DOE EPSCoR Proposal
A Positron Source for JLAB
Abstract
We propose to develop a partnership between Jefferson Lab's (JLab) Continuous Electron Beam Accelerator Facility (CEBAF) in Newport News, VA and the Idaho Accelerator Center (IAC) at Idaho State University. The partnership will initially be nurtured through a research and development project designed to construct a positron source for the CEBAF. The first year of this proposal will be used to benchmark the predictions of our current simulation with positron production efficiency measurements at the IAC. The second year will use the benchmarked simulation to design a beamline configuration which optimizes positron production efficiency while minimizing radioactive waste. The second year will also be devoted to the design and construction of a positron converter capable of sustaining the heat load from high luminosity positron production. The final year will be used to quantify the performance of the positron source and measure the source's radiation footprint. A joint research and development project to construct a positron source for use by the CEBAF will bring together the experiences of both electron accelerator facilities and solidify this partnership for future projects. Our intention is use the project as a spring board towards developing a program of accelerator based research and education which will train students to meet the needs of both facilities as well as provide a pool of trained scientists for others.
Project Objectives
We propose to develop a partnership between Jefferson Lab's (JLab) Continuous Electron Beam Accelerator Facility (CEBAF) in Newport News, VA and the Idaho Accelerator Center (IAC) at Idaho State University. The partnership will initially be nurtured through a research and development project designed to construct a positron source for the CEBAF. The first year of this proposal will be used to benchmark the predictions of our current simulation~\cite{HuntPos} with positron production efficiency measurements at the IAC. The second year will use the benchmarked simulation to design a beamline configuration which optimizes positron production efficiency. The second year will also be devoted to designing a positron converter capable of sustaining the heat load from high positron luminosity production. The final year will be used to measure the capabilities of the positron source and the source's radiation footprint. A joint research and development project to construct a positron source for use by the CEBAF will bring together the experiences of both electron accelerator facilities and solidify this partnership for future projects.
One of the more common methods used to create positrons is to bombard a target of high atomic number (Z), typically Tungsten, with electrons of sufficient energy to generate a shower of secondary electrons, photons, and positrons. Electron accelerators have used this technique to produce positron beams with intensities approaching /sec~\cite{Ito_1991} at 100 MeV energies that are at least an order of magnitude larger than traditional radioactive source based beams~\cite{Radbeam}. Positron beams have also been produced at GeV beam energies with intensities of /sec for use by the high energy physics community. The current conceptual design of a positron source for use at the CEBAF would rely on the production of positrons at relatively lower energies (MeV). Positrons would be produced using electrons from the current source which have been accelerated to energies between 10 and 20 MeV. Although these low energies produce less intense positron beams our current simulation predicts that we will be able to produce positron currents beyond a nanoamp which are sufficient for our needs. This energy selection has the feature of being close to the 10 MeV neutron production threshold~\cite{n_threshold} allowing us to reduce radioactive waste.
Year 1
The main objective of this proposal in the first year will be to perform positron
production measurements at the IAC in order to benchmark our simulation~\cite{HuntPos}.
As shown in Figure~\ref{fig:PosYieldBrightness-vs-Wthickness},
the simulation predicts an almost linear increase in positron
production efficiency, , as the tungsten converter target thickness
increases when using 10 MeV electrons.
In order to create a beam of positrons for acceleration in the CEBAF, one must also consider
the trajectories of the positrons from the converter.
The emittance () is defined as
where
$x$ is distance of the positron from the beam center along the horizon as it leaves the Tungsten converter
and
$x^\prime$ is the angle of the positrons momentum with respect to the beam axis
given by $\arctan(\frac{p_x}{p_z})$, an example is shown in Figure~\ref{fig:PosYieldBrightness-vs-Wthickness}.
Defined in this manner, the emittance simultaneously accounts for both the location of
emission and the direction. A small emittance will result in less transport loss of
the positrons into the accelerator.
Brightness, defined as
\begin{equation}\label{eq:brightness}
B={ N(e^+)\over \varepsilon_{x}*\varepsilon_{y} },
\end{equation}
is one of the parameters used to evaluate the optimal target thickness by simultaneously
taking into account positron production and emittance. The variable N(e$^+$) represents
the number of positrons from the converter and $\varepsilon_{y}$ represents the emittance in the
vertical beam direction.
The simulation predicts, see Figure~\ref{fig:PosYieldBrightness-vs-Wthickness},
a plateau in the Brightness when the Tungsten target thickness is 0.5 mm.
The simulation also showed this thickness to be optimal in terms of the
positron production efficiency per Watt of deposited energy when using
10 MeV incident electrons. Our objective in year one will be to evaluate
the veracity of the above predictions.
The transport of positrons from the Tungsten converter to the accelerator is envisioned as a three quadrapole system based on the work of Reference~\cite{Sarma2003}. Figure~\ref{fig:QuadPosTransSystem} shows the current conceptual design and the positron trajectories predicted by our simulation of the system. The quadrapole triplet system will select a positron momentum band as well as divert electrons and positrons which are out of the accelerator acceptance. Figure~\ref{fig:PosMomentum} shows the dependence of the positron momentum with positron efficiency as predicted by the simulation. The simulation predicts that a 0.5 mm thick Tungsten target placed in front of a 10 MeV electron beam will produce the most positrons in a momentum band between 3 and 5 MeV. Based on this prediction, the transport system has been designed to deliver positrons with a momentum distribution shown in Figure~\ref{fig:PosMomentum}.
