
DT4.00012: Closing Remarks Larry Grisham |
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