Joshua Eugene Coleman

Temporal response of a surface flashover on a velvet cathode in a relativistic diode

Surface flashover of a carbon fiber velvet cathode generates a discharge from which electrons are relativistically accelerated to γ ranging from 4.9 to 8.8 through a 17.8 cm diode. This discharge is assumed to be a hydrocarbon mixture. The principal objective of these experiments is to quantify the dynamics over the ∼100 ns pulse of the plasma discharge generated on the surface of the velvet cathode and across the anode-cathode (A-K) gap. A qualitative comparison of calculated and measured results is presented, which includes time resolved measurements with a photomultiplier tube and charge-coupled device images. In addition, initial visible spectroscopy measurements will also be presented confirming the ion species are dominated by hydrogen.

Spatially and temporally resolved measurements of a dense copper plasma heated by intense relativistic electrons

A 100-μm-thick Cu foil is isochorically heated by a ∼100-ns-long electron bunch with an energy of 19.8 MeV and current of 1.7 kA to Te > 1 eV. After 100 ns of heating and 20 ns of expansion, the plasma exhibits a stable, quiescent temperature and density distribution for >200 ns. Several intense Cu-I emission lines are observed after ∼20 J of electron beam energy is deposited. These lines have well known Stark widths providing a direct measurement of ne. The Los Alamos ATOMIC code [Magee et al., AIP Conf. Proc. 2004, 168–179 and Hakel et al., J. Quant. Spectrosc. Radiat. Transfer 99, 265 (2006)] was run in local-thermodynamic-equilibrium mode to estimate Te and ne. Spatially and temporally resolved measurements are presented in both the vertical and horizontal directions adjacent to the foil indicating temperatures >1 eV and densities ranging from 1–3 × 1017 cm−3 after expansion and cooling.

A spectral pyrometer to spatially resolve the blackbody temperature of a warm dense plasma

A pyrometer has been developed to spatially resolve the blackbody temperature of a radiatively cooling warm dense plasma. The pyrometer is composed of a lens coupled fiber array, Czerny-Turner visible spectrometer, and an intensified gated CCD for the detector. The radiatively cooling warm dense plasma is generated by a ∼100-ns-long intense relativistic electron bunch with an energy of 19.1 MeV and a current of 0.2 kA interacting with 100-μm-thick low-Z foils. The continuum spectrum is measured over 250 nm with a low groove density grating. These plasmas emit visible light or blackbody radiation on relatively long time scales (∼0.1 to 100 μs). The diagnostic layout, calibration, and proof-of-principle measurement of a radiatively cooling aluminum plasma is presented, which includes a spatially resolved temperature gradient and the ability to temporally resolve it also.

Warm dense matter experiments on DARHT

Summary form only given. An experimental facility has been developed to diagnose the properties of the backstreaming plasma and WDM produced due to interactions of monochromatic electrons with thin foils. A slow moving (<; 0.5 mm/μs) low temperature discharge (T <; 0.2 eV) is generated by a ~100 ns long intense relativistic electron bunch with energies of 19.1-19.8 MeV and currents of 0.2-1.7 kA interacting with 100 μm thick low-Z foils. The principal objective of these experiments is to quantify the dynamics of the different phases of plasma, including WDM, produced due to the vaporization of the foils and the evolution of the beam distribution due to space charge neutralization effects. The preliminary suite of diagnostics is presented, which includes time resolved plasma plume measurements, visible spectroscopy measurements indicating the evolution of the blackbody temperature, time resolved measurements of the beam spot size, and plans for the future.

Beam Breakup in an Advanced Linear Induction Accelerator

Two linear induction accelerators (LIAs) have been in operation for a number of years at the Los Alamos Dual Axis Radiographic Hydrodynamic Test (DARHT) facility. A new multipulse LIA is being developed. We have computationally investigated the beam breakup (BBU) instability in this advanced LIA. In particular, we have explored the consequences of the choice of beam injector energy and the grouping of LIA cells. We find that within the limited range of options presently under consideration for the LIA architecture, there is little adverse effect on the BBU growth. The computational tool that we used for this investigation was the beam dynamics code linear accelerator model for DARHT (LAMDA). To confirm that LAMDA was appropriate for this task, we first validated it through comparisons with the experimental BBU data acquired on the DARHT accelerators.

