Control electronics

The individual rods must be equipped with rf trapping voltages (Vrf), DC bias voltages

(UDC), and the ability for high voltage (UHV) pulse sequencing for extracting the ions into

the time-of-ight apparatus. These requirements are made more stringent by the need for phase and amplitude matching of the applied rf voltages, as well as proper synchronization of the extraction pulses across the four rods. If this synchronization is performed improperly, the ions may experience unnecessary micromotion or may be ineciently coupled into the mass-spectrometer.

These requirements inspired the development of a sophisticated set of electronics, known as MOTion drive units, that have now been exported to a variety of ion trapping groups throughout the nation. The reader is referred to Ref. [SSY16] for a complete discussion of the MOTion drive units, but a brief overview will be presented here.

Figure 6.3: Simplied circuit diagram of MOTion drive unit

The circuit is divided into a primary low-voltage (left) and secondary high-voltage (right) side, which are isolated from each other through a toroidal transformer. The low-voltage side consists of rf ampliers and the primary winding of the rf transformer (blue), as well as timing and damping circuitry (green). The high-voltage side consists of the secondary winding of the transformer along with capacitors that collectively form the resonator circuit (red). In addition, damping and HV pulsing circuitry (purple) and a UDC bias supply is also present on the secondary side. This gure

6.2.2.1 Drive units

There are twelve drive units total, one unit for each segment of the trap, and a simplied circuit diagram for the units is presented in Fig. 6.3 (borrowed from Ref. [SSY16]). Each drive unit is supplied an input rf voltage (supplied via a direct digital synthesis board (DDS), described in 6.2.2.2), which is initially RC-ltered before subsequently being amplied by both a preamplier and a power amplier. The power amplier then drives the primary side of a toroidal transformer with a total turn-number of a few tens.

The secondary side of the transformer consists of an LC circuit, where the total capac- itance of the resonant circuit is set collectively by the capacitance of the output cables, rf electrodes, vacuum feedthroughs, and lastly, an adjustable trim capacitor than can be used to ne-tune the resonant frequency (typically set to ≈ 2π × 680 kHz). One end of the sec- ondary side of the transformer is connected to the rf trap electrode, while the other end is rf grounded via a capacitor, which allows biasing of the rf trap segment with DC voltages, as is required for the HV pulsing and micromotion compensation.

UDC is supplied by a DC power supply that is fed through a low-pass lter and an HV

diode (as a protection from UHV to rf ground). Similarly, UHV is applied to rf ground

by activating a MOSFET which is supplied by a HV power supply and bypassed with a capacitor. As a result, not only the trap electrode but the entire secondary side of the drive

unit is biased with UDC permanently and UHV during pulsing.

Merely turning o the rf source prior to HV pulsing would result in a ring-down time of ∼ 10 µs on the secondary side of the transformer, causing problematic eects during ion extraction. To remedy these eects, active damping of the resonator on both its primary and secondary side is achieved using MOSFETs, which essentially short the windings of the rf transformer. Logic circuitry allows adjustment of the delay and duration of both the damping and HV pulsing sequences, and these parameters are further synchronized across all drive units for maximum ToF-MS coupling eciency.

6.2.2.2 Timing control

The rf signals are generated by four direct digital synthesis (DDS) devices, which each have four channels whose individual frequency, phase, and amplitude can be set programatically. All four DDS devices receive the same 500 MHz reference clock signal for synchronization of the outputs, and further, the output cables are chosen to be of similar lengths to ensure that the signals reach the drive boxes simultaneously.

Additional clocks are also introduced into the system to ensure the buer input/output sequences and the synchronization of the digital interface of the DDS devices are performed properly [SSY16]. Perhaps most crucially, the additional clocks help ensure that HV pulsing sequences are initiated at consistent rf phases. This helps ensure that the potential gradient the ions are accelerated through during ToF-MS extraction is consistently maintained even as other parameters associated with the rf signal are varied.

These synchronization devices are collectively managed by a microcontroller, which can be sent commands over a serial-to-USB interface coupled to the main experimental control computer or also accessed through the lab wi- network.

6.2.2.3 Wiring

The outputs of the drive units are connected to the vacuum chamber over ≈ 175 cm long low-capacitance coaxial cables. These cables have 75Ω mini-SMB connectors and are plugged into one of four printed circuit board (PCB) wiring units positioned on the 1.33 CF-anges on the vacuum chamber. The PCB interfaces possess receptacles that mate the electrical pins of the vacuum feedthroughs to SMB connectors, and the units also provide the ground connection to the vacuum chamber. On the vacuum side, three wires per feedthrough are connected to the three segments of an electrode (leaving one wire per feedthrough unused). The interface PCBs also include capacitive pickups, which sample a small fraction of the rf/HV voltage supplied to the segments and can be used for signal monitoring. The pickup signal allows the measurement of the input voltages at each segment with a probe ratio of ≈ 1000 : 1 (as measured with an oscilloscope with 1 MΩ impedance and typical cable

