Project

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Introduction

This is a prototype hollow electron gun for active beam halo control with electron lenses in HL-LHC.

The project is a collaboration between CERN, Fermilab, and the Beijing Vacuum Electronics Research Institute (BVERI).

The electron source is based on the cathode-anode geometry of the CHG-16-sc electron gun and features a similar scandate cathode. The main difference is the compact assembly, designed to fit in a smaller superconducting solenoid bore and to reduce materials and vacuum surfaces.

gs_20190513_075327_DSC_9442_LR.jpg gs_20190513_074827_DSC_9438-HDR_LR.jpg
Comparison between CHG-16-sc (left) and CHG-16-sc-e (right) CHG-16-sc-e, top view

An overview of the project was presented at IPAC19:

Perini et al., The Develpment Programme of Cathodes and Electron Guns for the Hollow Electron Lenses of the High Luminosity LHC Project, in Proc. of the 10th Int. Part. Accelerator Conf. (IPAC19), Melbourne, Australia, May 2019.

Measurements are recorded in the e-lens e-log .

Timeline

  • 2019-03-14: assembled at CERN
  • 2019-03-25: delivered to Fermilab
  • 2019-05-13: installed in the Fermilab electron-lens test stand
  • 2019-05-17: started filament heating
  • 2019-06-23: reached operating temperature
  • 2019-06-24: measured beam alignment, current yield, current-density profile
  • 2019-07-02: repeated yield and profile measurements
  • 2019-07-26: started filament cool-down
  • 2019-07-31: turned off filament heater
  • 2019-08-01: tested high voltage at room temperature
  • 2019-09-17: further high-voltage tests at room temperature

Design

3D views
EGun_Fermilab_ISO1.png EGun_Fermilab_ISO2.png
EGun_Fermilab_Section1.png EGun_Fermilab_Section2.png
Drawings
LHCNEHEP0001-AB.png LHCNEHEP0006-AB.png
[PDF] [PDF]

Expected Performance

Because of the identical cathode, electrode and anode structure, the expected current yield and current-density profiles are the same as those calculated for the CHG-16-sc source, which has a perveance of 6.9 uperv.

Assembly

Two identical prototypes were assembled at CERN on March 14, 2019: one for tests at Fermilab and one for CERN.

gs_20190314_115604_P1000708_LR.jpg

A photo gallery of the assembly can be found here.

Vacuum, Filament Heating and Cathode Conditioning

Summary of electron-gun vacuum and heating:

CHG-16-sc-e_overview.png

(The vacuum monitor calibration can be found here)

Vacuum and heating data, 2019-05-13@8:00 to 2019-08-03@8:00:

High-Voltage Tests

The resistance seen by the HV power supply (cathode to ground) was measured at various temperatures and compared with previous electron guns.

comparison_resistance_vs_voltage_and_temperature.png
Resistance vs. voltage and temperature [PDF]

The Fermilab 25-mm hollow gun (HG1c) had a resistance of about 6 Mohm at the operating temperature, approximately constant as a function of voltage.

The CERN electron guns CHG1b (25-mm) and CHG-16-sc (16-mm, scandium-based) had a similar resistance at voltages above about 2 kV. The resistance then decreased smoothly with voltage (a behavior that is not understood).

The high-voltage characteristics of CHG-16-sc-e are very different. At room temperature, the resistance is good, about 7 Mohm. However, vacuum activity is detected above 3.5 kV, with pressure spikes and power-supply trips above 4.5 kV. As the temperature is increased, the resistance drops significantly with voltage. At the operating temperature, the electron gun can only be operated up to 1.7 kV. Above this voltage value, vacuum activity is observed and the leakage current exceeds the rating of the HV power supply (2.5 mA).

The high-voltage characteristics of this compact design need to be reviewed and improved. Tests on the twin e-gun at CERN may help diagnose the issue.

Pulse Shape

The following oscilloscope traces show the trigger pulse (green), the cathode toroid (yellow, 1 A/V, with damped oscillations due to modulator switching) and the collector toroid (magenta, 1 A/V).

2019-06-24_1635_0p5kV.png 2019-06-24_1625_1p5kV.png
0.5 kV 1.5 kV

Measured Current Yield

The electron gun reached 0.49 A at 1.7 kV. The perveance is 7.0 uperv.

CHG-16-sc-e_yield_6p0A_20190702.png
Current yield vs. cathode-anode voltage [PDF] [data]

Measured Current-Density Profile

Current-density profiles were measured with the following magnetic configuration:

  • Solenoids: 2100 A (gun); 4800 A x2 (main); 600 A (collector), corresponding to 3.5-4-1 kG
  • Correctors: Z:HCOR02 = Z:HCOR06 = -4.0 A; Z:VCOR01 = Z:VCOR05 = -1.5 A
Filament heater 6.00 A, 7.10 V 6.00 A. 7.06 V
Vacuum 6 x 10 -8 torr 2 x 10 -8 torr
Pulse 10 us, 500 Hz 10 us, 50 Hz
HV power supply 0.5 kV, 0.79 mA 1.5 kV, 2.13 mA
Peak beam current 79.2 mA 412 mA
Current-density profile CHG16sce_190624_6p0A_3p5-4-1kG_500V_79mA_2D.png CHG16sce_190702_6p0A_3p5-4-1kG_1p5kV_412mA_2D.png
[PDF] [data] [PDF] [data]

Summary and Discussion

  • The perveance of the electron gun is very good, in agreement with simulations.
  • The current-density profile is smooth and uniform. There are no signs of surface defects.
  • Currently, this compact assembly cannot be operated at 10 kV. At room temperature, we could reach 4.5 kV before observing breakdowns. At the operating temperature (filament heater at 6.0 A), we could only reach 1.7 kV before exceeding the current rating of our HV power supply (2.5 mA max). Insulation of high-voltage electrodes needs to be improved.

Contact

For comments and questions on this project, please contact Giulio Stancari.

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