# Presentation: 008386

## Measuring Beam Intensity With Toroids

- The operation of toroids and their calibration
- Aisha Ibrahim
- July 28, 2004

## Introduction

- Toroids are intensity devices used to measure a pulsed beam current.
- The Toroid Intensity Monitor Integrator Module is designed to integrate the total area under a beam-induced toroid signal to determine the beam intensity.
- These provide a way to monitor transfer efficiencies between two accelerators and/or to ensure intensity are within safety or operational envelopes.

- Aisha Ibrahim <cadornaa@fnal.gov>

- July 28, 2004

## Basic Installation

- Basically, it consists of a vacuum tube with a ceramic piece and transformer cores.
- Pearson Models 3100 (3.5” ID) and 2864 (4.875” ID)
- Tap-wound cores
- 10Hz to 26MHz Bandwidth
- 0.5 Volts/Amp into 50 Ohms Load
- 0.033 Amp-Sec max / 41 Volts-Sec max at output
- Electrically isolated from beam pipe and tunnel ground
- 200-1300ft Cabling between toroid and integrator module
- 78 Ohms balanced twin-ax for induced signals
- 3/8” Heliax for calibration test pulses

- Aisha Ibrahim <cadornaa@fnal.gov>

- July 28, 2004

## Ceramic Break

- Since the magnetic field of the beam is attenuated outside a continuous, conducting vacuum chamber, a beam current monitor needs a “window to the beam”.
- Often a ceramic piece is inserted in series with an otherwise continuous beam section. This interruption along the beam tube forces wall currents to find a path outside the vacuum chamber.
- Zshunt can be added to limit the gap impedance and damp potential resonances

- Aisha Ibrahim <cadornaa@fnal.gov>

- July 28, 2004

## Gap /Wall Currents Model

- This is an example modeling a current monitor enclosed in a housing over a gap with shunting elements.

- Aisha Ibrahim <cadornaa@fnal.gov>

- July 28, 2004

## Gap Positioning Relative to Transformer

- All paths by which the wall currents bypass the gap MUST enclose the transformer.
- The transformer should be as close to gap as possible for best high frequency performance, but doesn’t have to straddle the gap physically.
- In each case, wall currents can pass around gap via a path connecting to the beam tube at either side of the break.
- A & B are acceptable positions because current bypass path enclose transformer.
- C&D are NOT acceptable because ground or other connections short-circuit the gap via paths not enclosed by the monitor. Wall currents interfere with beam current measurements.

- Aisha Ibrahim <cadornaa@fnal.gov>

- July 28, 2004

## Basic Installation

- 10MHz 3rd order low-pass filter used at Pearson output
- A RC network shunt the ceramic gap to further provide noise immunity.
- Straps or full-housing controls the side effects of gap impedance and guides wall/image currents from one side of the ceramic break to the other.
- Previously, toroids were electrically isolated with Kapton tape, while beam image currents were handled with braided conductors clamped to the beam pipe.
-  New mounting hardware mechanically supports, electrically isolates, provides a robust connection for the calibrate winding and output signals, and protects the toroid.

- Aisha Ibrahim <cadornaa@fnal.gov>

- July 28, 2004

## Theoretical Overview

- Passing through the center of the ferrite ring, the beam forms a single-turn primary coil of the transformer.
- An N-turn secondary coil is wound around the core (either a ferrite ring or tape-wound cores).
- Using  both Ohm’s law and transformer relationships			*N = *_*(# of secondary turns)*_* **	**       (# of primary turns)  **	**N* I** Secondary**	**= I **Primary** = I **Beam*
*	**	**V = I **Secondary** * R = I **Beam** * R / N*
- For a constant N, the output voltage is linear with respect to the beam current.
- The electronics is designed to integrate the total area under this beam-induced signal to determine the beam intensity.
- It is AC coupled to beam current and have no DC response.

- Aisha Ibrahim <cadornaa@fnal.gov>

- July 28, 2004

## Toroid Intensity Monitor Integrator Module (TIMI)

*<Schematic on Slide 12 >*
- The first stage receives a signal transmitted over twin-ax cable.
- This signal passes through a common-mode choke to filter any additive noise induced over the transmission lines, an impedance matching network to handle cable termination and reflections, and then a differential receiver amplifier.
- This differential-to-single-ended amplifier is characterized to have a high common-mode rejection ratio (70dB @10MHz).
- This minimizes the corruption by external noise sources or crosstalk.
- The amplifier also has tuneable gains to adjust for losses in the transmission lines or for different full-scale intensity ranges.

