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USE OF THE ISAC-II FLIGHT TIME MONITORSTOWARD AUTOMATED TUNING
S. Kiy∗ , P. Jung, T. Planche, O. Shelbaya, V. Verzilov, TRIUMF, Vancouver, BC, Canada
Abstract
A time-of-flight measurement system has been in use at
ISAC-II since 2006 for the phasing of cavities and accurate
ion beam velocity measurements across the nuclear chart.
This system is heavily relied upon as the primary energytime domain diagnostic downstream of the ISAC-II linac.
Ongoing High Level Applications (HLA) development at
TRIUMF has enabled the use of methods that are being
applied to these measurements - both for processing andautomation of data acquisition. An update will be provided
on operational experience with the system over the past 10
years including its recent re-calibration and error analysis.
A brief summary of the current HLA framework will begiven, including a database for beam measurements andthe ability to carry out sequential measurement processes.
Finally, the way in which these developments enable beambased calibration of cavity parameters and a shift to modelbased tuning methods is discussed.

Figure 1: ISAC-II Flight Time Monitor Canister showing
beam port and MCP.

OPERATIONAL EXPERIENCE
Maintenance

INTRODUCTION
The present configuration of the ISAC-II linac was installed in two phases, in 2006 [1] and 2010 [2]. Each phaseconsists of 20 superconducting quarter wave resonators designed to provide a total effective voltage of 40 MV forthe full linac. Phase-I houses two cavity types (β = 0.057
and β = 0.071) and consists of 5 cryomodules, while phaseII has a single cavity design (β = 0.110) and consists of
3 cryomodules. Each of the eight cryomodules houses asingle superconducting solenoid which provides transverse
focusing to the beam. The primary diagnostic devices formeasurements of beam velocity and phasing of cavities are
three flight time monitors (FTMs) downstream of the ISACII linac. The FTMs were originally installed in 2006 [3] andlater upgraded in 2011 [4] to have allow their positions tobe adjustable with respect to the ion beam.
Shown in Fig. 1, these diagnostics are composed of a
50 µm wire aligned along the axis of a grounded cylinder,
with a negative bias of ∼2 kV applied to the wire. The full
cylinder is actuated at 45 degrees from horizontal into thebeam, and the ions induce emission of secondary electrons
from the wire. These secondary electrons move away inthe electric field and are detected by a micro channel plate(MCP) detector with a time resolution of better than 100 ps.

∗ spencerkiy@triumf.ca

The FTMs have been regularly used since they were last
upgraded in 2011. During this time they have been regularly
used both for time of flight (TOF) measurements for calculating the beam velocity, as well as for setting cavity phases
during setup to experiments (phase-TOF scans). The latter
technique can be a fairly time consuming way to set cavity
phases, and with ISAC-II typically serving between 5 and
10 experiments per calendar year, the FTMs can often end
up being used for a total of 100 hours or more per year.
These monitors have proven to be very reliable over the
past 12 years. The only major repairs required over this time
has been the replacement of two of the three MCPs in 2016,
and the replacement of the tungsten wires due to increased
signs of field emission which can cause damage the MCPs.

Calibration
A calibration laser was installed in 2011 to measure andcorrect for small discrepancies of delays in cables of thethree separate monitors [4]. While it was initially envisioned
routinely re-check the calibration, the process was only repeated just recently during the first half of 2023. The sameprocedure described in Ref. [4] was carried out to measure
the arrival time of the laser at each of the three monitors,
with the resulting TOF spectrum shown in Fig. 2. Shown
below in Table 1, the recent calibration indeed showed smallchanges in the timing offsets relative to the typical flighttimes between the monitors.
The change in calibrations in Table 1 is attributable to2016 maintenance of the system, during which monitors
were removed and MCPs replaced. Plotting the relative

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1

1

1

0.8

0.8

0.8

0.6

0.6

0.6

0.4

0.4

0.4

0.2

0.2

0.2

0

0
-1

0

1

Rel. Intensity

regularly until 2020. The new method for low intensity TOF
measurements is:
1. Establish a tune with a stable beam at the desired energy.

0
7

1
0.8
0.6
0.4
0.2
0

8

9

37 38 39

Monitor 1
Monitor 2
Monitor 3

-5

0

5

10

15

2. Carefully document the necessary positions of the
FTMs for the established tune.
3. Scale the tune and switch to the radioactive beam.

20

25

30

35

40

4. Set MCP bias to its maximum allowable voltage andrestore FTM positions to their established values.
5. Collect for extended periods of time (up to 30 minutes).

Time [ns]Figure 2: Time-of-flight measurement of the calibration
laser.
Table 1: Summary of changes in FTM offsets, measured
with respect to the first monitor.
FTM

Change [ns]

2

-0.7

3

+0.04

difference of velocities calculated between different pairsof monitors (𝑉12 , 𝑉23 , 𝑉13 ) supports this, as it shows a step
function in the difference at the same date the work was
done.

