Diamagnetic loops (DML) provide a non-invasive method for measurement of beam size in electron beam accelerators that use solenoidal magnetic transport. The loop fundamentally measures the magnetic flux excluded by a diamagnetic object. A comprehensive theory relates the rms beam radius to the excluded flux measured by the DML. We have built and calibrated a DML apparatus. Recently, this DML has been used to measure the size of the electron beam near the final focus of the DARHT-I flash-radiography accelerator. Results agree with beam transport code predictions. In this article, we review and summarize the construction, calibration, and electron-beam testing of this DML.

Our non-invasive, time-resolved diamagnetic loop measurements of beam size during the electron current-pulse flattop at the exit of our 20-MeV linear induction accelerator revealed that the beam size varied by about 13% during the current flattop. This was an unexpected result, since both current and energy were constant during the interval of radius variation. One possible cause, poor vacuum, was investigated using a time resolved envelope equation. It was found that sufficiently high residual-gas pressure in the downstream transport region could result in the observed variation.

are compared with normal, full machine operation. A prescription for alternate operation with missing or reduced charge voltage on BCU22 is given.

Diamagnetic loops (DML) can be used as a noninvasive method for measurements of beam size in electron beam accelerators that use solenoidal magnetic transport. The loop fundamentally measures the magnetic flux excluded by a diamagnetic object. A comprehensive theory relates the rms beam radius to the excluded flux measured by the DML. We have built, and calibrated a DML apparatus. Recently, this DML has been used to measure the size of the electron beam near the final focus of the DARHT-I flash-radiography accelerator. Results are in agreement with beam transport code predictions. In this article, we review and summarize the construction, calibration, and electron-beam testing of this DML.

laser light in response to the voltage induced on a pickup electrode. Fiber coupling between the light source, Pockels cell and receiver ensures complete galvanic isolation with improved cost and performance as compared to conventional sensors fitted with fiber optic links. Furthermore, the EO approach requires that only the passive sensor element be located near the accelerator while the light source and receiver can be installed in remote locations. This paper describes the design and development of EO sensors for electron beam and pulsed power monitoring on the second axis of DARHT. Typical calibration and testing data for the sensors is also presented.

In this work, we present a new analysis method applied to revitalize permanent magnet Compton spectrometers used to measure photon energy spectra in the MeV range. The inversion of the measured electron distribution to determine the original photon distribution is achieved via a method of consistent coupled radiation transport and magnetic field mapping of the input photon spectra to the measured electron distribution. The method of linear least squares was used to perform the unfolding of the electron distribution to the initial photon spectra, without any assumptions made regarding the electron distribution. We present an application of this method to data from a nominal 19.4 MeV flash radiographic source (the first axis of the Dual Axis Radiographic Hydro-Test Facility) capable of generating 500 R @ 1 m in ∼60 ns and a medical therapy source (a Scanditronix M22, Microtron) capable of variable energies with nominal endpoints of 6, 10, 15, and 20 MeV and an output of ∼1000-2000 R/min @ 1 m. The results provide agreement between the modeled and unfolded experimentally measured photon spectra as quantified by statistical tests, from 1.5 to 20 MeV. Experimental results are presented as well as a discussion of the novel MCNP6-based simulations and methods for reconstruction of the spectra.

The Dual-Axis Radiographic Hydrodynamic Test (DARHT) facility provides flash radiography capabilities using two electron Linear Induction Accelerators (LIA’s). Axis-1 of DARHT produces a 20- MeV, 2-kA, 80-ns-FWHM electron beam. The strict requirements for flash radiography require a detailed understanding of the LIA’s performance, including precision measurements of the injector electron beam energy. The technique for time resolved measurement of the electron beam energy using electron Permanent Magnet Spectrometer (ePMSpec) has been developed. The electron energy then is used to infer the voltage produced by the injector pulsed power. The injector pulsed power of DARHT I consists of a 1.5-MV, glycol-insulated Blumlein that is pulsecharged by a step-up transformer and switched by four, laser-triggered spark gaps. A series of increasing impedance transmission lines are used to transform the output voltage of the Blumlein to a maximum of 3.8MV at the diode. Figure 1a shows a graphic of the Axis-1 prime power tank, Blumlein and downstream transmission lines. The injector pulse power is designed to produce voltage on a velvet cathode located in the vacuum vessel. The specifications of the voltage-pulse flat-top over 60ns is +/-1%. A key element in the high-voltage circuit is the ethylene glycol Blumlein. The outer volume, adjacent to the laser triggered switches of the Blumlein, is a peaking capacitor. This is an independent volume that contains a mixture of ethylene glycol and water to make it an adjustable capacitor of 380 to 750 pF.

