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This paper examines the reliability of a widely used method for temperature determination by multi-wavelength pyrometry. In recent warm dense matter experiments with ion-beam heated metal foils, we found that the statistical quality of the fit to the measured data is not necessarily a measure of the accuracy of the inferred temperature. We found a specific example where a second-best fit leads to a more realistic temperature value. The physics issue is the wavelength-dependent emissivity of the hot surface. We discuss improvements of the multi-frequency pyrometry technique, which will give a more reliable determination of the temperature from emission data.
This paper investigates prospects of utilizing a high-power laser-driven target-normal-sheath-acceleration proton beam for the experimental demonstration of the magnetic self-focusing phenomenon in charged particle beams. In the proposed concept, focusing is achieved by propagating a space-charge dominated ion beam through a stack of thin conducting and grounded foils separated by vacuum gaps. As the beam travels through the system, image charges build up at the foils and generate electric field that counteracts the beam's electrostatic self-field — a dominant force responsible for expansion of a high current beam. Once the electrostatic self-field is “neutralized” by the image charges, the beam currents magnetic self-field will do the focusing. The focal spot size and focal length depends on the choice of a number of foils and distance between foils. Considering the typical electrical current level of a target-normal-sheath-acceleration proton beam, we conclude that it is feasible to focus or collimate a beam within tens of millimeters distance, e.g., using 200–1000 Al foils, 0.5 µm thick each, with foil spacing ranging from 25 µm to 100 µm. These requirements are within technical capabilities of modern target fabrication, thus allowing the first possible demonstration of the pinch effect with heavy ion beams.
We describe a funnel cone for concentrating an ion beam on a target. The cone utilizes the reflection characteristic of ion beams on solid walls to focus the incident beam and increase beam intensity on target. The cone has been modeled with the TRIM code. A prototype has been tested and installed for use in the 350-keV K+ NDCX target chamber.
We describe the next set of experiments proposed in the U.S. Heavy
Ion Fusion Virtual National Laboratory, the so-called Integrated Beam
Experiment (IBX). The purpose of IBX is to investigate in an integrated
manner the processes and manipulations necessary for a heavy ion fusion
induction accelerator. The IBX experiment will demonstrate injection,
acceleration, compression, bending, and final focus of a heavy ion beam
at significant line charge density. Preliminary conceptual designs are
presented and issues and trade-offs are discussed. Plans are also
described for the step after IBX, the Integrated Research Experiment
(IRE), which will carry out significant target experiments.
For the High Current Beam Transport Experiment (HCX) at Lawrence
Berkeley National Laboratory, an injector is required to deliver
up to 1.8 MV of 0.6 A K+ beam with an emittance of
≈1 π-mm-mrad. We have successfully operated a 10-cm-diameter
surface ionization source together with an electrostatic quadrupole
(ESQ) accelerator to meet these requirements. The pulse length
is ≈4 μs, firing at once every 10–15 s. By optimizing
the extraction diode and the ESQ voltages, we have obtained
an output beam with good current density uniformity, except
for a small increase near the beam edge. Characterization of
the beam emerging from the injector included measurements of
the intensity profile, beam imaging, and transverse phase space.
These data along with comparison to computer simulations provide
the knowledge base for designing and understanding future HCX
The High Current Experiment (HCX) is being assembled at Lawrence
Berkeley National Laboratory as part of the U.S. program to
explore heavy ion beam transport at a scale representative of
the low-energy end of an induction linac driver for fusion energy
production. The primary mission of this experiment is to
investigate aperture fill factors acceptable for the transport
of space-charge dominated heavy ion beams at high space-charge
intensity (line-charge density ∼ 0.2 μC/m) over
long pulse durations (>4 μs). This machine will test
transport issues at a driver-relevant scale resulting from
nonlinear space-charge effects and collective modes, beam centroid
alignment and beam steering, matching, image charges, halo,
lost-particle induced electron effects, and longitudinal bunch
control. We present the first experimental results carried out
with the coasting K+ ion beam transported through
the first 10 electrostatic transport quadrupoles and associated
diagnostics. Later phases of the experiment will include more
electrostatic lattice periods to allow more sensitive tests
of emittance growth, and also magnetic quadrupoles to explore
similar issues in magnetic channels with a full driver scale
For the intense beams in heavy ion fusion accelerators, details
of the beam distribution as it emerges from the source region
can determine the beam behavior well downstream. This occurs
because collective space-charge modes excited as the beam is
born remain undamped for many focusing periods. Traditional
studies of the source region in particle beam systems have
emphasized the behavior of averaged beam characteristics, such
as total current, rms beam size, or emittance, rather than the
details of the full beam distribution function that are necessary
to predict the excitation of the collective modes. Simulations
of the beam in the source region and comparisons to experimental
measurements at Lawrence Berkeley National Laboratory and the
University of Maryland are presented to illustrate some of the
complexity in beam characteristics that has been uncovered as
increased attention has been devoted to developing a detailed
understanding of the source region. Also discussed are methods
of using the simulations to infer characteristics of the beam
distribution that can be difficult to measure directly.
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