One-Body Capillary Plasma Source for Plasma Accelerator Research at e-LABs

At a Glance

Metadata	Details
Publication Date	2023-02-16
Journal	Applied Sciences
Authors	Sihyeon Lee, Seong‐Hoon Kwon, Inhyuk Nam, Myung Hoon Cho, Dogeun Jang
Institutions	Pohang Accelerator Laboratory, Gwangju Institute of Science and Technology
Citations	2
Analysis	Full AI Review Included

Executive Summary

This analysis summarizes the development and characterization of a novel, sapphire-based, one-body capillary plasma source designed for plasma accelerator research (e-LABs).

Manufacturing Innovation: A compact, leak-free, one-body sapphire capillary was successfully manufactured using diamond machining, overcoming the assembly and gas leakage issues common in traditional two-plate laser-micromachined designs.
Gas Stability Validation: Computational Fluid Dynamics (CFD) simulations and Mach-Zehnder interferometry confirmed rapid gas distribution stability, achieving maximum density within approximately 50 ms of gas injection.
LWFA Performance: The capillary was validated as a stable acceleration column in Laser Wakefield Acceleration (LWFA) experiments using a 150 TW laser system, consistently producing reproducible electron beams with energies of 200 ± 25 MeV.
Active Plasma Lens (APL) Capability: The capillary was successfully operated as a discharge plasma source using a pulsed high-voltage system, achieving a peak current of 140 A at 10 kV applied voltage.
High Focusing Gradient: The resulting discharge plasma demonstrated a high magnetic field gradient of 97 T/m, confirming its potential utility as an Active Plasma Lens for focusing relativistic electron beams.
Engineering Relevance: The design provides a robust, durable, and compact plasma source suitable for high-repetition-rate operation in advanced accelerator facilities.

Technical Specifications

Parameter	Value	Unit	Context
Capillary Material	Sapphire	N/A	One-body construction via diamond machining
Capillary Diameter	1	mm	Used in both LWFA and APL tests
Capillary Length (LWFA Test)	7	mm	Used for 200 MeV electron beam generation
Capillary Length (APL Test)	15	mm	Used for pulsed discharge plasma lens test
LWFA Laser System Peak Power	150	TW	Used at IBS, GIST facility
LWFA Laser Pulse Duration	25	fs	FWHM
LWFA Gas Used	Helium (He)	N/A	Pure gas target
LWFA Gas Pressure	150	mbar	Operating pressure
Estimated Plasma Density (n_e)	7.2 x 10¹⁸	cm^-3	Full ionization assumed
LWFA Electron Beam Energy	200 ± 25	MeV	Stable and reproducible output
Gas Density Stabilization Time	~50	ms	Time to reach maximum density (CFD result)
APL Applied Voltage	10	kV	Pulsed high-voltage system
APL Peak Discharge Current	140	A	Measured current profile
APL Focusing Gradient	97	T/m	Calculated from 140 A discharge current
APL Input Beam Energy (Simulated)	70	MeV	Based on e-LABs photocathode gun parameters
APL Input RMS Beam Size (Simulated)	179	µm	Based on e-LABs parameters
APL Focused Beam Size (Simulated)	13	µm	Maintained regardless of current (at focal distance)

Key Methodologies

The development and testing involved specialized manufacturing and diagnostic techniques:

Capillary Manufacturing:
- Material: Single block of sapphire, chosen for its robustness against high temperature and intense laser/plasma exposure.
- Process: Diamond tool-based machining (diamond drilling) was used to create the capillary hole and internal gas feedlines, ensuring high precision and a seamless, leak-free structure.
- Assembly: Oxygen-free electrolytic copper electrodes were installed at both ends for discharge applications, isolated by PEEK material bolts and holders to prevent unexpected discharges.
Gas Density Characterization:
- Technique: Mach-Zehnder interferometry was employed using a continuous wave He-Ne laser (632 nm) to measure the phase shift induced by the gas.
- Simulation: Three-dimensional (3D) Computational Fluid Dynamics (CFD) simulations (ANSYS FLUENT) were used to model the continuous gas injection and determine the position-dependent pressure distribution and effective length (L_eff).
Laser Wakefield Acceleration (LWFA) Experiment:
- Laser System: 150 TW Ti:sapphire laser (25 fs pulse duration, 800 nm wavelength, normalized vector potential a₀ ~1.6).
- Target: 7 mm sapphire capillary filled with 150 mbar of pure helium gas.
- Diagnosis: Electron energy spectra were measured using a magnetic spectrometer setup consisting of a 1 T dipole magnet and two LANEX phosphor imaging plates.
Active Plasma Lens (APL) Discharge Test:
- System: Pulsed High-Voltage (HV) discharge system utilizing a thyratron switch (MA2440B) and a 10 kV DC power supply.
- Conditions: 15 mm capillary filled with helium gas at 300 mbar.
- Analysis: Discharge current (140 A peak) was measured, and the azimuthal magnetic field (B_θ) distribution was calculated using Ampere’s law, assuming a temperature-dependent current density profile.
- Simulation: Particle-in-Cell (PIC) simulations were used with Twiss matrix calculations to model the focusing of a 70 MeV electron beam, confirming the 97 T/m gradient capability.

Commercial Applications

The stable, high-gradient focusing capability and robust material construction of this capillary plasma source are critical for several high-tech engineering fields:

Advanced Particle Accelerators: Essential component for external injection Laser Wakefield Acceleration (LWFA) systems, particularly in facilities like e-LABs, enabling the production of high-quality, stable electron beams for fundamental physics research.
Compact Light Sources: Used to drive compact X-ray and Gamma-ray sources (e.g., synchrotron radiation and betatron sources) by providing the necessary high-energy, low-emittance electron beams.
Beamline Optics: Deployment as Active Plasma Lenses (APL) in electron beam transport lines, offering azimuthally symmetric focusing with high magnetic gradients (97 T/m) and low chromatic dependence, superior to traditional permanent magnet quadrupoles in certain applications.
High-Power Laser Systems: The sapphire material provides necessary durability for components exposed to high-intensity laser pulses (10¹⁹ W/cm²) and high-voltage plasma environments, increasing the lifetime and reliability of laser-plasma interaction chambers.
Industrial Electron Beam Processing: Potential for miniaturizing high-energy electron accelerators used in industrial applications such as sterilization, material modification, and non-destructive testing.

View Original Abstract

We report on the development of a compact, gas-filled capillary plasma source for plasma accelerator applications. The one-body sapphire capillary was created through a diamond machining technique, which enabled a straightforward and efficient manufacturing process. The effectiveness of the capillary as a plasma acceleration source was investigated through laser wakefield acceleration experiments with a helium-filled gas cell, resulting in the production of stable electron beams of 200 MeV. Discharge capillary plasma was generated using a pulsed, high-voltage system for potential use as an active plasma lens. A peak current of 140 A, corresponding to a focusing gradient of 97 T/m, was observed at a voltage of 10 kV. These results demonstrate the potential utility of the developed capillary plasma source in plasma accelerator research using electron beams from a photocathode gun.

Tech Support

Original Source

DOI: https://doi.org/10.3390/app13042564

References

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