dark current on vela -...
TRANSCRIPT
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.