Pulse-stretching out of the CANREB EBIS

The CANadian Rare isotope facility with Electron-Beam ion source (CANREB) at TRIUMF is set to deliver rare isotope beams in high charge states. In the Electron Beam Ion Source (EBIS) ions are charge-bred by collisions with an electron beam of up to 500 mA. A strong magnetic field (up to 6T) maximizes the overlap between ions and electron beam and increases the breeding efficiency. Ion confinement is maintained by a combination of an electrostatic field and the electron beam space-charge potential. Ions are released by lowering the trapping potential with a step function. The system is operated at a pulse repetition frequency up to 100 Hz. Due to the short trap length, this fast extraction scheme produces pulses shorter than 10 µs with high instantaneous rates that can saturate detectors in experiments. Stretching the pulse can be done using a slowly varying voltage function to modify trap electrode potentials instead of a step function. The ideal function produces a pulse with a flat top distribution and can be calculated by knowing the ion energy distribution inside the trap. The latest pulse-stretching results will be discussed including the latest pulse duration up to 1.4 ms that have been produced. The slow extraction scheme has also been used for a measurement of the effective energy distribution of the ions inside the trap.


Introduction and the CANREB facility
The CANadian Rare isotope facility with Electron Beam ion source (CANREB) is a part of the new Advance Radioactive IsotopE Laboratory (ARIEL) at TRIUMF [1].The goal of CANREB is to deliver radioactive beams in high intensities, high purity and high charge states to experiments.The Electron Beam Ion Source (EBIS) charge-breeds rare isotopes by electronion collisions.Fig. 1 shows a simplified sketch of the CANREB EBIS.The electron gun is designed to produce up to 500 mA of electron current.Two Helmholtz coils produce a magnetic field up to 6 T to compress the electron beam, increasing the electron current density inside the trap and therefore the charge breeding capabilities.The EBIS is a pulsed device with maximum repetition frequency of 100 Hz.Before injection into the EBIS continuous beams from the Isotope Separator and ACcelerator (ISAC), the OffLine Ion Source (OLIS) or in the future ARIEL target stations are bunched by the ARIEL Radiofrequency Quadrupole Buncher (ARQB).After the EBIS a Nier-type spectrometer is used to select the charge state of interest.A detailed description of the CANREB EBIS can be found in [2].Commissioning results of the CANREB facility have been presented in [3]. 1, between a high potential V high (470 V or 490 V depending on cases) to a low potential V low (160 or 290 V depending on cases).This extraction scheme is referenced as "switching" or "fast extraction".In practice the switching time is limited by the slew rate of the amplifier delivering the voltage, although it has been measured to be shorter than 1 µs.The ion bunch can contain up to 10 6 particles of the desired charge state.When extracted via switching, the natural length of this bunch has been measured to be shorter than 10 µs.The corresponding in-bunch frequency of about 100 GHz or larger is hard to handle for experiments.This is why "pulse-stretching", also known as "slow extraction" is being implemented.There a slow-varying function is used instead of a step function for extraction.In the ideal case this extraction function is attuned to the energy distribution of ions inside the trap and extracts them at a constant rate.

Setup
For slow extraction the CANREB setup is modified by adding an arbitrary function generator (AFG) before the extraction electrode voltage amplifier.The previously amplified signal (the step function used for fast extraction) is in that case the triggering signal for the AFG.

Diagnostics
Slow extraction measurements are done after the Nier spectrometer and therefore only include one charge state.The measurements discussed in the present work were conducted using stable 85 Rb from the ARIEL test ion source in charge states between 9+ and 22+.The pulse signal is measured using a Channeltron single channel Electron Multiplier detector (CEM), with a time resolution as low as 10 ns, coupled with a MCFC discriminator to convert the analog signal into a logical one.Due to the potential high rates in short pulses and the tendency of the CEM to saturation, only beams intensity under 0.5 epA, or ≈ 3000 ions per pulse or less, are used.Data acquisition is done with an oscilloscope during setup and for qualitative measurements, and with a VME scaler for quantitative measurements.For the latter data is saved in a recording window of 60 ms with a resolution of 1 µs and each measurement repeated 10 times.

