Proposal, procedure, and requirements for high-gradient studies of the Linac 4 RFQ

Anna Vnuchenko

CSIC / IFIC – Instituto de Fisica Corpuscular (CSIC-UV)

X-band group, CERN

August 9, 2018

Outline

Motivation to study BDs

Conditioning Procedure of CLIC Accelerating Structures

Identification of BDs

Review of the following structures:

RFQ ADAM

Technical specification of the RFQ

Preliminary BDs study

RFQ Linac 4

Proposal and requirements for high-gradient studies of the RFQ L4

Conclusions

CLIC structure, 12 GHz

Transfer Knowledge from High-Gradient CLIC acceleration structure to the RFQ.

RFQ LINAC 4, 352 MHz

Motivation

A study of the high gradient performance of the RFQ and definition of limiting factors.

Breakdown (BD) localization.

To achieve a high surface electric field (about 50 MeV/m) of the RFQ with a minimal risk of sparking and damages.

Using the acquired knowledge to optimize the design of structures.

Conditioning Procedure of CLIC acceleration structures

Conditioning algorithm

Conditioning of TD26CC in Xbox 2

Reduction in BDR during flat run

Acceleration structure after fabrication do not run right away at full specification: pulse length and gradient need to be gradually increased while pulsing.

Increase power if BDR is low.

Increase pulse width in steps when gradient target reached.

Algorithm limited itself by BDR of about 3x10-5 BDs per pulse.

CLIC accelerating structure specifications (12 GHz):

• surface electric field 200 MV/m,

• a pulse length 180 ns.

Transmitted RF pulseIncident RF Pulse

Beam Axis

Reflected RF Pulse

BD

Emitted Current

How to identify the BD at RF Accelerating Structures?

BD position can be resolved using timing of reflected and transmitted pulses.

BD diagnostics:

RF signals (directional couplers),

Field-emitted current, breakdown current (Faraday cups, BLMs).

Vacuum diagnostic signals (vacuum gauges).

RF signals in the structure with comparison to a nominal pulse before the BD event.

The scopeSignals from DAQ

Power Supply IOT 4 x 100 kW

Power Couplers 4Pickup Antennas 16

The RFQ ADAM: Technical specification

Saving average value of the signals: Reflected and forward signals from IOT DC, 4 Pick ups signal.

Full waveform of the signals

Source and RFQ parameters

Frequency 750 MHz

Length 2 m

Vane voltage 65 kV

Peak RF power 400 kW

Input Energy 40 keV

Output Energy 5 MeV

Duty cycle 0.40%

BD

The RFQ ADAM: preliminary BDs study

Normal

The main criteria for threshold should be signals from Pick Ups.

The measurement of the reflected signal must be carried out directly from the RFQ.

Only signals with vacuum activity and significantly different from the previous signals should be considered.

criteria for selecting the BD signal via LabVIEW

Problem with IOT

Comparison nominal and signal with vacuum activity (BD)

High power operation with constant power 400 kW.

Constraints:

Measurement of the reflected signal in upstream of IOT => no DC downstream,

Operation at maximum power from IOT (source limitation, power supply), few or no BD.

Next plan measurement:

Development of a high speed data acquisition system based on LabVIEW for measuring

RF signals directly from RFQ.

The RFQ ADAM:

• Elementary modules of 1 m;

• Maximum RF Power required: 0.8 MW toallow the use of one single LEP klystron;

• Operation: 480 kW, 250 usec, 1Hz;

• Max. Beam Pulse Length 1.2 ms.

The RFQ LINAC 4: Technical specification

RFQ LINAC 4

Linac4 RFQ Parameter Value

Frequency 352.20 MHz

Length 3.06 m

Vane voltage 78.27 kV

Max field on pole tip 34 MV/m

RF total peak power 600 kW

Kilpatrick value 1.84

Beam parameters ValueInput energy 0.045 MeV

Output energy 3.0 MeV Nominal norm. RMS 0.25 π mm mrad

Current 80 mA

HPRF system:

Power Couplers 1Pickup Antennas 16

The RFQ LINAC 4: Physical and Acquisition layers

RF signals: forward, reflected (DC), Vacuum signal.

1. DAQ of RFQ:

2. DAQ of LINAC 4:

0

100

200

300

400

500

600

700

800

0.0022 0.0024 0.0026 0.0028 0.003 0.0032

Pow

er (

kW)

Time (s)

Ant1(kW)

Ant2(kW)

Ant3(kW)

Ant4(kW)

Ant5(kW)

Ant6(kW)

Ant7(kW)

Ant8(kW)

Ant9(kW)

Ant10(kW)

Ant11(kW)

16 signal from pick ups, detect instability.

© Signals are not synchronized in time.

From the presentation about commissioning of the RFQ

The RFQ LINAC 4: Proposals and requirements for high-gradient studies

Perform ultimate RF conditioning of the RFQ up to 50 MeV surface electric field.

Develop the control and acquisition system: detection of BD events and data storage.

Adopt CLIC protocol.

Backup: meantime, capture exciting RF pulse and see if we can detect and identify BD position

Collect data from DAQ of RFQ and from DAQ of LINAC 4:

Directional coupler: incident and reflected signals;

Pickup: signal of field in the RFQ cavity;

Oscilloscope : additional record these signals (timing issues).

Control system: vacuum signals.

Conditioning can be done manually.

High power conditioning allow the ultimate high gradient performance of CLIC accelerating structures.

Single BDs are present in any high power test without significant structural damages.

Transfer knowledge of conditioning from CLIC acceleration structure to the RFQ.

Meantime measurement: perform high power test in manual mode.

Conditioning of the RFQ

o incrementally increasing the power to the structure: from … to … kW,

o start conditioning with short pulse length: from … to … ms.

Define an operating strategy:

o after the BD (vacuum spike and signals from pick ups) make a short stop (30 s) and again continues conditioning,

o If BDs are happened again, go down in power on some step ( … kW).

Conclusion and next steps

Thank you for your attention!

Back up slide How does the analysis works?

Responsible person: Luca Arnaudon and Antoine Benoit