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Workshop on Performance Variations in H- Ion Sources 2012: PV H-12
This paper briefly summarizes a workshop held in Jyvaskyla the day after NIBS 12. The half-day workshop aimed at globally capturing the issue of performance variations in H- sources. There was a focus on production facilities and facilities that work under production like conditions, because there are often high expectations to be met.
Plasma-Wall Interactions in the Cesiated SNS H- Ion Source
High-current H- ion beams are needed to drive high-power accelerators as well as to heat future fusion reactors with neutral beams. The most productive H- sources enhance the production of H- ions with caesium. Cs lowers the work functions of the metal walls, which increases the probability of ions to capture a second electron when bouncing back from the metal plasma walls. However, caesium also causes voltage break downs that can be severe and cause significant downtime. SNS has developed a frugal caesium management, which uses a single injection of ~4 mg of caesium to produce ~50 mA of H- beam without decay for up to 6 weeks. This paper presents calculations and experimental data, which suggest the persistence of the caesium enhanced H- beams are due to 1) thermally limiting the Cs emission, and 2) conditioning for high purity hydrogen plasma, which eliminates the sputtering of Cs by non-hydrogen ions, and 3) hydrogen ions being too light to sputter Cs.
Transients on Arc and Convertor Currents in the Multi-cusp Cesiated Surface Conversion H-Source at LANSCE
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Proposed mechanism to represent the suppression of dark current density by four orders with low energy light ion (H{sup −}) implantation in quaternary alloy-capped InAs/GaAs quantum dot infrared photodetectors
Graphical abstract: - Abstract: Here we propose a carrier transport mechanism for low energy H{sup −} ions implanted InAs/GaAs quantum dot infrared photodetectors supportive of the experimental results obtained. Dark current density suppression of up to four orders was observed in the implanted quantum dot infrared photodetectors, which further demonstrates that they are effectively operational. We concentrated on determining how defect-related material and structural changes attributed to implantation helped in dark current density reduction for InAs/GaAs quantum dot infrared photodetectors. This is the first study to report the electrical carrier transport mechanism of H{sup −} ion-implanted InAs/GaAs quantum dot infrared photodetectors.
An overview of the new test stand for H{sup −} ion sources at FNAL
A new test stand at Fermi National Accelerator Laboratory (FNAL) is being constructed to carry out experiments to develop and upgrade the present magnetron-type sources of H{sup −} ions of up to 80 mA at 35 keV in the context of the Proton Improvement Plan. The aim of this plan is to provide high-power proton beams for the experiments at FNAL. The technical details of the construction and layout of this test stand are presented, along with a prospective set of diagnostics to monitor the sources.
Upgrading the LANSCE accelerator with a SNS RF-driven H− ion source
The LANSCE accelerator is currently powered by a filament-driven, biased converter-type H− ion source that operates at 10%, the highest plasma duty factor for this type of source, using only ∼2.2 SCCM of H2. The ion source needs to be replaced every 4 weeks, which takes up to 4 days. The measured negative beam current of 12–16 mA falls below the desired 24 mA acceptance of the LANCSE accelerator. The SNS (Spallation Neutron Source) RF-driven, H− ion source injects ∼50 mA of H− beam into the SNS accelerator at 60 Hz with a 6% duty factor and an availability of >99.5% but requires ∼30 SCCM of H2. Up to 7 A h of H− have been produced during the 14-weeks-long source service cycles, which is unprecedented for small emittance, high-current, pulsed H− ion sources. The emittance of the SNS source is slightly smaller than the emittance of the LANSCE source. The SNS source also features unrivaled low Cs consumption and can be installed and started up in <12 h. LANSCE and SNS are working toward the use of SNS H− ion sources on the LANSCE accelerator because they could (a) fill the LANSCE accelerator to its capacity, (b) decrease the source replacement time by a factor of up to 7, and (c) increase source lifetime by a factor of about 4. This paper discusses some of the challenges that emerge when trying to match a different H− source into an existing injector with significantly different characteristics and operating regimes.
