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Hollow electron gun CHG-16-sc

Introduction

This is a hollow electron gun prototype for collimation and scraping in the High-Luminosity Large Hadron Collider (HL-LHC).

gs_20180605_102527_DSC_4258_LR.jpg
Assembled electron gun
It is based upon the CHG1 e-gun. Features include:
  • scandate cathode for higher current densities
  • smaller inner and outer cathode diameters, 8 mm and 16 mm, to better match the dimensions of the LHC proton beam halo
  • scaled electrode distances and dimensions to preserve same perveance as the CHG1 e-gun

Mechanical design

CHG-16-sc_3D-iso_2017-10-19.png
3D view

Drawings:

E-gun assembly [PDF]
Cathode assembly [PDF]
Cathode support [PDF]
Focusing electrode [PDF]
Control electrode [PDF]
Anode [PDF]
Anode shield [PDF]
Anode support [PDF]

The 16-mm scandate cathode

The cathode was manufactured at the Beijing Vacuum Electronics Research Institute (BVERI).

gs_20180112_035520_P1000041_LR.jpg gs_20180112_040022_P1000042_LR.jpg gs_20180112_035223_P1000037_LR.jpg gs_20180112_035403_P1000040_LR.jpg
Cathode in transport case Upright cathode Detail of emitting surface Detail of filament connections

Operating instructions from manufacturer [PDF]

18-0198-22.hr.jpg 18-0198-23.hr.jpg
Side view Close-up

More photos of the cathode taken by Reidar Hahn of Fermilab Creative Services.

Numerical simulations

Electron emission was simulated with the Warp particle-in-cell code.

Here is an example at 10 kV, 0.4 T, 6.91 A:

10kV_0p40T_n0032_overview.png 10kV_0p40T_n1024_I-vs-z.png 10kV_0p40T_n1024_j-vs-r.png
Overview [PDF] Current vs. z [PDF] Current-density vs. radius [PDF] [data]

Particle positions and velocities for a few cases can be found in the following table.

Each data table has 8 columns:
  • macroparticle number
  • x, y, z coordinates [m]
  • vx, vy, vz velocity components [m/s]
  • macroparticle weight
0.5 kV, 0.4 T, 78 mA Data
10 kV, 0.4 T, 6.91 A Data

The full output data tree (from commit 352fcc7b in the repository) is here (113 MB).

Yield vs. voltage and perveance

predicted_perveance.png
Predicted yield vs. voltage [PDF]

The predicted perveance is 6.91 uperv.

Emission vs. voltage/current and magnetic field

0.05 T 0.1 T 0.2 T 0.3 T 0.4 T 0.5 T 1.0 T
0.5 kV, 78 mA 00p5kV_0p05T_overview.png 00p5kV_0p10T_overview.png 00p5kV_0p20T_overview.png 00p5kV_0p30T_overview.png 00p5kV_0p40T_overview.png 00p5kV_0p50T_overview.png 00p5kV_1p00T_overview.png
6 kV, 3.22 A 06kV_0p05T_overview.png 06kV_0p10T_overview.png 06kV_0p20T_overview.png 06kV_0p30T_overview.png 06kV_0p40T_overview.png 06kV_0p50T_overview.png 06kV_1p00T_overview.png
10 kV, 6.91 A 10kV_0p05T_overview.png 10kV_0p10T_overview.png 10kV_0p20T_overview.png 10kV_0p30T_overview.png 10kV_0p40T_overview.png 10kV_0p50T_overview.png 10kV_1p00T_overview.png

Installation in Fermilab test stand

The electron gun was installed in the Fermilab electron-lens test stand on June 5, 2018 to characterize its performance.

Photographs of the installation.

Heating

The cathode is heated by controlling the filament current.

resistance-vs-current_2018-10-10.png resistance-vs-P14_2018-10-10.png
Filament resistance vs. filament current [PDF] [data] Filament resistance vs. filament power [PDF]

The filament resistance is an indication of temperature. Above a certain heating power, heating dissipation is mostly radiative.

Temperature distributions and infrared images were taken on Oct. 5, 2018 with filament heater at 5.8 A.

temperature_measurements.001.png temperature_measurements.002.png
Temperatures on contact Infrared image

Performance

Preliminary results are discussed in these slides

Current yield

After the first conditioning, the current yield was measured as a function of voltage in the Fermilab electron-lens test stand on Oct. 10, 2018.

CHG-16-sc_yield_6p6A_20181010.png
Measured current yield vs. voltage [PDF] [data]

During this test, the electron gun delivered 5.2 A at 10 kV.

At high voltage, the cathode is still temperature limited. Further conditioning is under way to extend space-charge-limited emission up to 10 kV.

Profile measurements

Preliminary 1-dimensional scans of the current-density profiles using 4 downstream correctors to check alignment.

2018-10-10_H_scan.gif 2018-10-10_V_scan.gif
Horizontal Vertical
(red = pinhole current; green = vacuum monitor)

First 2D profiles at 0.5 kV, 83 mA at 3.5-4-1 kG solenoid configuration.

CHG16sc_190213_7p0A_3p5-4-1kG_500V_83mA_2D.png
Measured current-density profile [PDF] [data]

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