Using this method, the energy of radioactive beams of

11 Li at 8 × 103 /s, 59 Cu at 5 × 103 /s, and 20 Mg at 9 × 103 /s

have been successfully measured so far.

UNCERTAINTIES AFFECTING FTMMEASUREMENTS
During the recent 2023 laser calibration, data was alsocollected to further investigate the uncertainties involved in
the FTM measurements. Previous data collected in 2022 [5]for repeat measurements of a beam of 20 Ne4+ showed nonnegligible deviations in calculated beam centroids at thethree monitors. This deviation increased when the transverse positions of the monitors were moved and there was aclear correlation observed between FTM position and beam
centroid for all three monitors as shown in Fig. 4.

Since 2020, the FTMs are now in regular use to measure
low intensity radioactive beams prior to delivery to experiments. An example of the TOF measurement for a 20 Mg
beam in 2023 is shown in Fig. 3.
1

1

1

0.8

0.8

0.8

0.6

0.6

0.6

0.4

0.4

0.4

0.2

0.2

0.2

0

0
4

5

1
0.8
0.6
0.4
0.2
0

284 285

Monitor 1
Monitor 2
Monitor 3

0

50

100

150

200

63.0
62.5
62.0
61.5
62
64
66
68
FTM transverse position [mm]058 59 60

Beam Centroid [ns]Low Intensity Measurements

Rel. Intensity

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250

300

Time [ns]Figure 3: Time-of-flight measurement of a beam of 20 Mg6+
at an intensity of 9 × 103 /s.
These monitors have in principle been capable of measuring intensities down to as low as 104 ions/s since their initialinstallation in 2006. However in practice this was not done

Figure 4: Correlation between transverse position of FTM
with measured beam centroid for a 20 Ne4+ beam.
As it can be difficult to assess whether the changes in centroid were due to drifts and correlations in the beam, some
repeat measurements were carried out with the calibration
laser for comparison to 20 Ne4+ data from 2022. The resulting
deviations in beam centroids at the three different monitors
are shown in Tables 2 (for 20 Ne4+ ) and 3 (for calibration
laser).
Table 2 shows a significant increase in deviation when the
FTM positions are varied vs fixed. This is apparent for bothcases of the ISAC-II linac off, and ISAC-II linac on (bunching cavity and 37 SC quarter-wave cavities on). In contrast
to this, the measurements with the laser show a much smaller

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Measurement

𝜎1 [ps] 𝜎2 [ps] 𝜎3 [ps]Linac off, static FTMs

300

300

400

Linac off, moving FTMs

700

700

800

Linac on, static FTMs

20

30

70

Linac on, moving FTMs

70

170

90

Table 3: Standard deviation of repeat measurements of thetime centroid of the calibration laser at each of the three
FTMs under various conditions in 2023. The conditions of
the laser are identical in each of the three cases.
Measurement

𝜎1 [ps]𝜎2 [ps]𝜎3 [ps]Static, over 2.5 hrs

6

0

5

Static, over 8.3 hrs

27

5

17

Moving, over 1.7 hrs

16

4

9

deviation. Although there does appear to be an increase indeviation if the monitor positions are varied, the effect ismuch smaller than for the beam-based measurements.
Based on this, the conclusion is that, due to the small
amount of beam that the 50 µm wire of the FTMs intersects,
it is likely measuring a real difference in the arrival time ofdifferent slices of beam as it is moved. There is a known chromatic bend in the upstream ISAC-I section that contributes
to (𝑥, 𝐸) coupling [6], which later due to the solenoids in
ISAC-II is mixed with the y dimension. Further to this, thequarter-wave resonators in use at ISAC-II are known to havea phase dependent vertical steering effect [7], that while
minimized may also be contributing to couplings between
the transverse and longitudinal coordinates.

Beam Database
Also described in Ref. [9], a beam database (beamDB)
has been developed which allows for a more structured andsearchable way to catalog measurements of an ion beam at
TRIUMF. Historically, built-in tools from EPICS and EDM(Extensible Display Manager) have been used to dump datafrom an EDM screen to a user-defined filename on a fileshareserver. Intensity measurements were saved solely as HTMLtables within an electronic logbook. This approach madeit difficult for others to find past measurements, to identify
which optics settings were in use for a measurement, or evento know the beam in use for a given measurement. In contrast to this, the implemented beamDB pairs the sequential
measurement process with a way to save data, collecting
multiple related diagnostic measurements together in a fixed
format together with the entire machine state for each measurement. In reference to operation of the ISAC-II linac,
this beamDB allows for saving of TOF profiles continuously
throughout a phase-energy scan.

