Willem Blokland

Active beam position stabilization of pulsed lasers for long-distance ion profile diagnostics at the Spallation Neutron Source (SNS)

A high peak-power Q-switched laser has been used to monitor the ion beam profiles in the superconducting linac at the Spallation Neutron Source (SNS). The laser beam suffers from position drift due to movement, vibration, or thermal effects on the optical components in the 250-meter long laser beam transport line. We have designed, bench-tested, and implemented a beam position stabilization system by using an Ethernet CMOS camera, computer image processing and analysis, and a piezo-driven mirror platform. The system can respond at frequencies up to 30 Hz with a high position detection accuracy. With the beam stabilization system, we have achieved a laser beam pointing stability within a range of 2 μrad (horizontal) to 4 μrad (vertical), corresponding to beam drifts of only 0.5 mm × 1 mm at the furthest measurement station located 250 meters away from the light source.

Performance of SNS Front end and warm linac

The Spallation Neutron Source accelerator systems will deliver a 1.0 GeV, 1.4 MW proton beam to a liquid mercury target for neutron scattering research. The accelerator complex consists of an H-injector, capable of producing one-ms-long pulses at 60 Hz repetition rate with 38 mA peak current, a 1 GeV linear accelerator, an accumulator ring and associated transport lines. The 2.5 MeV beam from the Front End is accelerated to 86 MeV in the Drift Tube Linac, then to 185 MeV in a Coupled-Cavity Linac and finally to 1 GeV in the Superconducting Linac. With the completion of beam commissioning, the accelerator complex began operation in June 2006 and beam power is being gradually ramped up toward the design goal. Operational experience with the injector and linac will be presented including chopper performance, transverse emittance evolution along the linac, and the results of a beam loss study.

Beam in gap measurements at the SNS front-end

The pulsed beam in the SNS accelerator has a fine time structure which consists of 695 ns long mini-pulses separated by 250 ns gaps in order to minimize transient beam losses in the accumulator ring which could arise during the ring extraction kicker rise time. This time structure is provided by a two stage Front End chopping system which must reduce the beam current in the gap to a level of 10/sup -4/ of the nominal current in order to satisfy requirements on the ring extraction losses. A Beam-in-Gap measuring system based on H/sup -/ stripping using Nd-YAG laser was developed and tested during the SNS Front-End commissioning period. This paper describes the Beam-in-Gap measurement system design and measured performance.

Laser stripping of H - beams: theory and experiments

Thin carbon foils are used as strippers for charge exchange injection into high intensity proton rings. However, the stripping foils become radioactive and produce uncontrolled beam loss, which is one of the main factors limiting beam power in high intensity proton rings. Recently, we presented a scheme for laser stripping an H- beam for the Spallation Neutron Source ring. First, H- atoms are converted to H0 by a magnetic field, then H0 atoms are excited from the ground state to the upper levels by a laser, and the excited states are converted to protons by a second magnetic field. In this paper we report on the first successful proof-of-principle demonstration of this scheme to give high efficiency (around 90%) conversion of H- beam into protons at SNS in Oak Ridge. In addition, future plans on building a practical laser stripping device are discussed.

Dynamic Visualization of SNS Diagnostics Summary Report and System Status

The Spallation Neutron Source (SNS) accelerator systems will deliver a 1.0 GeV, 1.4 MW proton beam to a liquid mercury target for neutron scattering research. The accelerator complex consists of a 1 GeV linear accelerator, an accumulator ring and associated transport lines. The majority of the SNS diagnostics platform is PC-based and runs Embedded Windows XP and LabVIEW. The diagnostics instruments communicate with the control system using the Channel Access (CA) protocol of the Experimental Physics and Industrial Control System (EPICS). This paper describes the Diagnostics Group’s approach to collecting data from the instruments and presenting live in a summarized way over the web. This data mining resulted in the “Diagnostics Status Page” that summarizes the insertable devices, transport efficiencies, and the mode of the accelerator in a compact webpage. The displays on the webpage change automatically to show the latest and/or most interesting instruments in use.

