# Presentation: 591475

## Preliminary Results of the  

**LEDA Beam-Halo Experiment**
** **

***R. Garnett, T. Wangler, J. Qiang, and P. Colestock***
***Los Alamos National Laboratory***
*** ****** ***
***SNS Mini-Workshop, ORNL***
***June 25-26, 2001***

## Halo Experiment Scientific Team

**P. Colestock**
**J. D. Gilpatrick**
**M. E. Schulze**
**H. V. Smith**
**T. P. Wangler**
**C. K. Allen**
**K. C. D. Chan**
**K. R. Crandall**
**R. W. Garnett**
**W. Lysenko**
**J. Qiang**
**J. D. Schneider**
**R. Sheffield**

## Experiment Motivation

- Computer simulations and linac operating experience show that a small fraction of particles can acquire a large transverse energy to form beam halo. *We want to validate our codes.*
- Search for** **evidence that halo formation is described by the particle-core model. *This implies that* *parametric resonances exist to drive particles to large amplitudes. Our codes should contain the correct space-charge physics.*
- Demonstrate that the simulation codes predict the mechanisms that produce beam halo and predict the maximum amplitude correctly. *We hope to be able to say we know how to select design parameters including apertures that will minimize beam loss and radioactivation.*

## x

- y

- z

- y

**Symmetric**
**(Breathing)**
**Mode**

**Antisymmetric**
** ****Mode**
**(not measured)**

**Quadrupole**
** ****Mode**

**Envelope Modes of Mismatched Bunched Beams**

## Beam-Halo Experiment

- 6.7-MeV, 75-mA pulsed beam from LEDA RFQ
- 	~30-sec pulse, 1-Hz rep-rate (slow data acquisition)
- FODO transport line with 52 quadrupoles + beam diagnostics.
- First 4 quadrupoles used for matching / mismatching.
- Wire scanners and scrapers used to make profile measurements. Large dynamic range ~ 105:1.
- Vary mismatch and current. Measure and compare with codes: 1) **Rms emittances, **2) **Maximum detectable amplitudes**,
-      3) **Kurtosis** (beam shape parameter).
- 	Data taken for : Breathing and Quadrupole Modes
- 				I=16 mA, 50 mA, 75 mA
- 			             0.65     2.00
- 				0.71  /o  0.93
- Also search for additional halo from other sources.

## Fully-Instrumented LEDA Beam-Halo Lattice

**52 Quadrupoles + 4 in the HEBT**
**9 Wire Scanners/Halo Scrapers (Projections) + 1 in the HEBT**
**3 Toroid (Pulsed Current) + 2 in the HEBT**
**5 PMT Loss Monitors (Loss) + 2 in the HEBT**
**10 Steering Magnets + 2 in the HEBT**
**10 Beam Position Monitors (Position) + 5 in the HEBT**
**2 Resistive Wall Current Monitors (Central Energy)**** **

**52 quadrupole FODO lattice**

**T**

**WS 4 /HS**

**BPMs**

**Steerers**

**PMTs**

**WS 45,47,49,51 /HS**

**WS 22,24,26,28 /HS**

**RFQ**

**HEBT**

**RWCs**

- First 4
- quadrupoles
- independently
- powered
- for generating
- mismatch modes.

**11 meters**

## LEDA Facility Halo Lattice

## Wire-Scanner / Scraper is our main halo diagnostic tool (J.D.Gilpatrick, et al.).

- Wire-scanner uses 33-m carbon fiber to measure core.
- Scraper is graphite plate brazed onto copper. Scraper measures halo-Graphite is 1.5 mm thick so protons stop in graphite.-Scraper bias voltage about +10V to suppress secondary electron emission.-Copper is water cooled.
- Dynamic range - 103:1 for wire alone (4 rms), 105:1 for wire + scraper (5 rms)
- Wire and scraper data acquired separately and combined later.

**Scrapers**

## Wire-Scanner / Scraper DataMatched Beam, 75 mA, WS22x

## Beam is quickly de-bunching.

**Beam is quickly de-bunching.**

- RMS Beam Sizes, 75 mA

## Measurement Cycle

- RF blanking pulse de-energizes RFQ.
- 75-keV beam injected into unpowered RFQ as injector beam approaches steady state.
- RF blanking pulse is removed and RFQ is excited. (T~5 s rise time)
- Beam profile monitors are in fixed position so only one wire or scraper is in beam at a time. All other wires or scrapers are outside beam pipe aperture.-Wire or scraper collects beam-induced charge over about 30 s-30-s limit is set by onset of thermionic emission of the scanner wire.-Integrated charge is digitized. -Only last 10 s of integrated charge is selected for data.
- After 30-s interval, dc injector turned off.
- Before next pulse (1 Hz), scanner wire and scraper are moved to next position.

