Graham Cooper

Paraxial gas-cell focusing of relativistic electron beams for radiography

A description of the underlying physics governing the operation of pulsed-power driven, gas-filled, paraxial diodes is presented. Gas-filled focusing cells are routinely used to transport high energy density electron beams for use in flash X-ray radiography experiments. The paraxial diode acts as a 1/4 betatron focusing element with a focal length F proportional to the square root of the beam energy and inversely proportional to the square root of the net current in the gas cell F/spl prop/(/spl gamma//I/sub net/)/sup 1/2/. Particle-in-cell simulations demonstrate that the time integrated radiation spot is determined both by focal sweeping due to time varying net currents and by finite beam emittance. The calculated radiation focal plane, spot, and dose are compared to data obtained by the Atomic Weapons Establishment, U.K., from a variety of experimental configurations and demonstrate good agreement between simulation and experiment. Suggestions to improve the focal properties of the diode are presented.

Comprehensive description of the Orion laser facility

The Orion laser facility at the atomic weapons establishment (AWE) in the UK has been operational since April 2013, fielding experiments that require both its long and short pulse capability. This paper provides a full description of the facility in terms of laser performance, target systems and diagnostics currently available. Inevitably, this is a snapshot of current capability—the available diagnostics and the laser capability are evolving continuously. The laser systems consist of ten beams, optimised around 1 ns pulse duration, which each provide a nominal 500 J at a wavelength of 351 nm. There are also two short pulse beams, which each provide 500 J in 0.5 ps at 1054 nm. There are options for frequency doubling one short pulse beam to enhance the pulse temporal contrast. More recently, further contrast enhancement, based on optical parametric amplification (OPA) in the front end with a pump pulse duration of a few ps, has been installed. An extensive suite of diagnostics are available for users, probing the optical emission, x-rays and particles produced in laser-target interactions. Optical probe diagnostics are also available. A description of the diagnostics is provided.

Observations of Light Emission From the Surface of the Paraxial Diode Cathode Fielded on the Superswarf 5-MV Pulsed Power Accelerator

Light from the region around a cathode in a radiographic diode, used for the generation of X-rays, was imaged using a gated camera array. Optical images of the resulting plasma are presented in this paper.

Remarx of the mevex X-ray machine

A replacement Marx generator for one of AWEs flash radiographic machines is required to mitigate component obsolescence. The design uses twelve 250nF capacitors to charge a 14nF Blumlein to 700kV. For ease of maintenance the generator is removable from the oil tank by use of a rail system. Electrostatic codes have been used to design a new earthing system with access to the hidden capacitor terminals. The paper describes the design and the results of the Marx construction and high voltage commissioning into a resistive load. This paper builds upon design work presented at the IEEE UK Pulsed Power symposium 2011, to include results of construction and testing.

20 TORR Gas Cell Focussing in the Paraxial Diode.

Summary form only given. The paraxial diode is routinely used to convert high intensity electron beams to bremsstrahlung X-radiation for use in experiments requiring flash radiography at AWE (Aldermaston). The paraxial diode is one of the diodes being considered for use on an inductive voltage adder (IVA) driver in the proposed core punch facility (CPF). The operation of the diode using a 20 torr air-fill in the gas cell as opposed to the traditionally used value of around 1 torr air will be presented. At present the performance of the diode is believed to be limited by significant focal sweeping which causes the radiographic spot to increase with time. It is anticipated that at the higher air pressures the magnetic diffusion timescale in the gas cell will be lower than the pulse length of the beam. Under these conditions the plasma return current will quickly diffuse away because of the high plasma resistivity. This causes the net current in the gas to quickly become equal to the beam current. It will be shown using particle in cell (PIC) modeling that under these conditions focal sweeping can be reduced which in turn results in smaller radiographic spot sizes. A comparison between the experiments carried out on Mogul D and the PIC models will be presented and the differences highlighted

