# Presentation: 004599

## LINAC BEAM QUALITY DEMANDS

**Jie Wei**
***SNS/ORNL Accelerator Physics***

**June 25 - 26, 2001**

**Workshop on Linac Space Charge**

**Notes:**
Good morning, everyone. I’m Jie Wei. I’m going to report to you on AP interface and integration activities.

## Key parameters

|  | Baseline | Back-up |
| --- | --- | --- |
| Kinetic energy, Ek [MeV] | 1000 | 975 |
| Uncertainty,Ek (95% probability) [MeV] | +/- 15 | +/- 15 |
| SRF cryo-module number | 11+12 | 11+15 |
| SRF cavity number | 33+48 | 33+60 |
| Peak gradient, Ep (=0.61 cavity) [MV/m] | 27.5 (+/- 2.5) | 27.5 (+/- 2.5) |
| Peak gradient, Ep (=0.81 cavity) [MV/m] | 35 (+2.5/-7.5) | 27.5 (+/- 2.5) |
| Beam power on target, Pmax [MW] | 1.4 | 1.7 |
| Pulse length on target [ns] | 695 | 699 |
| Chopper beam-on duty factor [%] | 68 | 68 |
| Linac beam macro pulse duty factor [%] | 6.0 | 6.0 |
| Average macropulse H- current, [mA] | 26 | 32 |
| Linac average beam current [mA] | 1.6 | 1.9 |
| Ring rf frequency [MHz] | 1.058 | 1.054 |
| Ring injection time [ms] / turns | 1.0 / 1060 | 1.0 / 1054 |
| Ring bunch intensity [1014] | 1.6 | 1.9 |
| Ring space-charge tune spread,Qsc | 0.15 | 0.20 |

- assuming 4% injection loss; 4% target window loss; linac max. -20o phase

## Linac Beam Quality Demands

- Output energy within +/- 5% window
- Key challenge: control transverse emittance and jitter
- Effective beam size growth & ring injection activation (< a factor of 2)
- Compared with other facilities (e.g. 5 –8 times growth at LANSCE), identified transverse jitter as main issue; easier for 402.5 MHz DTL
- New halo findings based on non-water-bag measured distributions
- Control momentum spread and jitter
- Facilitate longitudinal painting with a narrow paint brush
- Need to control total energy deviation within +/- 0.3% (3 MeV)
- Phase error an added complication
- Reduce uncontrolled beam loss across linac
- About 1 W/m and lower

**Notes:**
As a brief review of past activity:

We’ve chosen accumulator design for loss minimization, and it was endorsed by this committee in May 1999.

We’ve chosen SCL for its maintainability, technology and savings and it was endorsed by this committee in March 2000.

We’ve finally decided on 1 klystron per cavity, independent Rf control and it was endorsed by the mini-ASAC this July. 

## Transverse Requirements

- To facilitate transverse painting
- Ring injection foil miss, foil heating, foil traversal, radio-activation

## Emittance evolution	

| Device | Emittance (rms [m] | Jitter [mm] | Comments |
| --- | --- | --- | --- |
| IS/LEBT | 0.09 / 0.2 |  | Aberrations |
| RFQ | 0.21 |  |  |
| MEBT | 0.27 |  | Transverse space-charge growth due to chopper gap & longitudinal rebunching |
| Linac | 0.41 | +/- 0.2 | Transverse space-charge mismatch due to lattice & acc. gradient change |
| HEBT | 0.45 | +/- 0.2 | Energy spread (chromatic) |
| Ring | (44+44) |  | Painting; space-charge/magnet error  halo (Full UN = 120+120 m)  |
| RTBT | (44+44) |  | Full UN = 120+120 m |
| Target | 50 – 150 times |  | Window scattering |

**Notes:**
Second table is on emittance evolution. This is the nominal case, and we’ll study the consequence when the actual deviates from it. The emittance grows at various stages due to lattice change, space charge, etc. The key to preserve linac emittance is for ring injection foil/dump design. Then painting is used to reduce space charge. 

## Linac/Ring interface (transverse)

- Nominal norm. rms emittance ~ 0.4 mm mr
- Nominal jitter ~ 0.2 mm
- Larger emittance compromises power & loss/activation
- Injection dump accepts 10% power
- Movable transverse collimator for protection; takes 1% power
- Chamber aperture: HEBT rf cavity ...

- (equivalent; including jitter)

**Notes:**
This shows linac/ring interface. Blue is the nominal. Beyond that, the first consequence is the at injection foil: increased foil miss, reduced power, or increased foil hits. The dump takes 10%. The foil hit is nominally 6.5. Further beyond is protected by a movable HEBT collimators, which can stand 1% heat. It is designed to protect the HEBT and ring injection. 

## Longitudinal Requirements

- Facilitate longitudinal painting (instability, beam-in-gap cleaning, loss)
- No spreader, on energy
- No spreader, off energy
- With corrector & spreader

## Energy spread evolution

| Device | Spread [MeV] | Centroid Jitter [MeV] | Comments |
| --- | --- | --- | --- |
| Linac | 0.33 (rms) | +/- 1.5 | 0.5% amplitude, 0.5 deg. phase error (space charge, magnet errors, beam loading/transient, Lorentz detuning, microphonics, non-equipartition) |
| HEBT collimator | 0.72 (rms) | +/- 1.5
 | space charge growth due to lack of RF |
| Energy corrector | 1.3 -> 0.85 (rms) | +/- 1.5 -> +/- 0.2 | space charge growth due to lack of RF;
cavity corrects the energy error (jitter & spread) |
| Energy spreader | +/- 4 (full) |  | cavity broadens the energy core, without enhancing tail |
| Ring | +/- 10 (full) |  | gap cleanness versus beam stability |

**Notes:**
Similarly, evolution of beam energy spread. 

Stovall will show that the present RF control is at 0.5% 0.5 degree level, corresponding to an energy jitter of 1.5 MeV, and a spread of 0.33 MeV. This is corrected by the HEBT corrector, and then intentionally broadened without enhancing tail with the spreader. In the ring, it is a balance between gap cleanness and stability. 

