09/13/2011 1

Status of high intensity polarized electron gun project at MIT-Bates

Evgeni TsentalovichMIT

2

eRHIC (Linac-ring version)

Requires a polarized electron source with an extremely high current ( at least 50 mA).

• Average current of tens or even hundreds of mA is required• Modern state-of-the-art guns produce ~100-200 A• Average current of ~ 1 mA achieved in tests at JLab and

Mainz; lifetime ~ 20 h• Average current of up to 10 mA achieved at Mainz with very

short lifetime (needs active cathode cooling)

Main problem – ion backbombardment

05/20/2008 3

Ion damage mostly the center of cathode (Bates results)

0

0.2

0.4

0.6

0.8

1

-6 -4 -2 0 2 4 6

R,mm

Laser beam profile

Damage pattern

High Intensity Polarized Electron Gun

The principal points to achieve high average

current:

• Large area cathode.

• Ions tend to damage the central area of the cathode –

ring-shaped emission pattern.

• Active cathode cooling.

• Very small beam losses could be allowed near the

gun ( ).610

Phase I results: beam simulations.Three different initial emitting pattern were used:

Ring-shaped Gaussian Flat

The pictures represent the beam shape 400 mm from the cathode.

Beam line. Pipe aperture ~±34 mm.

Gun

DipolesSolenoidal

lenses

Beam dump

Losses estimates

10 11 12 13 14 150,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

j, m

A/c

m2

R, mm

- Beamline 3D - ExtraSAM

It is difficult to get a correct shape of the tails in regular simulations. Special simulations with electrons emitted only from the edge of the cathode (r>11.8 mm) have been performed. The resulting tail can be approximated by Gaussian distribution:

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max 2

( )( ) exp

2

r rj r j

R, mm 10 11 12 13 14 15 16 17

Losses .037 .012 3108.3 4109.7 4103.1 5101 7101 6101

Results for the ring-shaped beam at the entrance into the first dipole. r0=4.9 mm, σ=1.9 mm. Aperture is about 30 mm.

Similar calculations have been performed in several different locations in the beam line with all three initial emitting pattern. No substantial beam losses have been discovered.

Ion distribution

Electron current profile

Ion energy profile

Ion current profile

Simplification: the cross section ionization was independent of electron energy.

Interesting problem - beam dump

PUMPS

Current monitor

Water cooling

Beam

I~50 mA → P~6 kW !!!Outgasing in the dump could be serious problem.

Biased beam dump (energy recovery)-120 kV Gun power supply

-119 kV

GunBeam line

Beam dump

Electron energy in the dump drops from 120 kV to 1 kV.Heating power in the dump drops from 10 kW to 100 W

100 W in the dump still needs to be removed, and now dump is at 120 kV !Needs fluorinert chiller.

Isolated 1 kV power supply

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Cathode Cooling Test Chamber

Coolant in

Coolant out

HV

Laser

Manipulator

Cathode

Crystal

Cathode – anode assemblyFluorinert

(cooling agent)

Cathode – anode assemblyFluorinert

(cooling agent)

Cathode – anode assembly

Pack with a crystal

Heat exchanger

Preparation chamber

General assembly – top view

GUN

PREP. CHAMBER

LOAD LOCK

SECOND DIPOLE

LENSES

MANIPULATORS

General assembly – top view

GUN PREP. CHAMBERLOAD LOCK

FIRST DIPOLE

Tests results in the cathode cooling chamber-Vacuum manipulations works very well: excellent illumination with internal halogen bulbs, good visibility, reliable pack transfer.-High Voltage: processed the chamber to 125 kV, but electrical discharges happened. The ceramic pipes need better protection from the electrons produced by cold emission.-Temperature control: the pack temperature could be held at below 25°C even at the maximum laser power available (about 34 W on the crystal)

Vacuum features of the gun chamber• 100 l/s Ion pump with 400 l/s NEG.

• 4 additional 400 l/s NEGs (only two are currently installed).• The chamber walls are thin (~ 3 mm) to reduce outgasing.• The chamber and most of the parts have been prebaked to 400°C before the final assembly.• Bake-out at 250°C after the final assembly.• RGA readings after bake-out:

m 2 16 18 28 44

P, mbar 11109 13105 13104 13105 13102

HV processing

• The operating voltage is 120 kV

• The gun was processed to 150 kV

• After the processing no activity (measurable dark current, vacuum increase) could be detected at 120 kV

• The Fluorinert produces virtually zero conductivity (unable to measure).

Current status• The gun chamber: built and tested. It was vented 3 weeks ago to install the first dipole followed the gate valve and 2 additional NEGs. Rebaked and HV reprocessed.

• The preparation chamber: design is completed, the main chamber has been manufactured, many parts are already ordered.

• Load lock – the design is in progress.

• Beam line – conceptual design is completed, some parts are designed, the dipole vacuum chambers have been manufactured.