Dark Current on VELA

Frank Jackson

VELA Dark Current

• Measured several times in 2014, once* with original VELA cathode, and 2-3 times after cathode was replaced, mainly using WCM-01, ~1 m from gun.

• Effect of parameters gun gradient, BSOL, SOL.

• Some preliminary study of transport further from gun – using ED-FCUP (~ 10 m from cathode)

*one documented shift dedicated to dark current measurement, other shifts may have taken ad-hoc measurements.

Dark Current Intro

• Priorities change with time. – Firstly, important for conservative shielding

calculation, but less so now.

– Later, important for comparing cathode characteristics

• Some experiments very sensitive to dark current e.g. electron diffraction

• Other important issues will arise that are difficult to foresee (other experiments, emittance measurements etc). The method of DC investigation may depend on the issue.

Dark Current Intro

• Most people using machine experience dark current through observation on YAG screens

• Since our level of dark charge can be bunch charge, (especially near the gun) it can be often and easily noticed on screens.

• Could the screens be used more quantitatively – even pC of beam on YAG can be seen if focussed (see right) ?

• Most quantitative measurements so far use WCM rather than screens.

Original cathode, YAG-01 dark current image, Sep 18th 2013, (analogue camera, no gating or filtering ?)

Replaced cathode, YAG-01 beam(5 pc) + dark current image, Oct 01st 2014, (digital camera, gated?)

WCM-01 DC measurements

• Main tool for measuring dark current. • WCM has been used in different configs

(with/without electronics) in the different measurements. Now has a ‘head’ with switchable bunch/DC config. Signal observed and ‘integrated’ using scope.

• Signal/noise is the poor in some measurements (second cathode, moderate/low gun gradients). Averaging over many shots (100s) was normally required.

April 2014 Measurements (Original Cathode)

• WCM used with simple amplification of signal. Signal was usually >> noise

• Gun set to 6.5 MW (around 80 MV/m) with reflected power ratio of 1.9%. The RF pulse length was 3 μsec

Sep-Nov 2014 (#160,185,189)

• Repeated SOL/BSOL scan for previous RF parameters used, twice over 2 month gap

• DC vs gun gradient • At previous RF params used,

signal much smaller, noise-constant-drift , leads to error of 10s of pC on signal of same magnitude.

• Difficult to measure which sol values minimise DC at new low level.

• No gross difference in measured dark current between Sep and Nov ‘14.

Comparison with Other Guns.

Gun Dark Charge Per Pulse Reference

REGAE, 1.5 cell, 100 MV/m max,

3 GHz, 4 μs pulse.

10 pC @85 mV/m,

40 pC @ 100 mV/m,

0.5 m from gun, solenoid

settings?

H. Delsim-Hashemi, IBIC 2013

LCLS, 1.6 cell, 140 MV/m max,

2.9 GHz, 2 μs pulse.

< 50 pC @ 85 mV/m

0.5 nC @ 120 mV/m

Maximise charge using

solenoid, close to gun I think?

D. H. Dowell, PAC 2007

Waseda/KEK-ATF, 1.6 cell,

2.9 GHz, 2 μs pulse.

< 100 pC @ 6 MW

~ 0.7 nC @ 10 MW

FCUP directly after gun.

What solenoid setting?

A. Murata, EPAC 2008

FERMI (tested at MAXLAB), 1.6

cell, 3.0 GHz, 120 MV/m, 3 μs

pulse.

“few nC” from 90-120 MV/m

On ICT, how close to gun

M. Trovo, EPAC 2008

SPARC INFN, 1.6 cell, 120 MV/m,

2.5 μs pulse.

< 100 pC @ 85 mV/m

2.5 nC @ 120 MV/m

How close to gun

See link*. G. Gatti

presentation

SwissFEL injector Looked through JaCow, can’t

find any results on dark

current

Haven’t found any DC current for 2.5 cell guns so far Most ~1.5 cell guns found (table below) < 100 pC per RF pulse Details are not always clear on measurement params. See "\\dlfiles03\ASTeC\Projects\VELA\documentation\supporting\DarkCurrentWorldwide.docx"

Dark Current vs Gradient

• Dark current shows expected exponential increase with gradient/beam energy.

Data from shift #189, 12th Nov 2014

DC interpretation using Fowler- Nordheim

• Fowler-Nordheim model for dark current vs gradient parametrised in terms of field enhancement factor, work function, and emission area

PRL 109, 204802 (2012) SLAC-PUB-7684

From data taken on shift #189, 12th Nov 2014 = 132 , calculated from slope, using copper workfunction of 4.6 eV

Field Enhancement

• Was subject of recent study published in PRL (109, 204802 (2012)) Chen et al, Tsinghua University S-band RF gun (1.5 cell, 3 GHz)

• ‘traditionally assumed high value of = 50–500 does not provide a plausible explanation of the data, but incorporating a low value of [work function] at some sites does’

• They calculated = 130 from their dark current vs gradient data.

• But concluded, using additional photo-emission data that areas of low work function is more likely to account for measured data

Beta = 100 corresponds to a surface defect of h/a = 100 which is unphysical for typical surface roughness of 10 nm. Would imply a tower of 100 single atoms.

Summary

• Current cathode dark current is substantially lower that original cathode era.

• Max value measured ~ 400 pC per pulse at ~ 8.5 mW (> 80 MV/m)

• New cathode (3rd) promises yet lower dark current

• But cannot take this for granted, and important to check. Other factors like cathode enclosure may be significant.

• More documentation at – \\dlfiles03\ASTeC\Projects\VELA\documentation\notes

and \propsals

Fowler Nordeim Stuff

• Also see FEL 2013, Xiang, for Rossendorf FEL – beta = 307, 456.

Cathode Quality

• Energy spectrum.

• Machining marks on original cathode (e.g. 5th May 2014) visible in beam distribution.