Extraction with linear ramps
The simplest implementation of pulse-stretching is to use linear voltage ramps instead of a step function.For increasingly slower ramps, ions at a specific energy are released with an increasing delay with respect to the opening trigger, while an increase in pulse length due to slower ramps is a function of the energy spread of ions inside the trap.Such extraction was used as a preliminary pulse-stretching test and to estimate the trapped ions energy.Two measurement campaigns were performed, in June 2022 and September 2022.In the first, the ions were estimated to be distributed between between 450 to 470 V, with a maximum axial trapping potential of 470 V.This showed that ions were likely coming in with a significant surplus of kinetic energy, which was consistent with the poor trapping efficiency.After adjusting the injected beam energy, the second campaign data showed an ion energy distributed between 290 and 310 V, very close to the floor of the trap, which was consistent with the improved trapping efficiencies.This measurement repeated with 9+, 15+ and 22+ 85 Rb ions, leading to similar conclusions in each case.

Targeted extraction
With previous measurements showing that ions are mostly trapped at the bottom of the electrostatic potentials, the next step is to use an extraction function with a slow rate around that region.A succession of small and short step functions was used, resulting in a 85 Rb 9+ pulse length of about 1.4 ms (fig.2).While an improvement, this extraction function is not perfect, as shown by the presence of an additional peak ahead of the long pulse.

Energy distribution measurements
The energy distribution of 85 Rb 9+ ions inside the trap was measured with a method derived from [4].The idea is to split the ion bunch into two components by using a three-level extraction function: trap closed (V = V high ), partially open (V = V p ) over a time t p , and open (V = V low ).This way the first batch of ions measured is coming from trapping regions between V high and V p , while the second batch (separated from the first by a time t p ) comes from trapping regions between V p and V low .By repeating this measurement for successive voltages V p such that V high ≥ V p ≥ V low , we can measure the cumulative distribution of ions.Due to the potential saturation mentioned in 2.1, voltage was ramped down between plateau instead of simply using a 3-step function.An example of these waveforms and resulting time profile is shown on fig. 3.
From previous measurements we know that the upper region of the trap is mostly empty, therefore very few measurements where targeting those voltages.Conversely the lower region of the trap was scanned in smaller voltage steps to try and capture the features of the energy distribution.2.5 V steps where chosen, corresponding to 0.05 V in the extraction function applied by the AFG.The ratio of the number of ions in peak 1 and 2 with respect to plateau voltage is shown in fig. 4. Subtracting two subsequent values for peak 1 yield the energy distribution shown on the right-hand side.The associated uncertainty is however quite high, as shown by the non-physical negative intensity at the 302.5 V mark.

Constant rate extraction
With a known energy distribution, the "ideal" extraction function, i.e. the extraction function leading to a constant rate, can be calculated using the method from [4]: (i) Measuring the ion time distribution resulting from extraction with a reference function (ii) Convert to voltage distribution using the reference function (iii) Integrate the voltage distribution (iv) Invert the axis and scale to the desired rate A python code is being developed to that end.It is designed to take as an input either (1) an effective energy distribution of ions inside the trap or (2) an ion time profile obtained with a known reference extraction function.In both cases the other "missing" input is also calculated.The final outputs are the calculated extraction function and the predicted time profile resulting from extraction with that function.Fig. 5 shows an example output.In that case a known sum of two gaussian functions is used as an input energy distribution.

Summary and future work
Pulse-stretching has been implemented at the CANREB EBIS, with slow extraction functions being used both as a diagnostics tool and to increase the length of the ion pulse.Preliminary tests with a 85 Rb beam have shown ion energies very close to the floor of the trap.The pulse length has been increased from < 10 µs to ≈ 1.4 ms.A Python code has also been developed to calculate a function leading to a constant rate of extracted particles for a given energy distribution inside the

Figure 2 :
Figure 2: (a) Extraction function tailored to the estimated trapping region and (b) resulting ion time profile showing a pulse length of 1.4 ms

Figure 3 :
Figure 3: (a) Example of a waveform used to split the beam into two components and (b) with the measured CEM signal in yellow and the EBIS opening trigger in green

Figure 4 :
Figure 4: Ratios of the two peaks in the beam profile as a function of the plateau voltage V p (left) and resulting calculated energy distribution (right).

Figure 5 :
Figure 5: Output of python calculation code.Left: double gaussian energy distribution, reference extraction function and resulting pulse profile.Right: calculated extraction function for constant rate with assumed distribution, extraction function scaled to 2 ms length and resulting time pulse profile for the assumed energy distribution.