Exploring cesium properties internal to the LANSCE H - ion source using resonant absorption spectroscopy
The Los Alamos Neutron Science Center (LANSCE) H - ion source has provided stable output for decades of LANL mission needs, but its maximum beam output has remained the same at ~15 mA. A roadblock to improving beam output has been a lack of thorough understanding of the internal mechanisms of LANSCE H - ion source. The LANSCE H - Ion Source Laser Diagnostic Stand (HLDS) was recently built and commissioned to explore these internal mechanisms using laser absorption techniques, to measure and diagnose dynamic H - and cesium densities. The cesium density probe is based on resonant absorption of a continuous wave diode laser tuned though the D 2 line of cesium (~852 nm). The diagnostic capabilities of HLDS will be reviewed, and measurements using the cesium laser diagnostic will be presented.
Improvements to the internal and external antenna H{sup −} ion sources at the Spallation Neutron Source
The Spallation Neutron Source (SNS), a large scale neutron production facility, routinely operates with 30–40 mA peak current in the linac. Recent measurements have shown that our RF-driven internal antenna, Cs-enhanced, multi-cusp ion sources injects ∼55 mA of H{sup −} beam current (∼1 ms, 60 Hz) at 65-kV into a Radio Frequency Quadrupole (RFQ) accelerator through a closely coupled electrostatic Low-Energy Beam Transport system. Over the last several years a decrease in RFQ transmission and issues with internal antennas has stimulated source development at the SNS both for the internal and external antenna ion sources. This report discusses progress in improving internal antenna reliability, H{sup −} yield improvements which resulted from modifications to the outlet aperture assembly (applicable to both internal and external antenna sources) and studies made of the long standing problem of beam persistence with the external antenna source. The current status of the external antenna ion source will also be presented.
H - Ion Source RF Plasma Testing with 13 MHz and 27 MHz at the Spallation Neutron Source (SNS)
The baseline RF-driven H⁻ ion source configuration at the Spallation Neutron Source (SNS) facility uses a continuous wave (CW) 600 W 13 MHz RF system to ignite and maintain a low-density plasma inside the ion source vacuum chamber. After the continuous low-density 13 MHz plasma has been established, a pulsed (typical 1 ms pulse width and 60 Hz pulse repetition rate) 80 kW 2 MHz RF system is used to increase the plasma density to produce the pulsed H⁻ ion beam. Incremental upgrades and improvements to the SNS ion source systems have resulted in the ability to reliably operate an H⁻ ion source for SNS neutron production run cycles that can last up to four months. Conditions inside the SNS H⁻ ion source evolve throughout a four-month run cycle due to changes in impurity levels, sputtering, and erosion. As the internal ion source conditions change during the run cycle, there can also be changes in plasma stability and the 13 MHz RF power level required to ignite the plasma. This paper presents the preliminary results of testing performed on the SNS Ion Source Test Stand (ISTS) system where we looked at plasma ignition and plasma stability using 27 MHz RF in place of the baseline 13 MHz RF system.
Recent Operation of the FNAL Magnetron H- Ion Source
This paper will detail changes in the operational paradigm of the Fermi National Accelerator Laboratory (FNAL) magnetron H- ion source due to upgrades in the accelerator system. Prior to November of 2012 the H- ions for High Energy Physics (HEP) experiments were extracted at ~18 keV vertically downward into a 90 degree bending magnet and accelerated through a Cockcroft-Walton accelerating column to 750 keV. Following the upgrade in the fall of 2012 the H- ions are now directly extracted from a magnetron at 35 keV and accelerated to 750 keV by a Radio Frequency Quadrupole (RFQ). This change in extraction energy as well as the orientation of the ion source required not only a redesign of the ion source, but an updated understanding of its operation at these new values. Discussed in detail are the changes to the ion source timing, arc discharge current, hydrogen gas pressure, and cesium delivery system that were needed to maintain consistent operation at >99% uptime for HEP, with an increased ion source lifetime of over 9 months.