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 0 500 1000 1500 Time(
s) Volt age (V ) -0.4 -0.2 0.0 0.2 0.4 -150 -100 -50 0 50 100 150 Time(
s) Volt age (V ) -0.4 -0.2 0.0 0.2 0.4 -150 -100 -50 0 50 100 150 Time(
s) Volt age (V ) -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 0 500 1000 1500 Time(
s) Volt age (V )         amplitude/phase offset initiation phase too early unmatched HV overshoot rf zero-point

Figure 6.4: Optimized and unoptimized MOTion drive unit outputs

(a) and (c) display unoptimized signals from the MOTion drive boxes during both rf output and HV pulsing, as measured through pickup electrodes. In (a), the relative phase and amplitude between the rf applied to diagonally opposed rods in the LQT can be seen to dier. In (c), the asymptotic HV values between the front and back rod pairs both appear to diverge and the phase initiation time for the non-rf rods is chosen too early such that a suboptimal voltage `overshoot' occurs that will likely compromise the mass resolution and detection eciency of the ToF-MS. (b) and (d) display optimized rf output and HV pulse sequences, respectively, that avoid these issues to a large degree.

lengths). These ratios are calibrated against an HV probe (Agilent 10076B) to better than 1% (relative).

6.2.2.4 Optimization

While monitoring the pickup electrodes on an oscilloscope, commands can be sent to the microcontroller to adjust the phase and voltage amplitude of each rf channel such that the produced signals are properly matched. Further, the parameters of the HV extraction

sequence, such as the rf phase at which the sequence is initiated (see Ref. [SCR12] for further details), can also be controlled. Fig. 6.4 displays output traces for both rf output as well as HV extraction sequences both before and after such optimization has been performed.

For the voltage/phase rf optimization, the oscilloscope used to monitor the pickup traces can be interfaced with the experimental control computer through LabVIEW, allowing the phase and amplitude of the rf electrodes to be programmatically varied to minimize their mismatch with one another.

The HV extraction parameters can also be optimized to maximize the detection eciency and mass resolution of the ToF-MS. There are two main parameters to optimize here - the nal voltage each rod will reach and the phase at which the pulse sequence is initiated. The pulse initiation phases and amplitudes are initially coarsely chosen to match the following criteria.

Firstly, for the rods with rf applied (α and β), the initiation phases should be chosen such that the pulse sequence begins at roughly at the rf zero point, where the rf voltages cross the 0 V threshold with a positive slope.

Secondly, the HV values themselves are also adjusted so that each pair of front/back rods asymptote to approximately the same value, as this allows for the ions to be ejected most directly along the ToF-MS axis. Simulations conducted in the SIMION software suite demonstrate that, during an extraction sequence, the ions studied in this work exit the LQT in approximately ∼ 1 µs; however, the majority of their motion occurs in the latter 500 ns of this timeframe, when the HV pulses have essentially reached their steady state value (10%-90% rise time of ≈ 250 ns). Thus, choosing the HV amplitudes such that they are well-matched asymptotically is most critical for ToF-MS performance; however, slight osets from these values may be chosen to account for physical misalignments between the LQT and ToF-MS by `steering' the ions into the entrance of the spectrometer.

Lastly, the initiation phase of the non-rf rods (δ and γ) are adjusted such that the pulse signals for these rods intersect those of the rf rods (α and β) pulses at roughly half of their respective maximum amplitudes (see Fig. 6.4). This condition is chosen for the following

reason. The rise time of the HV pulse is slightly dierent for the dierent MOTion drive units. Primarily, this is because the the MOTion drive units supplying the rf voltage to the

αand β rods have a resonant frequency of ∼ 2π × 680 kHz, while the drive units that supply

the HV to the non-rf δ and γ rods have resonant frequencies of ∼ 2π × 1800 kHz. The boxes have dierent trapping frequencies to allow us to switch between the two settings if desired; however, in an approximate sense, their dierent associated capacitances can be understood as causing the two units to have dierent RC rise times.

Therefore, due to these dierences in rise times, for a given pair of front/back rods, if the initiation phase is chosen so that the pulses are well-matched during the beginning of the HV sequence, an `overshoot' will occur towards the end of the sequence (see Fig. 6.4c), and vice versa. In practice, choosing the initiation phase such that the voltages intersect at roughly half of their asymptotic values balances the amount of `overshooting' that occurs during early and late portions of the pulse sequence; however, ner optimization is typically performed by observing experimental mass-spectra and adjusting HV phase and amplitude parameters to maximize the corresponding ToF-MS mass-resolution. Ultimately, however, initiation phase selection is a relatively minor consideration when compared to the asymptotic amplitude matching criteria, for reasons mentioned above.

The HV and low-voltage DC signals needed for biasing of the rf electrodes are provided by separate homebuilt, low-noise DC (0-50 V) and HV (0-2 kV) power supplies, engineered by Peter Yu and Christian Schneider. These power supplies are heavily low-pass ltered to ensure that the resultant output signals have minimal frequency components at secular resonances of the ion trap. Table 6.1 provides typical voltage values at which the LQT is operated as well as other parameters related to the trap.