O.R. Sander

Experience with the ground test accelerator beam‐measurement instrumentation

During the past two years, the Ground Test Accelerator (GTA) has used a variety of off‐ and on‐line beam diagnostic measurements to understand and verify the transverse and longitudinal phase space characteristics of a 35‐mA, low‐energy (2.5‐ to 3.2‐MeV) H−‐beam. For the transverse phase‐space characterization measurements, a slit and collector device samples of the x−x’ and y−y’ phase space, to determine the transverse emittance and Courant–Snyder parameters. The longitudinal phase‐space data are acquired by a laser neutralization technique developed at Los Alamos know as the laser induced neutralization diagnostics approach (LINDA). The transverse and longitudinal phase‐space centroids of the low‐energy, 425‐MHz‐bunched beam are directly measured using the microstrip probe systems. Beam current and transmission are measured by various toroid systems. Beam‐loss‐detection techniques have just been installed and a non‐interceptive beam‐profile measurement has been commissioned. All of these measurement sys...

Measurement of longitudinal phase space in an accelerated H− beam using a laser-induced neutralization method

Abstract Laser-induced neutralization of H − ions is a process that can be used to measure the longitudinal phase space of accelerated H − beams. The laser-induced neutralization diagnostic approach (LINDA) measures the longitudinal emittance of an H − beam by photoneutralizing different phase slices of beam microbunches and analyzing the energy distribution of the neutral slices. A LINDA system utilizing a pulsed laser and time-of-flight analysis has successfully measured longitudinal emittance of the 5 MeV H − beam exiting the drift-tube linac of the Los Alamos Accelerator Test Stand. Design considerations associated with the LINDA laser-based emittance measuring system are given. The present LINDA system is described and its limitations are discussed. Experimental results are given from an application of the LINDA system to the measurement of longitudinal emittance growth in a drift space and following insertion into the beamline of beam transport elements comprising a single-arm funnel. A new system is proposed which uses a mode-locked laser and spectrometer to improve resolution and shorten measurement time.

Recent Improvements in Beam Diagnostic Instrumentation

In high-current machines, such as LAMPF and the envisioned Hanford Engineering Development Laboratory (HEDL) Fusion Materials Irradiation Test (FMIT) Facility linac, hands-on maintenance is desired. Beam spill must be kept extremely low; therefore, attention must be given to beam fringes (tails). Beam matching to the structure becomes increasingly important. We describe equipment capable of accurately measuring transverse beam profiles over a range spanning more than four orders of magnitude and longitudinal phase profiles over ranges spanning more than three orders of magnitude. Errors in 100-MeV transverse emittance measurements are explored and experimental emittance measurements made with three different methods are compared. Advantages of one nondestructive method are developed.

Noninierceptive Techniques for the Measurement of Longitudinal Parameiers for Intense H- Beams

With increasing brightness, beam diagnostic techniques requiring interception of the beam become impractical. For H- particle beams, solutions for this problem based on the phenomenon of photodissociation are currently under investigation at the Los Alamos National laboratory accelerator test stand (ATS). A laser can be used to selectively neutralize portions of the beam that can be characterized after the charged particles have been swept away. We have used this technique for the measurement of current density versus longitudinal phase and the longitudinal phasespace distribution at the output of the ATS radiofrequency quadrupole (RFQ). The results of our measurement are compared with the predictions of the particle-dynamics code, PARMIEQ.

Comparison of Simulation with Experiment in an RFQ

The accelerator test stand (ATS) RFQ has provided an opportunity to compare the predictions of the RFQ beam-dynamics code PARMTEQ with actual operation of an RFQ. For this comparison, the code was adapted to simulate the measured operation parameters, which are somewhat different from those of the ideal design. A Monte Carlo code was written to provide input to PARMTEQ, based on measured input beam distributions. With these refinements, the code has given results that are in good agreement with measurements1 and has provided information leading to an explanation of an unexpected set of measurements. This paper describes the method used to generate a pseudo particle beam based on the measured transverse properties of the RFQ input beam and describes some of the comparisons between simulation and experiment. An explanation is provided for the energy-spectrum structure observed in the RFQ output beam during low-voltage operation.

Four-Dimensional Beam Tomography

A computer code has been developed to reconstruct the 4-D transverse phase-space distribution of an accelerator beam from a set of linear profiles measured at different angles at three or more stations along the beam line. The code was applied to wirescan data obtained on the low-intensity H- beam of the LAMPF accelerator. A 4-D reconstruction was obtained from 10 wire-scan profiles; 2-D projections of the reconstruction agree fairly well with slit-and-collector measurements of the horizontal and vertical emittance distributions.

Operating Characteristics of a 2.0-MeV RFQ

A second radio-frequency quadrupole (RFQ) accelerator has been designed, constructed and operated at Los Alamos National Laboratory. The accelerators design parameters represent a major extension from the original Los Alamos RFQ, with the new accelerator being 2.5 times as long, having three times the output energy, and with 2.5 times the current limit. The new accelerators operating characteristics were studied for 3 months before disassembly to incorporate design modifications. Results are discussed.

Transverse Emittance of a 2.0-MeV RFQ Beam with High Brightness

One major purpose of the accelerator test stand (ATS) is to see if particle simulation codes can correctly predict the output beam parameters of our radiofrequency quadrupole (RFQ) linac. By injecting a very bright 100-keV beam (100 mA, 0.02 ¿.cm.mrad) and comparing the measured output beam parameters with those predicted (using PARMTEQ), we can investigate PARMTEQs performance in a high space-charge regime. PARMTEQ has successfully simulated the RFQ beam transmission and energy distribution; it is more difficult to simulate output transverse emittance. Transverse emittance is also difficult to measure because of the high-power-density beam. Special modification of our interceptive slit and collector equipment was required. This paper will compare measured transverse emittances with those predicted.

Beam-dynamics design and performance of the RF deflector in the Los Alamos single-beam funnel experiment

A single-beam funnel experiment exploring the beam dynamics and physics issues of a discrete-element funnel has been completed at Los Alamos National Laboratory (LANL). The only known beam dynamics issue of concern in a two-beam funnel not addressed was the beam-beam interaction, which is negligible. The RF deflector, in which beam merging occurs, is a key component of a discrete-element funnel. The beam dynamics design procedure and experimental results for the RF deflector are reported.<<ETX>>

Unexpected matching insensitivity in DTL of GTA accelerator

The intertank matching section (IMS) of the Ground Test Accelerator (GTA) contains four variable-field quadrupoles (VFQs) and is designed to match beam exiting the radio-frequency quadrupole to the first tank of the Drift-Tube Linac (DTL-1). By varying the VFQ field strengths to create a range of beam mismatches at the entrance to DTL-1, one can test the sensitivity of the DTL-1 output beam to variations in the DTL-1 input beam. Experimental studies made during commissioning of the GTA indicate an unexpected result: the beam exiting DTL-1 shows little variation for a range of mismatches produced at the entrance. Results of the experiment and simulation studies are presented.