VELA-EP-dark-02. Proposal for Dark Charge Measurement in VELA
14 May 2014
VELA-EP-dark-02. Proposal for Dark
Charge Measurement in VELA.
Frank Jackson.
1 Document History.
Version
VELA-EP-DC-00
VELA-EP-dark-01
VELA-EP-dark-02
Comments
First version for discussion in 27 March 2014
meeting.
Many small changes following 27 March 2014
discussion, too many to summarise. Main change
inclusion of ‘quick method’ to measure dark
current.
New section to include the more sensitive
electronics to measure the dark current.
2 Motivation
The amount of dark current inputs into shielding requirements for CLARA. It is also interesting to
compare level of dark current against other guns worldwide. Some of the measurement process may
indicate the best way to mitigate dark current if required (e.g. the solenoid setting required to
minimise dark current). For example, it may be desired to reduce dark current due to their effect on
BPMs; (dark current should increase the noise on BPM measurements, but should not affect the
position measurement averaged over noise1 ). For reference, dark current measurements on other
guns worldwide are recorded in a supporting document2.
3 Methods of Measuring Dark Current.
3.1 Using Wall Current Monitor (WCM)
The wall current monitor has been equipped with electronics to measure the dark current . Different
electronics have been developed leading to differences in the details of measurement; but generally
the method is to observe the WCM signal on a LeCroy scope and integrate it (using the convenient
in-built scope functions of the LeCroy) to measure the charge. The background level reading on the
WCM must be subtracted by closing the VALV-01.
3.1.1 Sensitive WCM Electronics (used from late April 2014)
Following first experience of using WVM for dark current measurement, sensitive electronics were
developed for the WCM. This consists of a simple amplification3. This should be the apparatus used
in future to measure the dark current. The WCM trace for these electronics is shown in Figure 1. The
1
A. Kalinin.
\\dlfiles03\ASTeC\Projects\VELA\documentation\supporting\DarkCurrentWorldwide.docx
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A. Kalinin.
2
VELA-EP-dark-02. Proposal for Dark Charge Measurement in VELA
14 May 2014
WCM-1 calibration should be checked in the Wiki, or checked with A. Kalinin (at the time of writing,
the calibration was that an area of 1 Volt second, multiplied by 0.07 is equivalent to 1 Coulomb4).
Figure 1. WCM signal with sensitive electronics (cyan trace). The total dark charge is calculated by the backgroundsubtracted area of the signal calculated between the dashed cursors (t  2 μs). In this example the RF pulse length is
3μs. The calibration of this signal, at the time of writing, is the integral[nVs]*0.07 = charge [nC]. The yellow trace shows
the ‘old electronics’ dark current measurement, the pink trace shows the signal on the spectrometer FCUP.
The WCM signal area is automatically computed using scope functions in the time interval that the
dark current is judged (by eye) to be emitted over. A more systematic method of position the
cursors should be developed. The time interval is shown by the cursors in Figure 2 (shift #124) and is
approximately 2 μs (RF pulse is 3 μs long here). To compute measurements, the average of many
shots (e.g. 500 shots) should be taken.
The background in this signal is negligible compared to the size of the maximum dark current signal,
thus background subtraction is not required.
3.1.2 Less Sensitive WCM Electronics (used before April 2014)
Before the sensitive electronics were installed, the WCM was used without electronics amplification
and the WCM had a larger signal/noise ratio. This configuration should no longer be used for dark
current measurements, but the information is kept below for reference.
As above, the dark current per machine pulse is measured by the area under the WCM- 1 signal
scope trace as shown in Figure 2. The WCM-1 calibration was that an area of 1 Volt second is
equivalent to 1 Coulomb5.
4
5
Wiki page http://projects.astec.ac.uk/VELAManual/index.php/Wall_Current_Monitor.
Wiki page http://projects.astec.ac.uk/VELAManual/index.php/Wall_Current_Monitor.
VELA-EP-dark-02. Proposal for Dark Charge Measurement in VELA
14 May 2014
The area is automatically computed using scope functions in the time interval that the dark current
is judged (by eye) to be emitted over. A more systematic method of position the cursors should be
developed. The time interval is shown by the cursors in Figure 2 (shift #95) and is approximately 5 μs
(RF pulse is 3 μs long here). To compute measurements, the average of many shots (e.g. 500 shots)
should be taken.