Future Work
The central part of the HLA framework is an internal
web API for accessing EPICS. This web API handles multiple functions including: authentication, writing to EPICS,
maintaining process variable (PV) monitors with EPICS,
long-running sequential measurements, and read/write access to the beam database. An upgrade to the web API isunderway that will modularize the code, separating many of
the functions into different services that can be maintained
and re-deployed independently. Along with this will be theaddition of a new service to allow for optimization of EPICS
PVs, that can then be run within a sequential measurement.
This will be a significant improvement to the use of the
ISAC-II FTMs, as it will allow the count rates on the FTMs
to be maximized via optimization of their positions before
each set of profiles is saved (Fig. 5).

AUTOMATIC DATA COLLECTION
Automation
A High Level Application (HLA) task force was implemented at TRIUMF [8] in 2017 to try and automate various
tasks where possible and to utilize real-time physics simulations to expedite machine configuration changes as wellas improve reliability. This has led to the development of
sequential measurements [9], which consist of a series ofcommands that are managed by the HLA server and sent
to EPICS [10]. This allows for automated measurementsthat previously required highly repetitive manual changes
and saving of data either by operators or beam physicists.
One example of a task that this expedites are standard phaseenergy scans of the ISAC-II quarter wave resonators either
for calibration of cavity amplitudes, or for identifying thecorrect accelerating phase during setup to an experiment.

Figure 5: Sequential measurement process for collecting
calibration data for ISAC-II cavities.

MODEL BASED PHASING
OF ISAC-II LINAC
A recently completed MSc thesis [5] has utilized the
FTMs and other work summarized in this paper to begin
implementing model based phasing of the ISAC-II linac.
This involved two main parts - the first was to collect phaseenergy calibration data for each of the 40 cavities in the
ISAC-II linac. From this, the scaling between the control
system amplitude and the physical field of a cavity could be
calculated, as well as the phase offsets of all cavities. These

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Table 2: Standard deviation of repeat measurements of thetime centroid of the 20 Ne4+ beam at each of the three FTMs
under various conditions in 2022 (reproduced from Ref. [5]).

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E/u [MeV/u]

1.8

Model
Measured

1.7
1.6
1.5
1.4
1.3

Residuals

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0.02
0
-0.02
-200

-150

-100

-50

0
Phase [deg]

50

100

150

200

Figure 6: Energy-phase scan and resulting model fit for first linac cavity.
are then input into a model of the machine in an envelopecode, TRANSOPTR [11]. This model was then used for thesecond part of this project, verification of the new approach- using the model to predict the necessary phases for given
beam properties (mass, charge, input energy, and requested
output energy).
The phase-energy calibration data during this work (andhistorically in general) has been difficult to collect due tothe relative positioning of the FTMs with respect to the 40
linac cavities. The 40 cavities are spaced over approximately
18.6 metres, with the three FTMs at distances of 2.74, 4.93,
and 13.99 metres downstream of the last cavity. There are
no longitudinal diagnostics within the linac, and as a result
the beam must be transported by up to 32.6 metres (in thecase of the first cavity) between the cavity being adjusted
and the last FTM. Further to this, the relative energy gain isquite significant at the start of the linac, with the first cavity
increasing the beam energy by between 10 and 30%. For the
first half of the linac, especially the first few cavities, thismeans that as the phase is changed, the necessary settings
for the solenoids for transport are changing significantly
due to energy alone, on top of which the RF focusing effectis changing by a non-negligible amount. The Facility for
Rare Isotope Beams (FRIB), has implemented automatedcalibration of the linac model and instant phase setting [12]that has a number of similarities to the approach used at
ISAC-II. However by contrast, FRIB has a BPM after eachcryomodule, and a diagnostics-per-cavity number that ismuch higher than ISAC-II.
Recently, semi-automated phase-energy scans have beensuccessfully tested, using the model to calculate necessarychanges to optics as the phase is scanned, and automaticallysaving data before moving to the next phase. The only manual adjustments still in use were occasional optimizationof the FTM positions and steering through the linac. This
allows for much more detailed scans, such as that shown in
Fig. 6, which in turn improve the accuracy of the model. The
data shown in Fig. 6 took approximately one hour to collectcompared to a manual scan for the same cavity a year early
which took over four hours. The two calibrations taken one
year apart using different ion beams and amplitude setpoints
agreed to within 2%.

CONCLUSION
The flight time monitors in ISAC-II have been reliablediagnostics over the past 17 years for both measurement of
the beam velocity as well as phasing of cavities. Recent
calibrations have been done to ensure their continued accuracy, and various other improvements summarized here
have contributed to their use in automated tuning tools. The
ISAC-II flight time monitors are now being used to collectdetailed phase-energy scans which are automated and saved
to a beam database. The resulting calibrations from thesedetailed scans are expected to continue increasing the accuracy of the ISAC-II model and hence the model-basedtuning techniques which rely on it.

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