VELA and CLARA

Vacuum System Designs

Keith Middleman

Vacuum Science Group

ASTeC, Daresbury

UK

Introduction • VELA – Versatile Electron Linear Accelerator

• CLARA – Compact Linear Accelerator for Research and

Applications

• VELA Overview

• 3 main areas of vacuum design:

– VELA Modelling + Results

– Differential Pumping system for Laser Transport

– Shielding of total pressure gauges

• CLARA plans and latest design

The Versatile Electron Linear Accelerator Beam Energy 4 - 6 MeV

Bunch Charge 10 - 250 pC

Bunch length (σt,rms) 1 - 10 ps

Normalised emittance 1 - 4 m

Beam size (σx,y,rms) 1 - 5 mm

Energy spread (σe,rms) 1 - 5 %

Bunch repetition rate

1 - 10 Hz (ALPHA-X gun)

1 - 400 Hz (with high rep. rate gun in the future;

klystron and laser specified for 400 Hz)

£2.5 million capital investment from

UK government

S-band RF Photoinjector using a Cu

photocathode

Allow development of new

accelerator technologies within

ASTeC

Available for industry to test their

technologies and take them from

prototypes into market products

Industrial and Scientific support

(e2v, Rapiscan, Strathclyde University

etc)

1st industrial users September 2013

– Rapiscan

Further industrial use this year –

Siemens, AWE and Rapiscan

Versatile Electron Linear Accelerator

VELA conductance limited

VELA Modelling

VELA Injector Pictures

Reality of seeing the photoinjector was different to the drawings supplied.

Injector area had to be remodelled and an additional SIP added due to the trapped volumes

and conductance limitations.

Laser Transport Path

Initial Design

Boundary Conditions:

• 1E-5 mbar for Mirror Box. Sticking Coefficient = 1

• VELA Lightbox – Sticking Coefficient = 1

• 10mm tube diameter

Pressure at VELA Lightbox for Initial

Design

10-8

10-7

10-6

0 0.2 0.4 0.6 0.8 1

Pressure at EBTF Lightbox for Initial Design

Sticking Coefficient

2nd Design

Boundary Conditions:

• 1E-5 mbar for Mirror Box. Sticking Coefficient = 1

• VELA Lightbox – Sticking Coefficient = 1

• 10mm tube diameter

Pressure at VELA Lightbox for 2nd

Design

10-9

10-8

10-7

10-6

0 0.2 0.4 0.6 0.8 1

Pressure at EBTF Lightbox for 2nd Design

Sticking Coefficient

3rd Design

Boundary Conditions:

• 1E-5 mbar for Mirror Box. Sticking Coefficient = 1

• VELA Lightbox – Sticking Coefficient = 1

• 10mm tube diameter

Pressure at VELA Lightbox for 3rd

Design

10-9

10-8

10-7

10-6

0 0.2 0.4 0.6 0.8 1

Pressure at EBTF Lightbox for 3rd Design

Sticking Coefficient

4th Design

Boundary Conditions:

• 1E-5 mbar for Mirror Box. Sticking Coefficient = 1

• VELA Lightbox – Sticking Coefficient = 1

• 8mm tube diameter

Pressure at VELA Lightbox for 4th

Design

10-9

10-8

10-7

10-6

0 0.2 0.4 0.6 0.8 1

Pressure at EBTF Lightbox for 4th Design

Sticking Coefficient

Pressure at VELA Lightbox

Compared for all Designs

10-9

10-8

10-7

10-6

0 0.2 0.4 0.6 0.8 1

Pressure at EBTF Lightbox Compared for all Designs

Initial Design2nd Design3rd Design4th Design

Sticking Coefficient

Results and Recommendations

• Maximum pressure difference achievable between Mirror Box and

VELA is just under 4 orders of magnitude.

• Further improvement only possible if we can increase the distance

between the Mirror Box and VELA or further reduce the laser

aperture requirement down from 10mm.

• Recommendation would be to improve the vacuum in the Mirror Box

down to 1E-6 mbar. This should allow us to achieve E-10 mbar in

the VELA lightbox and give us some contingency.

Total Pressure Gauge Shielding

• Given the low energy of the electron beam there was some concern about the impact of stray magnetic fields

• Vacuum equipment involves SIP’s and IMG’s both of which have stray fields.