## Procedures for Matching and Mismatching

- Initial matched-beam quad settings determined from TRACE3D.
- Quad scans used to determine partial derivatives of the beam size as functions of the matching quad strengths.
- Least-squares fitting procedure is used to determine quad settings that produce equal rms sizes in x and y at WS 22-28 (middle of the channel) — The beam is rms matched.
- TRACE3D is used to determine quad values that give the appropriately scaled ellipse parameters at WS 22-28 for the pure-mode mismatches.
- Mismatch parameter,   ratio of initial mismatched rms size  to matched beam rms size.

## Matching procedure appears to work! 

**Matching procedure appears to work!**

## Other Beam Measurements / Calculations 

- Rms Emittance: Calculated from quad scan data using a least- squares fitting procedure and rms beam sizes measured at the profile monitors
- Maximum Detectable Amplitude: Determined from intersection of transverse profile curve with background noise level.
- Shape of Distribution: Characterized using a “kurtosis” parameter defined in terms of ratio of 4th moment to 2nd moment of the measured distributions .

## Combined Data - 75 mA, =1.0 and =1.5

- Combined Data - 75 mA, =1.0 and =1.5

- =1.0

- =1.5

- Scanner 22

- Scanner 51

- Scanner 22

- Scanner 51

- Our simulations do not predict distributions like this.

## Max. amplitude vs. z shows good agreement with simulations.

- Measurement Results

- Simulation Results

## Maximum Amplitude Particle-Core Model and Simulation Predictions, 100 mA

**Maximum Amplitude**** ****Particle-Core Model and Simulation Predictions, 100 mA**

## Maximum AmplitudeComparison of Data and Simulation

**Maximum Amplitude****Comparison of Data and Simulation**

- Results independent of beam current (tune depression).
- Code correctly predicts trend of the data including minimum for the matched-beam case.
- Supports particle-core model.

- WS/HS 51 Data

## Comparison of Transverse Emittance vs. z

**Comparison of Transverse Emittance vs. z**

- IMPACT Simulation Results

- Measurement Results

***Simulations using the nominal RFQ output beam from PARMTEQM appear to underpredict the measured emittance growth.***

## Kurtosis Definition

- -Similar definition applies for y and z coordinates.
- -Dimensionless shape parameter. Independent of beam intensity.
- -Easily calculated from moments of measured or simulated beam profiles.
- -Zero for uniform-density 2-D elliptical (KV) or 3-D ellipsoidal beam.
- -Equal to or near unity for Gaussian profile. -Matched beams without halo have values between 0 and 1. Increases as tails develop, but can decrease and go negative
-  if beam profile becomes square.

## X-Kurtosis Data - 75 mA

- General trends -
- 	-Kurtosis decreasing as a function of .
- 	Plausible since distribution     is seen to become square.
- 	-No particular z-dependence.
- Kurtosis may not be a good figure-of-merit for halo.

## Low-energy tail on RFQ output beam?

**Assumed PARMTEQM output distribution for core (100 keV energy spread).**
**Superimpose 10% low-energy tail:**
**	****	****Uniform distribution**
**	****	****6.0 MeV ******** W ******** 6.7 MeV**
**Assume low-energy beam is mismatched:**
**	****	******** ****=1.5 to get agreement ****	****	****with measured widths.**
**Good agreement with data.**
**Seems physically plausible.**
**Channel transports beam as low as ********2 MeV.**
**Preliminary measurements do not support hypothesis.**

- =1.5,  75-mA Simulation Results

## Matched beam distribution shape also not predicted by nominal PARMTEQM distribution.

- Measured Profile
- Matched Beam, WS 22

- Simulation Results
- Matched Beam, All WS

## Transverse tails in RFQ output beam explain our observations?

- Shoulders produced at Wire-Scanner #49

- X-profile

- Y-profile

- Artificially superimposed tails

- Simulation Results

## Summary

- Measurements have been made at 15, 50, and 75 mA. Initial analysis for 75 mA data.
- Rms-emittance grows along the channel; Growth rate increases as mismatch increases.
- Kurtosis decreases with increasing mismatch strength as shoulders develop.
- Profile measurements are _not_ in good agreement with simple multiparticle simulations using the nominal output beam from the RFQ.
- 	 Unexpected halo structure: shoulders and asymmetries
- General trends of the data are predicted by the codes and the Particle-Core Model.
- Better understanding of profile data will require better characterization of RFQ output beam:  spectrometer, slit & collector emittance measurement?