Electron recombination of rotationally cold D 2 H + ions
Dissociative recombination (DR) of electrons with small molecular ions is a fundamental process for the physics and chemistry of the interstellar medium and planetary atmospheres. In previous DR studies, detailed analysis of the experimental rate coefficients has been hindered by the difficulty of preparing the ions in well-defined quantum states. For polyatomic ions in particular, truly state-selective measurements have been elusive, allowing only qualitative benchmarks of theory. Here, we present DR studies of the deuterated triatomic hydrogen ion D 2 H + , where the molecular ions were stored for up to 1000 seconds inside the Cryogenic Storage Ring (CSR) prior to the DR measurements. Our experiments with rotationally cold D 2 H + ions allow for detailed comparison to state-of-the-art theoretical calculations. We obtain very good agreement between experiment and theory even in the important collision energy range from 1 meV to 0.5 eV, where a multitude of Rydberg resonances reveal their imprint on the rate coefficient.
Modeling of tungsten filament in gas discharge in H - ion source
Sources of negative ions such as H - are essential elements of proton accelerators and tokamaks. They have limited lifetime. The replacement of an ion source is a costly process causing delays in the operation of the entire machine. The hot tungsten filament is the key element that limits the lifetime of the H - ion source at the Los Alamos Neutron Science Center (LANSCE) facility. An accurate model, which describes the filament physics and provides a reliable estimate on its lifetime, may significantly improve the operation of the facility. In this report we describe such a model, including a comprehensive list of relevant physical processes from first principles. The model can be used to describe different regimes of operation using diagnostics data as input parameters. It has been benchmarked against the data collected during the production cycle at LANSCE and shows good agreement with experimental data.
H- Ion Sources for High Intensity Proton Drivers
Existing RF Surface Plasma Sources (SPS) for accelerators have specific efficiencies for H + and H - ion generation around 3 to 5 mA/cm 2 per kW, where about 50 kW of RF power is typically needed for 50 mA beam current production. The Saddle Antenna (SA) SPS described here was developed to improve H- ion production efficiency, reliability and availability for pulsed operation as used in the ORNL Spallation Neutron Source . At low RF power, the efficiency of positive ion generation in the plasma has been improved to 200 mA/cm 2 per kW of RF power at 13.56 MHz. Initial cesiation of the SPS was performed by heating cesium chromate cartridges by discharge as was done in the very first versions of the SPS. A small oven to decompose cesium compounds and alloys was developed and tested. After cesiation, the current of negative ions to the collector was increased from 1 mA to 10 mA with RF power 1.5 kW in the plasma (6 mm diameter emission aperture) and up to 30 mA with 4 kW RF power in the plasma and 250 Gauss longitudinal magnetic field. The ratio of electron current to negative ion current was improved from 30 to 2. Stable generation of H- beam without intensity degradation was demonstrated in the aluminum nitride (AlN) discharge chamber for 32 days at high discharge power in an RF SPS with an external antenna. Some modifications were made to improve the cooling and cesiation stability. The extracted collector current can be increased significantly by optimizing the longitudinal magnetic field in the discharge chamber. While this project demonstrated the advantages of the pulsed version of the SA RF SPS as an upgrade to the ORNL Spallation Neutron Source, it led to a possibility for upgrades to CW machines like the many cyclotrons used for commercial applications. Four appendices contain important details of the work carried out under this grant.