• Testing done on items of vacuum equipment to verify whether there was a problem. Items tested include: – 400 l/s Sputter Ion Pump (SIP) – used in various locations

– 40 l/s Sputter Ion Pump (SIP) – used in one location, on the waveguide connected to the RF photoinjector

– Inverted Magnetron Gauge (IMG) – used in various locations

Total Pressure Gauge Shielding

•The shielding is 6mm thick

•The downside of such a large piece of magnetic shielding is that it has an

impact on gauge performance and affects pressure readings

•Unfortunately the effect is non linear across the pressure range, as a result

calibration of the IMG gauges with the shields on has been done.

Calibration Run for IMG + Shielding

• Results for the calibration of multiple gauges with shielding.

PTB IMG_1 IMG_1 C/F IMG_2 IMG_2 C/F IMG_3 IMG_3 C/F

2.98E-09 1.14E-08 3.82 3.70E-08 7.42 2.40E-08 8.05

5.07E-09 1.84E-08 3.62 5.59E-08 7.02 3.38E-08 6.67

9.14E-09 3.21E-08 3.52 8.35E-08 6.14 5.22E-08 5.71

3.01E-08 1.15E-07 3.81 1.81E-07 6.02 1.81E-07 6.02

5.20E-08 1.59E-07 3.06 2.82E-07 5.42 3.18E-07 6.12

9.09E-08 2.67E-07 2.93 4.35E-07 4.78 4.35E-07 4.78

3.14E-07 7.58E-07 2.42 8.96E-07 2.85 8.96E-07 2.85

5.06E-07 1.16E-06 2.30 1.42E-06 2.81 1.42E-06 2.81

9.15E-07 1.95E-06 2.13 2.33E-06 2.54 2.46E-06 2.69

2.78E-06 5.52E-06 1.99 6.44E-06 2.32 6.44E-06 2.32

5.04E-06 1.02E-05 2.02 1.15E-05 2.29 1.15E-05 2.29

9.00E-06 1.80E-05 2.00 2.08E-05 2.31 2.08E-05 2.31

2.00E-05 4.08E-05 2.04 4.35E-05 2.17 4.65E-05 2.33

CLARA

CLARA front end terminating

module – to be discussed

New CLARA Service trays

VELA ….existing New laser extended

from existing VELA

CLARA Front End Overview

MODULE 1 RF screen Bellows

~5mm squeezed too

tight – but note valve!? LINAC1

draft model received

from R.I. 02-03-14

LINAC 1 support

design?

Solenoids… no input or

guidance on requirements so

far…assume VELA gun type

YAG screen

New design – or

alternate VELA

design to be decided

RF screen Bellows

Waveguide coupler

…to be designed

BPM

Stripline

VELA type

WCM

VELA type

HV Corrector

Type 1

HRR Gun

(with DLS solenoid)

Photocathode

loadlock

RF Screen

Valve HV Corrector

Type 1

HRRG Modelling

HRRG Model – NEG coating

CLARA NEG coating

140 160 180 200 220 240 260 280 300 3200.01

0.1

1

CO

stic

king

pro

babi

lity

140 160 180 200 220 240 260 280 300 3201 10

4

1 103

0.01

0.1

Ti-Zr-Hf-V

Hf-Zr-V

Ti-Zr-Hf

Ti-Hf-V

Ti-Zr-V

Ti-Zr

Zr-V

Zr

Activation temperature [ C]

H2

stic

king

pro

babi

lity

140 160 180 200 220 240 260 280 300 3200.01

0.1

1

10

CO

pum

ping

cap

acity

• Ti-Zr-Hf-V is the best

• Hf-Zr-V, Ti-Zr-Hf, Ti-Hf-V and

Zr are comparable

• Ti-Zr-V is lower

• Zr-V (best binary alloy) has the

lowest activation temperature

O.B. Malyshev et al Vacuum 100 (2014) p26-28

Conflat space savings

U Slots – for each flange

reduce by 1 flange length

(12mm)

U Slots Vacom QCF

ISO standard CF knife edge UHV

flange but no bolt extraction – uses

chain ring clamp system instead.

Uses annealed copper CF gaskets.

Tested at CERN & Frascati

What are people’s experiences? A good solution?

New heater coating development for bakeout

Any other such solutions, experiences?

• Developed by 2D heat

• Partial oxidation of Ni-

Cr-Fe alloy

• Advantages over

conventional

techniques

• Permanently in place

• Coating 0.3mm thick

• Intimately bonded