# Presentation: 363826

## REVIEW OF FAST BEAM CHOPPINGF. Caspers CERN AB-RF-FB

- Introduction
- Review of several fast chopping systems
- ESS-RAL
- LANL-SNS
- JAERI
- CERN-SPL
- Discussion
- Conclusion

## Introduction

- Beam choppers are typically used for β =  v/c values between 0.05 and 0.1
- Chopping is oft often done by deflecting the undesired part of the beam into a dump or against of wall of the MEBT (medium energy beam transfer line)
- Deflecting elements can be implemented as slow wave structures (meander type delay line)
- Also properly phased individual electrodes are used for chopping
- Fast Energy modulation is another means to cut out parts of the beam (driving it beyond momentum acceptance of a downstream section)
- There is no unique solution! Large range of parameters and parameter setting requirements depending on application

## Review of several fast chopping systems: RAL-ESS

| Key parameters for RAL-ESS | Prechopper | Chopper |
| --- | --- | --- |
| Beam energy | 2.5 MeV |  |
| Chopping factor | 30 % (ring stacking regime) |  |
| Electrode voltage |  2.2 kV |  6.0 kV |
| Electrode length | 340 mm | 360  mm |
| Electrode gap | 14 mm | 11 mm |
| Deflection angle | 16 mrad | 66 mrad |
| Pulse transit. (10-90%) | ~ 2 ns | ~12 ns |
| Pulse duration | 12 ns | 240 ns-0.1 ms |
| Pulse repetition freq. | 2.4 MHz | 1.2 MHz |
| Burst duration | 1.5 ms |  |
| Load impedance | 50  | 35 pF / 60 nH |
| Repetition rate | 50 Hz (two systems @ 25 Hz) |  |
| Beam power on dump | 2.5 kW (2 systems @1.25 kW) |  |

## The RAL-ESS system: ESS Front-end 

## The RAL-ESS system: ESS-MEBT linewith “Tandem” chopper

## The RAL-ESS system: ESS-MEBT linewith “Tandem” chopper details

## The RAL-ESS system: ESS-MEBT linewith “Tandem” chopper  timing

## The RAL-ESS system: Tandem chopperfast and slow chopping scheme

- GPT (General Particle Tracer )
- Simulations for fast chopping
- Bunch 1-3 and 63-66 chopped

- GPT (General Particle Tracer )
- Simulations for slow chopping
- Bunches 4-62 are chopped

## The RAL-ESS system: Tandem chopperdriver (pulse generation) scheme

- The slow-wave electrodes (meander type structure) of chopper 1 are driven by fast transition (~ 2 ns), short duration (~ 12 ns) quasi-trapezoidal, uni-polar high voltage pulses of ( 1.4 kV) into 50  loads. The modular configuration makes extensive use of high power transmission line transformers (TLT’s) for efficient wide-band impedance transformation and combination of the outputs of 18 solid-state high voltage pulse generator cards, consisting of two, nine card modules
- With additional modules the output voltage can be increased to  2.2 kV
- The lumped element slow wave electrodes of chopper 2, are driven by  12 ns transition, uni-polar, trapezoidal, high voltage pulses (+6.0 and –6.0 kV). Pulse duration will be programmable in the 240 ns to 0.1 ms range. The 120W air-cooled modules will be close-coupled to the electrodes, to preserve pulse fidelity.

## The LANL-SNS system: key parameters

| Parameter | Value | Comments |
| --- | --- | --- |
| Beam energy | 2.5 MeV | β=0.073 |
| Structure length | 35 cm |  |
| Meander width | 96 mm |  |
| Gap | 18 mm | Adjustable |
| Pulser voltage | ±2350 V | Max. ±2500V |
| Deflection angle | 18 mrad |  |
| Chopping period | 945 ns |  |
| Duty factor | 32 % | Beam on:68 % |
| Structure rise/fall time | 1.5 ns |  |
| Pulser rise / fall time | 10 ns | 2-98 % |

## The LANL-SNS system: deflector structure (1)

- Part of the meander  type structure model:
- notched metal meander strip (dark-blue) on dielectric supports (green),
- metal separators (red) are connected to the ground plate (light-blue, below).

- beam

## The LANL-SNS system: deflector structure (2)close up view (photograph)

## The LANL-SNS system: deflector structure (3)time and frequency domain response

- Input
- 2 ns

- Input
- 1 ns

- Frequency domain transmission measurement
- 0-2 GHz ;2 dB/div vert.scale
- 50 cm structure length, delay =22.65ns
- Note the typical notch for meander type
- structures around 1.9 GHz

## The LANSCE deflecting structure

## Pulsers used for LANL-SNS and LANSCE

**The chopper pulser for the SNS-MEBT has a peak voltage of 2.35 kV, higher by a factor 2.6 compared to its original specification of 900 V, so that the peak current through the meander line is about 47 A.**
**The requirements for this pulser voltage was set to 2.35 kV with a  rise and fall time below 10 ns, thus allowing up to 3 partially-chopped bunches in the beginning and at the end of each chopper pulse, which lasts around 300 ns. **
**The pulser was developed by Directed Energy Inc. (Fort Collins, CO); it uses 4 FETs in series for each voltage source, positive and negative.**
**The SNS MEBT chopper system was tested with beam at ORNL in 2003. All the results satisfy the chopper system requirements **
**The LANSCE pulse modulator was developed in the early 80s as a fast vacuum-tube-driven device. In the 90s it has been revised to a solid-state model, which is easier to maintain at peak performance, but with a slower rise time. ****Both LANSCE pulsers give 500 V. **
**The old LANCSE pulser used 8 vacuum tubes in parallel; it was very fast (about 2.5 ns rise / fall “on a good day”, typically 50% slower), but its tuning was fairly difficult. Its reliability was a serious issue, so it has been replaced later with a slower, but more reliable solid-state pulser.**

