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SCHPhotocathode, schematicA photocathode releases free electrons when light strikes it, turning an optical pulse into an electron beam. This is an extremely simplified representation of the photocathode experiment described below. Drag to orbit.

Photocathode Experiment & Field Probes for Ultra-wideband Radiation

Los Alamos National Laboratory · Applied Electrodynamics

Role
Mechanical Engineering Intern
Group
Accelerator Operations & Technology — Applied Electrodynamics
Location
Los Alamos, NM
Dates
May – December 2025

During my summer at Los Alamos National Laboratory, I designed and fabricated a novel ultra-high-vacuum photocathode test system, including custom vacuum, high-voltage, and radiation-shielding hardware. Developed, calibrated, and tested ultra-wideband electromagnetic field probes using waveguides, VNA measurements, MATLAB signal processing, and CST simulations. Designed and fabricated a MACOR ceramic photocathode assembly, balancing vacuum compatibility, dielectric performance, manufacturability, and high-voltage electrostatic loading.

This page focuses on the development of the field probes, which is the portion of the work covered by the public-release poster.

Background

The work required accurate characterization of ultrawideband electric and magnetic fields. A photocathode converts incident light into electrons through the photoelectric effect, providing a controllable source of electrons. Accurate measurement of the resulting electromagnetic fields is critical for understanding wave propagation and validating measurement performance.

D-dot probes (dipoles) measure electric fields with capacitance and B-dot probes (loops) measure magnetic fields with inductance. The probes were designed for ultrawideband measurements and calibrated across multiple frequency ranges using waveguides, with the resulting signals processed after the fact. This work provided the experimental infrastructure needed to characterize ultrawideband fields in the laboratory.

Probe design and fabrication

These probes were modeled first before being fabricated from COTS parts. 3D modeling of the experimental ultra-high vacuum chamber assembly and the RF evaluation chamber lent to the design of the overall measurement system. The probe designs were developed to integrate with the experimental hardware while maintaining the required electrical, mechanical, and vacuum constraints.

Sectioned CAD view of a rectangular waveguide with a D-dot monopole probe entering through the broad wall on an SMA flange and a B-dot loop probe mounted downstream.
01Calibration fixture, sectionedI sectioned the model to settle probe length. Deeper gives more sensitivity and more reflection, so the depth is a performance trade off.
Two fabricated probes on a grey bench. Left: an SMA flange connector with a soldered semi-rigid copper loop forming a closed circuit. Right: an SMA flange connector with a straight conductor projecting from its face.
02Fabricated B-dot and D-dot probesThe loop senses dB/dt inductively, the monopole dE/dt capacitively. Both are built on stock SMA panel-mount flanges, so the exposed conductor is the only geometry that changes between builds.

Calibration in waveguide

Each probe was calibrated by placing it within a waveguide and measuring its frequency response using a microwave Vector Network Analyzer (VNA). S-parameter measurements were collected across the operational frequency range of each waveguide, with 10 averaged sweeps and smoothing applied to reduce measurement noise. The measured response was analyzed in MATLAB using the RF Toolbox and fit with a rational function using a selected number of poles, achieving a fitting error below −40 dB. The resulting transfer function characterized the probe’s frequency-dependent response and provided the calibration needed to reconstruct the electric or magnetic field from measured signals.

Calibration bench: a handheld microwave analyzer displaying a swept trace, cabled to a blue-mounted waveguide section with probes installed, alongside a benchtop network analyzer.
03Calibration benchS-parameters recorded through the probe under test.
CAD assembly view of a waveguide section with a D-dot probe exploded above its mounting holes and a second probe positioned at the guide's end flange.
04Probe mounting assemblyMounting uses a custom machined bolt pattern on the waveguide itself.

Reconstruction chain

The calibrated transfer function was applied to the raw probe signal in the frequency domain to correct for the probe’s frequency response. Cable and oscilloscope attenuation were then accounted for before transforming the corrected signal back into the time domain to reconstruct the incident electric or magnetic field pulse.

Block diagram in three panes. Experiment: source drives probe. Hardware: probe feeds attenuator, attenuator feeds oscilloscope. Software: raw data, correct for attenuator and offset, FFT, then divide by cable attenuation transfer function 1/S12(jw), then by probe transfer function 1/H_probe(jw), then IFFT.
05Measurement and reconstruction chainDiagram of the total experimental setup for probe measurements. This follows the signal path from collection in the experimental chamber through the software post-processing.

MACOR photocathode assembly

The photocathode stack holds the cathode at high voltage inside the vacuum chamber. It had to electrically insulate the cathode from the anode, remain vacuum-compatible, and be machinable within the project timeline. I specified MACOR, a machinable glass-ceramic, for its dielectric properties, vacuum compatibility, and ability to be machined with conventional tooling.

I completed the detailed CAD for the assembly and sourced the standoffs and insulated fasteners as COTS components, making the ceramic the only custom part. This minimized custom manufacturing and kept the assembly to a single lead time.