There is substantial background (noise) on the WCM signal, equivalent to about 10 nC (shift #95)
that must be subtracted by measuring it with the gun turned off (or valv-01 closed). This means that
 90 % of the measurement is background (shift #95). Some noise is due to the impedance of the
WCM and cannot be reduced by scope settings6, altering the scope bandwidth in principle can help
but in practice doesn’t. This means it will be difficult to measure dark charge at the pC level using
this device. Measuring the background over increasing larger samples would reduce the error as n,
so this is a possibility of achieving pC resolution if it is required.
Figure 2. WCM-1 signal trace from a single machine pulse (yellow). The total dark charge is calculated by the
background-subtracted area of the signal calculated between the dashed cursors (t  5 μs). In this example the RF
pulse length is 3μs.
3.2 Using Faraday Cup
Faraday cup with standard electronics (used to measure bunch charge) cannot measure dark
current. FCUP can possibly be used to measure dark current using the WCM electronics. In
performing these measurements, the transport ‘efficiency’ to the FCUP must be considered. It has
not yet been decided the role of dark current measurements with the FCUP.
3.3 Using Integrated Current Transformer
The ICT is a ‘natural’ device to measure dark current with. However the calibration of this devices is
not known (given as 5V=1A, see ref7, but this is difficult to interpret) and attempts to perform beam
based calibration against the WCM may be difficult unless the transport of dark current from WCM
to ICT is understood.
6
7
A. Kalinin.
Wiki page http://projects.astec.ac.uk/VELAManual/index.php/ICT
VELA-EP-dark-02. Proposal for Dark Charge Measurement in VELA
14 May 2014
4 Measurement Method
Dark current is a multidimensional observable. Can depend on: location in the machine; gun
gradient; RF pulse length; machine magnets (particularly SOL and BSOL).
4.1 Quick Evaluation of Maximum Dark Current Level.
To simply measure the maximum dark current near the gun at the nominal operating parameters,
use the WCM. There are no adjustable apertures between the gun and the WCM. The dark current
measured depends strongly on the SOL and BSOL values.
Set the gun to its nominal gradient (probably the maximum gradient at which the gun is stable) and
with the nominal RF pulse length. Ensure the reflected power is low (~ 1 % level).
Record the GUN FWD PWR, GUN REV PWR, RF PULSE LENGTH (all readable from the RF EPICS GUi).
Measure the dark current on WCM using the method described in section 3.1. First maximise the
dark current level using SOL-1, then maximise using BSOL-18. Record the SOL/BSOL values and level
of dark current.
4.2 Dark current at WCM-1 as a function of gun gradient and solenoid.
This seems to be sort of measurement done by other machines (see appendix), a dark charge
measurement close to the gun, vs gun gradient.
1. Start at nominal working point of gun, (just for ref, in Nov 2013 this was 3 μs RF pulse length,
~ 7 MW Gun FWD power)
2. Measure dark current vs. SOL and BSOL using WCM procedure described above.
3. Change gun power down, by -1 MW forward power. Wait until rev power has stabilised.
Note FWD-REV power. Measure the dark current vs. SOL and BSOL.
4. Repeat step 3 for some more points until a decent range of net forward power
measurements is achieved.
For speed, do not retune gun via temp each time the gradient is changed, as long as the detuned net
FWD-REV power can cover a decent range.
The above assumes we will be using a 3 μs pulse for the foreseeable future. If we are considering
changing the RF pulse length for any other reason, repeat the whole experiment at the new pulse
length.
4.3 Other Experiments
Dark charge in spectrometer, to deduce energy profile of dark current? Use YAG and FCUP with
WCM electronics? Dark charge in BA1?
4.4 Other Issues
Does the WCM measure current lost on pipe?
Does the maximum dark current depend on BSOL and SOL in an uncorellated way? i.e. does the SOL value for
the maximum dark current depend on BSOL, or is it independent of it?
8
This assumes the overall maximum dark current depends on BSOL and SOL in an uncorellated way. This has
not been established experimentally.