## HEBT energy corrector & spreader

- Function: longitudinal painting with a narrow paint brush
- Control space charge tune spread (20 - 30 %)
- Enhance Landau damping (a factor of 2 - 3 in p/p FWHM)
- Avoid local non-uniformity (holes & spikes); reduce tail loss
- Possible instabilities in the absence of the cavities
- Electron-proton instability (PSR like): scales with (p/p)2
- Transverse instability (kickers): estimate 3.6 -> 0.9 x 1014
- Study conclusions:
- Estimate < 0.5 MW in the absence of cavities
- Functions not easily replaceable by the last linac cavity with fast feedback
- Cavity response ~ 10 kHz (<< 100 kHz); very challenging
- Ideal: correction before bend; spreading after bend; collimate

**Notes:**
As a brief review of past activity:

We’ve chosen accumulator design for loss minimization, and it was endorsed by this committee in May 1999.

We’ve chosen SCL for its maintainability, technology and savings and it was endorsed by this committee in March 2000.

We’ve finally decided on 1 klystron per cavity, independent Rf control and it was endorsed by the mini-ASAC this July. 

## Linac/Ring interface (longitudinal)

- Nominal energy jitter limit:      +/- 2.2 MeV
- Nominal rms energy spread:     0.3 MeV
- Impact of beam loading, Lorentz detuning, and microphonics (under study) 1-on-1 controlled
- Movable energy collimator; corrector limit & ring acceptance
- Two locations of energy corrector reserved (present & future)

## Beam loss distribution

## Controlled Losses

| Mechanism | Location | Fraction | Energy | Power |
| --- | --- | --- | --- | --- |
|     FRONT END |  |  |  |  |
| Unbunched beam | LEBT chopper | 27.7 e-2 | 0.065 MeV | 36 W |
| Unbunched beam | MEBT chopper | 3.4 e-2 | 2.5 MeV | 215 W |
|     HEBT |  |  |  |  |
| H0 ionization in the Linac | Linac dump | 1.0 e-5 | < 1 GeV | 20 W |
| Linac transverse tails | x-y collimator | 3.0 e-5 | 1 GeV | 600 W |
| Energy jitter/spread | z collimator | 1.0 e-3 | 1 GeV | 2 kW |
|     RING |  |  |  |  |
| Beam in gap | Collimators | 1.0 e-4 | 1 GeV | 200 W |
| Excited H0 at foil | Collimator | 1.3 e-5 | 1 GeV  | 26 W |
| Partial ionization/foil miss | Injection dump | 1.0 e-2 | 1 GeV | 20 kW |
| Space-charge halo | Collimator | 1.9 e-3 | 1 GeV | 3.8 kW |
| Energy straggling at foil | Collimator | 3.0 e-6 | 1 GeV | 6 W |
|     RTBT |  |  |  |  |
| Kicker miss-fire | Collimator | 1.0 e-5 | 1 GeV | 20 W |
| Nuclear scattering at target window | Target window | 4 e-2 | 1 GeV | 80 kW |

## Uncontrolled Losses (I)

| Mechanism | Location | Fraction | Energy [MeV] | Length [m] | Power [W/m] |
| --- | --- | --- | --- | --- | --- |
|    Front End |  |  |  |  |  |
| RFQ transmission | Uniform | 2.0 e-1 | 0.75 | 3.7 | 80.7 |
| H- stripping | MEBT | 1.1 e-3 | 2.5 | 3.6 | 1.6 |
|   Drift Tube Linac (DTL) |  |  |  |  |  |
| Emittance growth | End of Tank 1 | 6.6 e-5 | 2.5 – 7.5 | 4.2 | 0.24 |
| Emittance growth | End of Tank 2 | 2.4 e-5 | 7.5 - 22 | 6.1 | 0.18 |
| H- stripping | continuous | 5.8 e-4 | 2.5 - 87 | 37 | 0.3 – 0.6 |
|   Couple Cavity Linac (CCL) |  |  |  |  |  |
| Double H- stripping | CCL module 1 | 1.9 e-5 | 86.5 - 107 | 12 | 0.35 |
| Emittance growth  | CCL module 1 | 4.7 e-5 | 86.5 - 107 | 12 | 0.86 |
| H- stripping | continuous | 4.6 e-5 | 87 - 186 | 55 | 0.22 |
|   Super Conducting Linac (SCL) |  |  |  |  |  |
| Emittance growth | Warm sections | 3.7 e-6 | < 1000 | 37 | <0.2 |
| Emittance growth | Supl. 9 modules | 1.2 e-5 | < 1000 | 71 | 0.35 |
| H- stripping | Medium beta warm | <5 e-8 | 186 - 379 | 18 out of 63 | <0.002 |
| H- stripping | High beta warm | <3 e-8 | 379 - 1000 | 19 out of 94 | <0.002 |
| H- stripping | Supl. 9 modules | <5.3 e-6 | 1000 | 71 | <0.15 |

## End-to-end evolution & simulation (2000)

**Notes:**
This is a busy table. There will be 3 zoomed-in tables following this.

This is mainly to show you that 
1) There is a consistent design and parameter set from FE to Target
2) It is section-by-section connected but not completely continuous
  a) LEBT to RFQ    and  b) HEBT to Ring
3) Some are included, more not included but separately studied.
4) There are open issues identified.