One consideration was the brittle nature of MACOR and the electrostatic load carried by the stack at the design voltage. I calculated the electrostatic load and increased the ceramic thickness as a factor of safety rather than machining it to the geometric minimum. The additional material added no manufacturing cost while reducing the risk of ceramic failure inside the vacuum chamber, which would require venting, rebuilding, pump-down, and bake-out.

Supplier property table for MACOR, in three groups. Mechanical: density 2.52 gm/cc, zero porosity, flexural strength 94 MPa, compressive strength 345 MPa, elastic modulus 66.9 GPa, maximum use temperature 1000 C with no load. Thermal: conductivity 1.5 W/m-K, coefficient of expansion 12.6 per million per C. Electrical: dielectric strength 40 ac-kV/mm, dielectric constant 6.0 at 1 kHz, loss tangent 0.005, volume resistivity above 1e14 ohm-cm.
06MACOR published propertiesThe three rows that imformed material selection: 40 ac-kV/mm dielectric strength, zero porosity, and a volume resistivity above 1e14 ohm-cm. Zero porosity is what makes it hold vacuum, and the resistivity is what keeps the cathode isolated from the anode.
Stock photograph. An end mill cuts a circular pocket into a white ceramic block held in a vise, with white chips scattered across the machine table.
07Machining MACORA stock photograph, NOT a picture of my part. MACOR can be cut on a conventional mill with carbide tooling by in-house machinists.

Vacuum chamber and shielded enclosure

I designed and modeled the ultra-high-vacuum photocathode chamber assembly: multiport flanges, a custom electrical passthrough, and mounting hardware. Several custom multi-port vacuum flanges and passthrough had to be designed and sent out for machining.

I modeled and built the RF test enclosure, an anechoic chamber with integrated shielding, so measurements could be taken effectively and safely. I sourced and worked with lead shielding panels for the enclosure while following LANL's radiation safety procedures and site requirements for handling and installing toxic heavy metals.

(https://www.researchgate.net/figure/Left-Diagram-of-the-photocathode-activation-vacuum-chamber-internal-components-showing_fig1_337232985) Photograph of an ultra-high-vacuum chamber on a bench. A vertical central cylinder carries a gold-plated bolted flange at the top and radiates six ports, several fitted with bellows, gate valves, and a viewport with a dark window. A large horizontal vessel sits behind it, an optical breadboard with mounts stands in front, and a turbo pump and a monitor are at the right.
08Photocathode chamber, installed @ CEBAFThis is a photocathode activation vacuum chamber from the CEBAF facility at the Thomas Jefferson National Accelerator Facility. It is similar to the model I built and is here to ilustrate the scale and complexity of the vacuum hardware.
Stock photograph. A room lined floor to ceiling with blue pyramidal RF absorber. A horn antenna on a white mast points at two benchtop instruments on a table, one displaying a spectrum trace, cabled together with coaxial leads.
09Generic anechoic enclosureA stock photo of a generic anechoic chamber, NOT the enclosure I modeled. The chamber uses specialized foam pyramidal absorber to kill the reflections that would otherwise interfier with the measurements.

Tools

  • SOLIDWORKS CAD: vacuum chamber, the photocathode assembly, and the probe models.
  • CST: electromagnetic simulation of the probe and waveguide geometry.
  • MATLAB: RF Toolbox for the fitting and the reconstruction chain.

The analysis is system dynamics applied to hardware: transfer functions, Fourier transforms, and a measurement path to interpret data.

Known limitation

The probes reflect power back into the waveguide, which shows up as elevated S11 in the calibration data. The calibration assumes the probe samples the guide's field. A probe that reflects has altered that field, so some of the fixture ends up inside the measured transfer function.

The next step is a G-TEM cell, which produces a known far-field environment without a waveguide's boundary conditions. That would bound the uncertainty and allow functional testing across voltage levels and time scales. The calibration is usable now.

Skills applied

Instrumentation and test
  • D-dot and B-dot field probe design
  • Ultra-wideband far-field measurement
  • Waveguide-mode calibration
  • Vector network analyzer, S-parameter measurement
  • High-speed oscilloscope capture chains
Analysis and simulation
  • CST electromagnetic simulation
  • MATLAB, RF Toolbox
  • Transfer functions and Fourier methods
  • Rational-function fitting, RMSE assessment
  • Electrostatic load calculation
Mechanical design
  • 3D CAD assembly design
  • Ultra-high-vacuum hardware, multiport flanges
  • Custom high-voltage vacuum passthrough
  • Anechoic enclosure and radiation shielding
  • Design for manufacture under schedule
Materials and fabrication
  • MACOR glass-ceramic specification
  • Dielectric, vacuum, and machinability trade-offs
  • COTS sourcing and integration
  • Hands-on lab fabrication

Note on redaction

This entry stays inside the boundary of LA-UR-25-26601, the public-release approval covering the poster.

Sources