- Aisha Ibrahim <cadornaa@fnal.gov>

- July 28, 2004

## Toroid Intensity Monitor Integrator Module (TIMI)

*<Schematic on Slide 12 >*
- The second stage addresses the baseline of the AC-coupled signal. It is composed of a sample-and-hold (S/H) amplifier and a differential amplifier.
- A 300nsec minimum acquisition time must be allotted for acquiring and sampling the baseline between integrations.
- Further, the S/H amplifier is characterized with a slow 0.02μV/μs droop rate, allowing the sampled baseline to be held relatively steadily.
- Also, due to the noise contribution of the S/H amplifier, this sampled signal is filtered at 10KHz.  Assuming that the baseline drifts very slowly or not at all, this effectively samples the baseline much slower than the actual beam signal.
- Once sampled, the baseline is subtracted from the original signal using a differential amplifier.
-  The differential input range of the differential amplifier must accommodate differences between peaks in the beam bunches and the baseline.

- Aisha Ibrahim <cadornaa@fnal.gov>

- July 28, 2004

## Toroid Intensity Monitor Integrator Module (TIMI)

*<Schematic on Slide 12 >*
- Next, this baseline-corrected signal is feed into the integrator in a switched-capacitor configuration.
- The time constant determined by the feedback resistor and capacitor needs to be much greater than the typical gate width.
- This minimizes the intrinsic exponential droop error of non-ideal integrators during the hold state.
- In addition, errors due to noise also vary proportionally to the square root of the gate width.
-  Serving as an input buffer to the A/D converter, this amplifier has a fast settling time (90nsec to 0.1%) as well as a high slew rate (230V/msec uncompensated).

- Aisha Ibrahim <cadornaa@fnal.gov>

- July 28, 2004

## Toroid Intensity Monitor Integrator Module (TIMI)

*<Schematic on Slide 12 >*
- From this integrated signal, there 2 distinguishable intensity outputs.
- One is a full 16-bit digital intensity reading. The integrated signal is then passed to the next stage, where is it converted to a digital 16-bit equivalent (A/D) and then back to analog (D/A). With a 250 kHz sampling rate, the A/D acquisition and conversion time is at most 4μsec. The 16-bit D/A has a bipolar output rate of _+_10V and has a typical settling time for 1 LSB step is 2.5μsec.
- The other is an analog intensity reading. This analog output is put through a non-inverting unity operational amplifier. This low noise op-amp has a maximum offset voltage drift of 0.1μ/oC and a maximum offset voltage of 25μV at 25oC. This eliminates the need of external offset voltage adjustments and increases system accuracy over temperature.

- Aisha Ibrahim <cadornaa@fnal.gov>

- July 28, 2004

## TIMI Schematic Summary

- Aisha Ibrahim <cadornaa@fnal.gov>

- July 28, 2004

## Triggering & Gating

- Each toroid integrator has at least one trigger.
- Each trigger is determined from a list of Reflected TCLK Events as well as delay in μsec.
- The corresponding toroid is set to start its integration window “some delay” after an event occurs.
- Also, each trigger can be enabled or disabled by toggling the asterisk at the end of the line.
- When integration window is active, the integrator module starts integrating the beam signal received from the toroid.
- “Local Gating Mode”: 0.1-99.9 μsec
- “Remote Gating Mode”: Follows width of triggering pulse with about 180nsec delay and 500nsec minimum size
- Typically, transfer line toroids use “local mode”, set to 11.1usec.
- The RMS noise output behaving proportionally to the gate
- Baseline Subtraction is available
- Requires a separate TTL timing signal and 300 nsec min acquisition time

- Aisha Ibrahim <cadornaa@fnal.gov>

- July 28, 2004

## Example of Gating/Timing

- Channel 2 :  ~2.8μsec Integration Window produced using “Remote Mode”
- For transfer line toroids, this window would typically be a 11.1μsec wide.
- Channel 4 : 300nsec Gate for Baseline sample and hold
- Channel 3 : Beam signal from Wall Current Monitor (WCM) consisting of 4 bunches in RR
- For transfer line toroids, the signal would be a 1.6μsec pulse.