The goal in year 1 will be to confirm these predictions with measurements at the IAC using different Tungsten target thicknesses. A Faraday cup will need to be purchased and installed early in year 1 for these measurements. The magnet system will be provided by our DOE lab partner, the CEBAF, and the remaining beamline components will be provided by the IAC. The IAC has several HP-Ge detectors to measure the energy spectrum at low intensities as well as neutron sensitive scintillators (BC420), and position sensitive ionization chambers to measure attributes of the positrons exiting the converter at higher intensities and perform a preliminary evaluation of the source's radiation footprint.
\begin{figure}[htbp] \centerline{ \scalebox{0.25} [0.4]{\rotatebox{0}{\includegraphics{Graphs/Brightness-vs-Momentum_W0.5.eps}}} %\scalebox{0.15} [0.32]{\rotatebox{0}{\includegraphics{Graphs/PosMom_AfterTarget.eps}}} \scalebox{0.15} [0.28]{\rotatebox{0}{\includegraphics{Graphs/PosMom_AfterColl.eps}}} } \caption{The above figures are predictions from our current simulation. The left figure illustrates the positron production Brightness as a function of the positron's momentum leaving a 0.5 mm thick target. The right image shows the positron momentum distribution leaving the quad tripled transport system shown in Figure~\ref{fig:QuadPosTransSystem}.
Year 2
The second year will use the benchmarked simulation to design an optimal beamline configuration, perform heat load studies at the IAC, and construct a positron converter target which can sustain a high heat load. We expect at least another iteration of measurements using the IAC in order to benchmark the simulation during the first part of year 2. During those measurements we will also perform heat load studies by spanning the accelerator's instantaneous current range from dark current (nA) to 100 mA and increasing the energy from 5 MeV to beyond 25 MeV. Table~\ref{tabl:HeatLoad} is the simulation's prediction for the heat load on each transport element when using a 0.5mm Tungsten target and a 10 MeV electron beam to generate a 20 nA beam of positrons. The final checks between the simulation and data are expected to be completed midway through year two as well as the heat load measurements. A chamber for the Tungsten converter will be designed and constructed which will also house the brushless motor used to rotate the target. A conceptual design of the system's general features is shown in Figure~\ref{fig:CoolingSystem}. A quote for the typical cost of a brushless motor to rotate the converter was used to determine the budget allocation of \$15,000 for the motor.
\begin{figure}[htbp] \centerline{ \scalebox{0.3} [0.2]{\rotatebox{0}{\includegraphics{Graphs/cooling_system.eps}}} } \caption{Conceptual cooling system. Electrons from the CEBAF's present source would impinge a Tungsten converter disk which is rotating to improve its heat load capacity. Water cooled copper plates are heat sinked to the converter and triplet magnet system. The 3 MeV positrons are then injected into the CEBAF. } \label{fig:CoolingSystem} \end{figure}
\begin{tabular}{|c|l|}\hline \multicolumn{2}{|c|}{ Heat Load per Transport System Element}\\ \hline Element & Power (kW)\\ \hline \hline Tungsten Target & 22 \\ First Quad Magnet & 25 \\ Second Quad Magnet & 15 \\ Third Quad Magnet & 5 \\ Collimator & 28 \\ \hline \label{tabl:HeatLoad} \end{tabular}
- Measure production efficiency for different Tungsten converter target thicknesses (100 microns -> 1mm in 100 micron steps)
- Measure positron emmitance and compare to simulation predictions
- Measure brightness as a function of quad setting (quads change emmitance)
- measure positron output as a function of beam current
- Begin designing tungsten converter capable of handling high heat load
- CEBAF can accept particles into acceleration stage as long as
- Electrons are injected at 500 MeV (1 GeV) for a 6 GeV (12 GeV) CEBAF. Currently
Year 3
The final year will be used to quantify the performance of the positron source, measure the source's radiation footprint, and hold a workshop to review the positron source's performance in order to determine a path to implementation at the CEBAF. The first half of the year will be devoted to measuring the positron production rates and emittance at the IAC with in house detectors. The Faraday cup installed in year one will be used to measure the positron current. We expect to only spend a small amount of the beam time match provided by the IAC on this measurement since it was the focus of measurements made during years 1 and 2. The remaining beam time will be spent measuring the amount of extraneous radiation which will not be injected into the CEBAF in order to determine the environmental impact of using the source at JLab. The positron emittance will be measured using two ionization chambers equipped with Gas Electron Amplifiers~\cite{Sauli} that enable spatial resolutions of least 100 $\mu$m~\cite{GEM}. The radiation footprint of the source will be measured using the above ionization chambers for charged particles, a HP-Ge detector for photons, and a BC420 scintillator for neutrons. A workshop will be organized at the end of this project which will bring together CEBAF personnel, IAC personnel, and experimentalists interested in using this positron source. One of the main goals of this workshop will be to present the positron source design for review and formulate a path forward for its implementation in the CEBAF.
- Conceptual Positron source design level 1 (CD1)
- IAC beam is x 40 less power than what will be needed at JLAB
Bibliography
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Parts list
Latex File
The LaTex file for this proposal is under Media:Positrons.txt