L -Value and Energy Dependence of 0.1–50 keV O + , He + , and H + Ions for CME and CIR Storms Over the Entire Van Allen Probes Era
Our studies report the first observation of L -value and energy sorted correlation of differential fluxes of 0.1–50 keV O + , He + , and H + ions with different geophysical parameters for 29 Coronal Mass Ejected (CME) and 40 Corotating Interaction Region (CIR)-driven geomagnetic storms during the entire Van Allen Probes era. For both solar wind drivers, ions with ≥1 keV energies show more variability in response to the solar wind changes, while the lower energy (<1 keV) ions are relatively stable. During the in-storm interval, O + ions show maximum flux enhancement and become further prominent during CME storms. O + ion (≥10 keV) fluxes show good correlation with – $V_{sw}B_z$, and Sym-H index during CME-driven storms in the L ~2.5–5.5. Apart from this, the average duration of persistence ($\langleΔt\rangle$) for enhanced fluxes is higher for CIR-driven storms with $\langleΔt\rangle_{O^+}>\langleΔt\rangle_{He^+}>\langleΔt\rangle_{H^+}$ at E ≤ 50 keV in the L ~2.5–5.5. Moreover, the observed value of $\langleΔt\rangle_i$ (where $\textit{i}$ is O + , H + or He + ) increases with the increasing L . Further, we discuss the plausible mechanisms to provide a comprehensive overview of L -values and energy sorted O + , He + and H + ion dynamics for two different categories of solar wind drivers.
Plasma emission spectroscopy for operating and developing the Spallation Neutron Source (SNS) H- ion sources
An RF-driven, Cs-enhanced H- ion source feeds the SNS accelerator with a high current (typically >50 mA), ~1.0 ms pulsed beam at 60 Hz. To achieve the persistent high current beam for several weeks long service cycles, each newly installed ion source undergoes a rigorous conditioning and cesiation processes. Plasma conditioning outgases the system and sputter-cleans the ion conversion surfaces. A cesiation process immediately following the plasma conditioning releases Cs to provide coverage on the ion conversion surfaces. The effectiveness of the ion source conditioning and cesiation is monitored with plasma emission spectroscopy using a high-sensitivity optical spectrometer. Plasma emission spectroscopy is also used to provide a mean for diagnosing and confirming a failure of the insulating coating of the ion source RF antenna which is immersed in the plasma. Emissions of composition elements of the antenna coating material, Na emission being the most significant, drastically elevate to signal a failure when it happens. Plasma spectra of the developmental ion source with an AlN chamber and an external RF antenna are also briefly discussed.
Intrabeam Stripping of H- Ions in the JLEIC Ion Linac with PyORBIT
Jefferson Lab is designing an electron-ion collider (JLEIC) to meet the experimental needs of the nuclear physics community. In JLEIC, a potential mechanism for beam loss is intrabeam stripping of the H- ions in the linear accelerator (linac). As the ions interact with each other, there is a chance that electrons will be stripped from the ions. This creates neutral particles that are unaffected by the electromagnetic fields and are thus lost. This project determined whether intrabeam stripping is a relevant form of beam loss for JLEIC. The pyORBIT code was modified to simulate the beam dynamics of the JLEIC linac. Then plots were created of relativistic velocity to determine the likelihood of intrabeam stripping. From these plots, the relativistic velocity values of the ion beam were found to be similar to values from previous studies on other linacs; however, the calculated average power of JLEIC is significantly lower than the powers of other linacs. Previous predictions have shown that intrabeam stripping is negligible below a certain average power value. Our value for average power was many orders of magnitude below this average power value and thus confirmed that intrabeam stripping of the H- ions will have negligible effects on the amount of beam loss in JLEIC ion linac.
Optimization of the cesiation process for the spallation neutron source (SNS) H - ion sources
RF-driven (internal and external antenna), Cs-enhanced H - ion sources at the Spallation Neutron Source (SNS) facility are capable of producing high current (>60 mA), high duty-factor (1 ms, 60 Hz) H - beams lasting multiple months. A solid reaction Cs dispenser system is used to cesiate the ion source. This work reports on our continuous effort to optimize the ion source cesiation process guided by the Cs signal observed with the plasma emission spectroscopy to mitigate the Cs induced high voltage sparking while assuring to enable high H - beam currents