An optical scattering experiment to measure the timing of x-ray converter target debris in the DARHT II accelerator

The DARHT II accelerator utilizes a fast closing vacuum valve to block x-ray converter target debris from entering the accelerator. An optical scattering diagnostic was developed to measure the arrival time of the debris at the fast valve to verify that the valve closure time is adequate to block the debris.

Characterizing the photon spectrum generated in the DARHT AXIS-I diode

Summary form only given. An array of photon diagnostics have been deployed on a high power relativistic electron beam diode. Surface flashover of a carbon fiber velvet cathode generates a discharge from which electrons are accelerated through a 17.8 cm diode. This discharge is assumed to be a hydrocarbon mixture. A small portion of the off energy electrons accelerated during the rise and fall of the 80 ns FWHM pulse impact the beam pipe and generate a flux of 0.1-4 MeV photons. The principal objective of these experiments is to quantify the dynamics of the hard X-ray and gamma-ray flux generated in the diode region, in addition to quantifying dynamics of the visible photons generated, and the ion species of the surface discharge on the velvet cathode. A qualitative comparison of different diagnostic results are presented, which include time resolved measurements with X-ray PIN diodes, a PMT, and CCD images. In addition initial visible spectroscopy measurements will also be presented.

Characterizing the photon spectrum in the DARHT Axis-I diode

An array of photon diagnostics have been deployed on a high power relativistic electron beam diode. Surface flashover of a carbon fiber velvet cathode generates a discharge from which electrons are accelerated through a 17.8 cm diode. This discharge is assumed to be a hydrocarbon mixture. A small portion of the off energy electrons (1-3 MeV) accelerated during the rise and fall of the 80 ns FWHM pulse impact the beam pipe and generate a Bremsstrahlung spectrum of 0.1-3 MeV photons. The principal objective of these experiments is to quantify the dynamics of the hard X-ray and γ-ray flux generated in the diode region, in addition to quantifying dynamics of the visible photons generated, and the ion species of the surface discharge on the velvet cathode. A qualitative comparison of different diagnostic results are presented, which include time resolved measurements with X-ray PIN diodes, a PMT, and CCD images. In addition initial visible spectroscopy measurements will also be presented.

PPPS-2013: Explosive emission and gap closure from a relativistic electron beam diode

Explosive emission cathode studies are currently being conducted on a high power relativistic electron beam diode. A black velvet cathode is driven with a 75 Ohm graded transmission line, which provides a 3.8 MV, 80 ns (FWHM) pulse across a fixed A-K gap of 17.8 cm. Cathode sizes range from 1.9-7 cm with space-charge limited currents of 0.26-3 kA. The principal objective of these experiments is to quantify the current emission limits and the dynamics of the gap closure process. A qualitative comparison of experimental and calculated results are presented, which included the I-V relationship, impedance and perveance curves, plasma expansion velocity, and the time-resolved light emission on the surface of the cathode.

PPPS-2013: Design of a paraxial inverse Compton scattering diagnostic for an intense relativistic electron beam

An inverse Compton scattering diagnostic is currently being developed for a 60 ns, intense relativistic electron bunch with an energy of 19.8 MeV and nominal current of 1.7 kA. The principal purpose of this diagnostic is to provide a measurement of the 6-D phase space distribution of the electron beam in a single shot without disrupting its axial propagation. The electron beam is intercepted by a 450 mJ of green light, which is upscattered into the soft X-ray range by the relativistic electrons. The diverging, scattered photons are diffracted onto an X-ray framing camera by an X-ray crystal concentric to the beam pipe utilizing an elongated von Hamos geometry1. The experimental configuration is presented, which includes the electron and photon interaction dynamics, crystal design, and the expected time resolved longitudinal and transverse distributions.An inverse Compton scattering diagnostic is currently being developed for an 80 ns, intense relativistic electron bunch with an energy of 19.8 MeV and nominal current of 1.7 kA. The principal purpose of this diagnostic is to provide a measurement of the 6-D phase space distribution of the electron beam in a single shot without disrupting its axial propagation. The electron beam is intercepted by 450 mJ of green light, which is upscattered into the soft X-ray range by the relativistic electrons. The diverging, scattered photons are diffracted onto an X-ray framing camera by an X-ray crystal concentric to the beam pipe utilizing an elongated von Hamos geometry [1]. The experimental configuration is presented, which includes the electron and photon interaction dynamics, crystal design, X-ray framing camera design, and the expected time resolved longitudinal and transverse distributions.