The Dual-Axis Radiographic Hydrodynamic Test (DARHT) facility provides flash radiography capabilities using electron Linear Induction Accelerators (LIA’s). The strict requirements for flash radiography require a detailed understanding of the LIA’s performance, including precision measurements of the injected electron beam energy. The DARHT Axis 1 injector of produces a 3-4 MeV, 1-2 kA, 80-ns-FWHM electron beam. Injector capacitive monitors (EVACSUM) are summed to give the injector beam energy. Calibration of EVACSUM was last done in 1999 and is needed. In addition, the flatness of the injector drive voltage is controlled by a peaking capacitor in a Blumlien that is charged by the prime power tank. Time resolved measurements are used to optimize the value of this capacitor.

The Compact Electron Permanent Magnet Spectrometer (CEPMS) shown in Figure 2 consists of collimation at the electron beam entrance, a vacuum chamber for beam transport and a removable 60 degree sector magnet constructed by SABR Enterprises (Ref 2) that is used to momentum analyze the incoming electron beam. There are electron detection planes located perpendicular to straight through and the 60 degree trajectory ports. The planes are designed to accept phosphor screens for time integrated measurements. The screens can be replaced by scintillator for time resolved measurements using gated or streak cameras. The length of the spectrometer along the beamline without the collimator is about 400mm (14”). The spectrometer requires no power supply with controls for cycling or cooling for the magnet. The entire assembly, with magnet installed, weighs about 35kg (75lbs) Figure 3 depicts central and extreme trajectories through the spectrometer for various electron energies. The chamber accommodates a +/-50mm wide detection field relative to the center trajectory. Trajectories with larger momentum (energy) are bent the least. Since the permanent magnet slides off and on with reference to the upstream edge of the vacuum chamber, it can be adjusted to various positions to slightly increase or decrease the central energy thereby changing the range of available energies. Figure 4 shows the effect of changing the location of the permanent magnet on the electron trajectory. A small inward change in r produces a longer trajectory in the magnet therefore, the same energy trajectory is bent more.

The Dual-Axis Radiographic Hydrodynamic Test (DARHT) Facility is a two-axis, linear induction electron accelerator designed to produce Bremsstrahlung radiation for imaging hydrodynamic physics within rapidly compressed targets. The radiation dose code DOSECALC, used to obtain approximate results for experiments, was modified to include new materials and renamed DOSECALCX. Calculations were compared with results from the MCNP code. Photon spectra was found to agree visibly and quantitatively while angular dose data was only comparable within an order of magnitude.

Our team at Los Alamos National Laboratory has successfully employed Compton spectrometers to measure the x-ray spectra of intense radiographic sources, both continuous and flash. In this method, a collimated beam of x-rays incident on a convertor foil ejects Compton electrons. A collimator may be inserted into the entrance of the spectrometer to select the angular acceptance of the forward-scattered electrons, which then enter the magnetic field region of the spectrometer. The position of the electrons at the magnet's focal plane is proportional to the square root of their momentum, allowing the x-ray spectrum to be reconstructed. Two spectrometers have been fielded since 2013; a neodymium-iron-boron permanent magnet with an energy range of 500 keV to 20 MeV, and a new samarium-cobalt magnet with an energy range of 50 keV to 4 MeV. Measured spectra were produced by x-ray generating machines of various intensities (~5 rad at 1 m per 50 ns pulse to >2000 rad/min at 1 m) and different endpoints (range of 2.25 to 20 MeV). Preliminary analysis of the electron spectra produced at two different facilities with various beryllium converter foil thicknesses is presented in these proceedings.

Accurate knowledge of the x-ray spectra used in medical treatment and radiography is important for dose calculations and material decomposition analysis. Indirect measurements via transmission through materials are possible. However, such spectra are challenging to measure directly due to the high photon fluxes. One method of direct measurement is via a Compton spectrometer (CS) method. In this approach, the x-rays are converted to a much lower flux of electrons via Compton scattering on a converter foil (typically beryllium or aluminum). The electrons are then momentum selected by bending in a magnetic field. With tight angular acceptance of electrons into the magnet of ~ 1 deg, there is a linear correlation between incident photon energy and electron position recorded on an image plate. Here we present measurements of Bremsstrahlung spectrum from a medical therapy machine, a Scanditronix M22 Microtron. Spectra with energy endpoints from 6 to 20 MeV are directly measured, using a CS with a wide energy range from 0.5 to 20 MeV. We discuss the sensitivity of the device and the effects of converter material and collimation on the accuracy of the reconstructed spectra. Approaches toward improving the sensitivity, including the use of coded apertures, and potential future applications to characterization of spectra are also discussed.