Emittance and its Role on the Radiographic Spot in a Paraxial Diode

Summary form only given. The paraxial diode is one of several candidate diodes being investigated to meet the X-radiography requirements of the CPF at AWE. The long term requirement is to produce a 2 mm X-ray source with a dose of 1000 rads at 1 metre. The paraxial diode currently produces around 500 R @ 1 m in a 5 mm spot. To help meet the long term requirement stated above the paraxial diode is currently being studied in detail with the aid of the large-scale-plasma PIC code Lsp in order to see if improvements can be made to spot and dose. It will be shown how the concept of emittance can be used as a powerful tool in understanding the behavior of the beam as it propagates in the diode. Emittance results in degradation of an electrons trajectory, which ultimately leads to an increased spot on the high Z, target. Previous work suggests that focal sweeping, because of time-varying net currents, is the main contributor. However, it will be demonstrated with the aid of Lsp simulations that emittance growth from non-linear focussing and foil scatter will set fundamental limits on the spot size achievable (typically 5 mm when operating at 1.1 torr air in the drift cell). Suggestions to reduce emittance and spot are presented by using a high pressure air fill ~20 torr inside the drift cell

Improved Experimental Techniques to Determine the X-Ray Source Intensity Distribution Produced from Intense Electron Beam Diodes

Summary form only given. The experimental measurement of the source distribution of flash X-rays produced by intense electron beam diodes for use in deep penetration flash radiography is important. Data on the intensity distribution of the X-ray source is required, both for the development of improved electron beam diodes, and also in order to provide more quantitative information for iterative reconstruction methods used to interrogate the data from explosively driven hydrodynamic experiments which use flash transmission radiography as a key diagnostic. A description of the current methods by which edge spread distributions of the source can be inferred from what is known as a knife edge measurement is given. Potential sources of error in the edge spread distribution derived from this measurement are described. These sources of error include effects due to photon scattering in knife-edge experimental geometries, detector resolution effects and the process by which knife-edge experimental data is processed. Guidelines in the provision of improved knife-edge measurements are given. These guidelines include quantification of how scattering effects degrade edge-spread determination in knife edge experiments, how detector blur considerations can be used to specify minimum radiographic magnifications for these experiments and how improved analysis techniques applied to knife edge transmission radiographs can minimise the errors introduced in the quantification of the source intensity distribution

The potential of the paraxial diode as a focussing element for X-radiography

Summary form only given. The paraxial diode is one of several candidate diodes being investigated to meet the X-radiography requirements of the HRF at AWE. The short term requirement is to produce a 5 mm X-ray source with a dose of 600 Rads at 1 metre. To help meet this requirement the paraxial diode is currently being studied in detail with the aid of the pic code Lsp. The paraxial diode is a gas filled focussing cell which transports and focusses a relativistic electron beam to a high atomic target producing bremsstrahlung radiation. The diode typically generates currents of the order of 30 kA at voltages up to 10 MeV. The diode acts as a 1/4 betatron focussing element with a focal length F proportional to the square root of the beam energy and inversely proportional to the square root of the net current in the gas cell F /spl prop/ (/spl gamma//I/sub net/)/sup 1/2/. The time integrated radiation spot is determined both by finite beam emittance and by focal sweeping due to variations in net current. The Lsp code was used to calculate the electron beam time integrated spot, dose, mean angle, focal plane and emittance. Lsp calculations for the spot, dose and focal plane compare well with experimental data obtained from a variety of diodes fielded at the Atomic Weapons Establishment and from diodes fielded on the RITS3 accelerator at Sandia National Laboratories. Suggestions to improve the X-ray intensity are presented.

Recent X-ray diode and magnetically insulated transmission line experiments performed on the 5.5 MV Superswarf machines at AWE Aldermaston

The Pulsed Power Group at AWE is interested in the generation of small, intense, multi-megavolt X-ray sources for radiographic applications. Two of them, the Superswarfs, are maintained as facility machines and used by other groups for radiographic experiments, but upon which we are permitted to conduct diode research experiments. These machines have been using a paraxial diode for a number of years. This is a passive method of focussing the electron beam to provide a small X-ray source. Within the last few years this has been enhanced to provide a smaller source, but with a slight loss in dose. The magnetically immersed diode is being investigated as an alternative method to focussing the electron beam onto a tantalum target. The external magnetic field, up to 25 T, is produced by a solenoid which is driven by a 418 /spl mu/F, 22 kV capacitor bank. The best Bz shot to date has produced 82 R @ 1 m with a 4.2 mm diameter spot size.