Timing measurements of the extraction kicker system at the Spallation Neutron Source

The Spallation Neutron Source (SNS) extraction kicker system is a high power 60 hertz pulsed power system [1]. The system consists of fourteen identical modulators, each driving a magnet to extract a proton beam from the accumulator ring through the beam transfer line to the target. The kickers are required to reach maximum magnetic flux density during a 200-250 ns beam gap in the accumulator ring. Timing variations in the modulator and/or trigger system could cause firing outside of the beam gap causing beam loss resulting in residual activation and possible equipment damage. Each modulator is a Blumlein Pulse Forming Network (PFN) with a fast high current switching thyratron and low inductance capacitor banks. This paper describes measurements made to characterize the jitter and drift associated with the trigger system, thyratron and PFN. Furthermore, the effect of firing asynchronously to the line voltage on jitter of the thyratron trigger system is discussed.

Measurements of Beam Pulse Induced Mechanical Strain Inside the SNS* Target Module

Because several of the SNS targets have had a shorter lifetime than desired, a new target has been instrumented with strain sensors to further our understanding of the proton beam’s mechanical impact. The high radiation and electrically noisy environment led us to pick multi-mode fiber optical strain sensors over other types of strain sensors. Special care was taken to minimize the impact of the sensors on the target’s lifetime. We also placed accelerometers outside the target to try correlating the outside measurements with the internal measurements. Remote manipulators performed the final part of the installation, as even residual radiation is too high for humans to come close to the target’s final location. The initial set of optical sensors on the first instrumented target lasted just long enough to give us measurements from different proton beam intensities. A second set of more rad-hard sensors, installed in the following target, lasted much longer, to give us considerably more data. We are developing our own rad-hard, single-mode fiber optic sensors. This paper describes the design, installation, data-acquisition system, the results of the strain sensors, and future plans.

The Spallation Neutron Source Beam Commissioning and Initial Operations

The Spallation Neutron Source (SNS) accelerator delivers a one mega-Watt beam to a mercury target to produce neutrons used for neutron scattering materials research. It delivers ~ 1 GeV protons in short (< 1 us) pulses at 60 Hz. At an average power of ~ one mega-Watt, it is the highest-powered pulsed proton accelerator. The accelerator includes the first use of superconducting RF acceleration for a pulsed protons at this energy. The storage ring used to create the short time structure has record peak particle per pulse intensity. Beam commissioning took place in a staged manner during the construction phase of SNS. After the construction, neutron production operations began within a few months, and one mega-Watt operation was achieved within three years. The methods used to commission the beam and the experiences during initial operation are discussed.

Status of various sns diagnostic systems

The spallation neutron source (SNS) accelerator systems are ramping up to deliver a 1.0 GeV, 1.4 MW proton beam to a liquid mercury target for neutron scattering research. Enhancements or additions have been made to several instrument systems to support the ramp up in intensity, improve reliability, and/or add functionality. The beam current monitors now support increased rep rates, the Harp system now includes charge density calculations for the target, and a new system has been created to collect data for the beam accounting and present the data over the web and to the operator consoles. The majority of the SNS beam instruments are PC-based and their configuration files are now managed through the Oracle relational database. A new version for the wire scanner software was developed to add features to correlate the scan with beam loss, parking in the beam, and measuring the longitudinal beam current. This software is currently being tested. This paper also includes data from the selected instruments.

SNS Ring and RTBT Beam Current Monitor

The SNS Diagnostics Group has implemented Beam Current Monitors (BCM) for the Ring and RTBT (Ring to Target Beam Transferline). In the Ring, the BCM must handle a thousand‐fold increase of intensity during the accumulation, and in the RTBT, the BCM must communicate the integrated charge of the beam pulse in real‐time for every shot to the target division for correlation with the produced neutrons. This paper describes the development of a four channel solution for the Ring BCM and the use of FPGA for the RTBT BCM to deliver the total charge to the target over a fiber optic network. Both system versions are based on the same commercial digitizer board.