- Aisha Ibrahim <cadornaa@fnal.gov>

- July 28, 2004

## Calibration Procedure

- Dedicated Equipment : HP 33120A
- 15MHz Function/Arbitrary Waveform Generator.
- Its internal resistance was verified to be 50.540Ω.
- Verified accuracy from equipment manual: There a 0.5% change in gain for 1% change in output termination accuracy.
- It is used to send a known pulsed waveform to a single turn winding around the toroid in tunnel.
- CALPULSE was created and consists of 11000 points and models a 1.6usec pulse at 91KHz.
- Although the calibration winding is terminated with 50Ohms, this resistance is measured and verified using a DVM.
- $0F triggers are used to time in the integration gate and the pulsed test signal.

- Aisha Ibrahim <cadornaa@fnal.gov>

- July 28, 2004

## Calibration Procedure

- The voltage is typically stepped from 0-5Volts in 1 volt increments.
- Primarily for calibration/testing scenarios, a time-averaged ACNET reading is available.
- This is the last 100 data points of the MADC reading sampled at 15Hz.
- It will take approx 7-10 seconds for the reading to level out; a fast-time-plot can be used to verify this.
- A “least squares fit” is done between the ACNET measured value and the calculated, expected value.
- Error Deviation of the measurements and %change is analyzed
- Gain/Offset adjustments are DABBELED into ACNET database

- Aisha Ibrahim <cadornaa@fnal.gov>

- July 28, 2004

## TYPICAL Transfer Line Toroids

| Full-scale Intensity * | 1.03e13 (9.7V/1e13) |
| --- | --- |
| Integrating Time Constant* | 330nsec |
| Integrator Low Corner Frequency* | 48.2Hz ( ~21msec) |
| Integrator Droop Rate  1 – e ( - gate /  ) | 0.05% |
| * Can be modified for a given application |  |
| Magnetic Field Sensitivity | 6E9/Gauss (f>100Hz) |
| Temperature Sensitivity | -0.01/oF error from Integrator
+0.02%/oF error from Pearson  |
|  |  |
| Short-term or Pulse-pulse “RMS” variation/error  | ±1E9  |
| Observed long-term systematic drift | ±1E11 over several months  |

- Aisha Ibrahim <cadornaa@fnal.gov>

- July 28, 2004

## 08/01/03 Calibration Studies using TIMI in RR

- As part of a partial calibration effort, high intensity beam is injected in to a 1.6μsec wide RR barrier bucket and scrapped down from 120e10 to about 0.5e10.
- Calibration curves calculated R:DBBIN1 to vary with R:IBEAM by about 4%

- Aisha Ibrahim <cadornaa@fnal.gov>

- July 28, 2004

## 08/11/03 Response Studies using TIMI in RR

- Stacked about 30e11protons in RR.
- Debunched them in a barrier bucket and set the gating properly to look at its response.
- Found that the R:DBBIN1 and R:IBEAM follow very closely within 1%

- Aisha Ibrahim <cadornaa@fnal.gov>

- July 28, 2004

## 08/27/03 Response Studies using TIMI in RR

- As pbar shots were injected into the RR barrier bucket, the integrator module showed intensity. As the beam was moved and fell out of the buckets, the intensity dropped.
- Notice that injected beam was not on target on previous transfers. Hence we expected some dc beam in RR which affected the S/H signals and  caused the module to underestimate the beam intensity. For cleaner beam transfers this problem should go away.

- Aisha Ibrahim <cadornaa@fnal.gov>

- July 28, 2004

## 01/08/04 Response Studies using TIMI in RR

- Pbar beam in RR before spreading the beam around the machine.
- R:ADBBI1 provides only the intensity at the barrier bucket for injection
- R:ADBBI2 is positioned to read the intensity for the entire RR

- Aisha Ibrahim <cadornaa@fnal.gov>

- July 28, 2004

## Future Developments

- Planned for August Shutdown
- Install improved mounting hardware for MI/RR transfer line toroids
- Pull in Trumpeter twin-ax signal cables to replace RG108
- Modify electronics to improve temperature sensitivity and long-term stability
- Track toroid efficiencies (Lumberjack vs. SDA)
- Cross-calibrate toroids along a single transfer line
- Devise an automatic calibration process
- Looking into how Columbia module is used for EBERM

- Aisha Ibrahim <cadornaa@fnal.gov>

- July 28, 2004