## The J-PARC chopping system; key parameters

| Beam energy | 3 MeV |
| --- | --- |
| Type | RFD |
| Operation frequency | 324 MHz |
| Operation mode | TE11 |
| Number of cavities | 2 |
| Number of gaps per cavity | 1 |
| Cavity length  | 172 mm |
| Bore radius | 5 mm |
| Gap length  | 20 mm |
| Full rise / fall time | 15 ns |
| Deflection field voltage | 1.6 MV/m |
| Beam-on duty factor  | 56 % |
| RF power source type | Solid state amp. |
| Number of cavities per RF source  | 2 |
| Peak RF power per cavity | 30 kW |

## The J-PARC chopping system; key parameters

- The J-PARC linac has a two-stage chopping system with an LEBT pre-chopper and an MEBT chopper.
- The LEBT pre-chopper is an energy modulation induction cavity, with which one can drive bunches beyond the longitudinal acceptance of the RFQ. The MEBT chopper is an RFD (RF deflection cavity) with short time-constant or low Q –value (around 10) driven by fast RF bursts from a high power RF amplifier.
- Bunches are kicked horizontally with the RFD and removed with a collector downstream.
- With the cavity modulating the LEBT beam energy, the downstream RFQ operates as an “energy filter” by letting the off-momentum beam arrive outside the momentum acceptance of the RFQ.
- The MEBT chopper has been tested [see next slide] and the design rise/fall time of 10 ns has been achieved
- It has also been confirmed that the residual current during “chopper-on” periods is less than 10-4 of the nominal current (5 mA).
- Further testing of the chopper performance is now in progress during beam commissioning of the J-PARC linac front-end at KEK

## The J-PARC chopping system; performance

**Waveform of a chopped beam obtained with a beam position monitor in MEBT. The horizontal scaling is 10ns/div. The pulse duration is shortened to demonstrate the chopper performance **

## The CERN-SPL system parameter list

| Beam energy | 3 MeV |
| --- | --- |
| Overall length | 3.7 m |
| Number of chopper structures | 2 inside quads |
| Number of quadrupoles | 11 |
| Chopper plate length | 400 + 400 mm |
| Chopper plate distance | 20 mm |
| Separation chopped/ un-chopped beam | 15 mm |
| Chopper structure rise-and fall time  | <2ns (10-90%) |
| Chopper voltage pulse (per plate) | 500 V |
| Effective chopper voltage pulse | 400 V |
| Max. chopper frequency | 44 MHz |
| Pulse length | 8-1700 ns |
| Max chopping factor (duty cycle) | 40% |
| Repetition rate  | 1-50 Hz |
| Output transverse emittance (rms, norm) | 0.27 π mm mrad with collimator |
| Chopper deflection angle | 6.8 mrad |

## The CERN-SPL system deflecting structure

**The alumina ceramic plates with printed meander structure (MoMn + 30 micron Ag); **
**mounting holes for M 2.5 screws are at the sides **

**beam**

## The CERN-SPL system deflecting structurepulse transmission properties

**Attenuation vs frequency**

**Pulse distortion vs. travel-time**

**The nonlinear phase shift (dispersion)**
** ****is the main reason for the pulse distortion**
**as well as for ringing and overshoot**

## The CERN-SPL system deflecting structure“coverage factors”

**SPL 50 Ohm meander, max 82%**

**LANL meander, max 89%**

**The coverage factor is defined here as the ratio of deflecting electric field **
**from the meander structure as compared to the field from a homogeneous infinite**
**metallic plane (ideal capacitor without fringe field)**

## The CERN-SPL system deflecting structure3D views

## The CERN-SPL system power amplifiers

**Since high power dissipation during long flattops is to be avoided, the chopper plates are operated in a high impedance quasi-static mode below about 10 MHz and simultaneously in traveling wave mode above 10 MHz;  2 amplifiers! Triax feed-throughs**

**High frequency( >10 MHz ) vacuum tube amplifier performance**

**80 ns pulse burst**

## Discussion

- A certain number chopper concepts developed or implemented  in the past years are based on slow wave structures (meander like delay lines) which are synchronous with the beam and/or using  electrostatic deflection.
- In the case of the RAL-ESS chopper the slow-wave line and electrostatic deflection are physically separated and for the CERN-SPL chopper there is simultaneous traveling wave and electrostatic deflection foreseen.
- Cavity type (low Q) structures are in use for the JAERI design,  both for beam deflection( deflecting mode, used in JAERI MEBT) and particle momentum change (accelerating mode in LEBT) for application of a longitudinal kick to bring particles beyond the momentum acceptance range.

## Conclusion

- The main technological challenges for present developments are the pulse driver or power amplifier.
- The actual deflecting (or accelerating) structures are often designed following what is available from the driver units.
- Depending on the technology used, reliability of the driver may be an issue.
- Progress in fast solid state switching technology can be expected
- This may lead in turn to modifications of the deflector hardware
- Management of beam losses in the MEBT line is very important

## Acknowledgements

- This paper would not have been written without the extensive discussions with and contributions in particular from M. Clarke-Gaither, S. Kurennoy as well as R. Garoby, M. Vretenar, M. Paoluzzi, A. Lombardi and M. Ikegami. The author would also like to thank R. Garoby and T. Linnecar for support.

- RAL-ESS:
- Supported by EC Research Infrastructure Activity FP6 “Structuring the European Research Area” program, CARE –RII3-CT-3003-506395

- CERN-SPL
- Supported by EC:(HIPPI inside CARE), contract number RII3-CT-2003-506395