# Presentation: 323017

## Beam Modulation at the Ion Source?

- In the present scheme we use a split extractor to deflect the beam
- With a 50 ohm transmission line.  The deflection time is limited by HV pulse, and ion transit time through deflection plates, expect ~ 50 ns rise and fall times.  For more details see: AIP Conf. Proc. 763, pg. 89

- 5/25/06

##  Beam Modulation at the ion source?

-  Beam Modulation at the ion source?
- What have we achieved?

- Chopping 106 ns out of ~ 2.2 μs
**Problem:  Long tail 1.3 ****μ****s**

- Long tail gets better inversely with chop time and evacuation depth

- 61 mA beam
- Chopping 85%

- 5/25/06

## Here’s the Problem

- In the Linac Pre-Accelerator we use space charge neutralized beam.
- Roughly the same number of positive ions as negative beam ions.
- The positive ions do not travel with the beam but are trapped by the beam potential.
- The neutralization time is 1-50 us depending on background pressure and beam energy.
- _In the chopped section of beam the neutralizing charge is lost!_
- Between the deflection plates the positive ions are swept out in the opposite directions as the H- ions
- Further up stream the positive ions repel each other as the chop moves through their location.
- The long tail is caused by space charge blow up of the H- beam ions as they move back into “void” of positive charge.  This continues until neutralization is re-established.

- 5/25/06

## Here’s the Solution

- Use a space charge dominated beam!
- The SNS LEBT chopper achieves 20 ns beam rise and fall times in front of the RFQ at 65 kV.
- Their beam is Space charge dominated!
- A Space charge dominated beam is achieved by passing the beam through electric fields strong enough to over come the beam potential.
- These fields could be provided by electrostatic focusing similar to the SNS LEBT
- Advantages of Electrostatic LEBT
- Can be very short
- Leaves a space charge dominated beam
- Disadvantages of Electrostatic LEBT (no show stoppers)
- Needs to be strong enough to handle space charge which varies depending on our operating mode 15 mA (3 ms) or 47 mA (1 ms)
- HV requires good isolation between electrodes and good vacuum
- Cesium flow needs to be controlled
- Small apertures and a cold plate before RFQ

- 5/25/06

## Ion Source Test Bench uses an electrostatic LEBT of sorts

- 5/25/06

## How to achieve shorter chop lengths?

- Modulate the anode cover on the ion source
- Plasma frequency:  fpe=8980(ne)1/2 = 2.8e1010 Hz
- Electron density (ne)~1013-1014:
- Cathode to anode voltage is ~150V negative
- Modulation voltage needs to be slightly higher to stop beam ~180-200 V negative
- Possible supplies:
- Technical division HV pulser (Robyn, Madrak)
- _[AVRL-1-PS-T1 pulse generator](http://www.avtechpulse.com/speed/avrl-1)_-200 V1 ns3 ns5-1005 kHz ~ $6510
- Kentech pulser can already achieve +/- 500 V and do the duty factor.
- 50 ohm transmission line: use quartz cable near ion source to handle 270 oC.

- Chopped beam

- Unchopped beam

- Cathode: -150 V
- Anode: 0 V
- Pulsed anode cover
-  0 V or  -180V

- Extraction 20 kV

- 5/25/06

## Benefits
No need for chopping after RFQ
Reduces complexity of Linac design
No longer have to worry about bunching through MEBT
Eases space requirements 
Unknowns
Does the plasma meniscus (the plasma vacuum interface) recover as quickly as the plasma frequency?
Need to test single notch and prove we can over come long tail
How far can a notch be propagated in LEBT
Short beam chops may tend to close up due to space charge?
Don’t know effects of increased beam current?
Is this good at 15 mA but not 47?

- No need for chopping after RFQ
- Reduces complexity of Linac design
- No longer have to worry about bunching through MEBT
- Eases space requirements
- Unknowns
- Does the plasma meniscus (the plasma vacuum interface) recover as quickly as the plasma frequency?
- Need to test single notch and prove we can over come long tail
- How far can a notch be propagated in LEBT
- Short beam chops may tend to close up due to space charge?
- Don’t know effects of increased beam current?
- Is this good at 15 mA but not 47?

- Beam Modulation at the Ion Source?

- 5/25/06