# Presentation: 137420

## Peter Seidl 
Lawrence Berkeley National Laboratory
and the  Heavy Ion Fusion Science Virtual National Laboratory

***Lawrence Berkeley National Laboratory***
***and the  ******Heavy Ion Fusion Science Virtual National Laboratory***

**Progress in neutralized beam compression and focusing**

_**Outline**_
- Neutralized drift compression
- Pulse Line Ion Accelerator
- Solenoid transport

**VNL-PAC**
**LLNL**
**August 9, 2006**

**Notes:**
Give context for each major topic

	What were our objectives in 2005?
	Significance of each measurement presented

## Collaborators - 

**J. Armijo,****a,g**** D. Baca,**** a**** F.M. Bieniosek,**** a**** J. Coleman,**** a,f**** R. C. Davidson,**** d**** ****P.C. Efthimion,**** d**** A. Friedman,**** b**** E.P. Gilson,**** d**** D. Grote,**** b**** I. Haber,**** c**** ****E. Henestroza,**** a**** I. Kaganovich,**** d**** M. Kireeff-Covo,**** b,f**** M. Leitner,**** a**** ****B.G. Logan,**** a**** A.W. Molvik,**** b**** ****P.A. Seidl**,a  **D.V. Rose,**** e**** P.K. Roy,**** a**** ****A.B. Sefkow**** a**** W.M. Sharp,**** b**** J.L. Vay,**** a**** W.L. Waldron,**** a**** D.R. Welch**** e**** ****and S.S. Yu**** a**** **
***a******Lawrence Berkeley National Laboratory, Berkeley, CA 94720, U.S.A.***
***b******Lawrence Livermore National Laboratory, Livermore, CA 94550, U.S.A.***
***c******University of Maryland, College Park, MD 20742-3511 CA, U.S.A.***
***d******Princeton Plasma Physics Laboratory, Princeton, NJ 08543-0451, U.S.A.***
***e******Voss Scientific, Albuquerque, NM 87108, U.S.A.***
***f******University of California, Berkeley, CA 94720, U.S.A.***
***g******Universite ENS, Paris, France.***

## Neutralized drift compression

**Acceleration and velocity ramp for compression**
**induction core(s) or other (Pulse Line Ion Accelerator?)**
**Need to cancel out space charge **
**plasma column with n****p**** >> n****b**
**Preservation of emittance**

## Huge advantage of neutralized compression for high perveance (K ≈ 10-3)

**Beam density**

**3x10****12**** cm****-3**** plasma**

**vacuum**

**3x10****12**** cm****-3**** plasma**

**vacuum**

**vacuum**

**3x10****12**** cm****-3**** plasma**

**Beam Conditions at Focus **

**~500 x enhancement of intensity on target is possible**

**E = 300 keV, K****+**
**  ****I = 44 mA**
**Solenoids **
**  ****B1=2.445 T**
**  ****B2 = 2.6 T**
**Bunching core**
**  ****200 kV, 200 ns**

**Notes:**
Left plots are for z=279.0 cm, while right shows peak @ 279.5 cm for 1400 ns.

STX parameters:
K+ ions
  300 keV
  44 mA
Solenoids 
  B1=2.445 T
  B2 = 2.6 T
Bunching core
  200 kV
  200 ns

## Before WDM user facility, we plan a modest upgrade ... NDCX-2

**Solenoid **
**matching**

_***Alternative:***_

**2.8 MeV Li****+**** -- Possibly less costly.**
**	****Issues****: source, beam formation, higher ********beam**** at injection, T****ll**

## 1. Neutralized drift compression experiments 

**2006 Goal: Experiments and modeling on combined transverse and longitudinal focusing of intense heavy-ion beams. **

- injected 10-sec, K+, 280-310 keV, 22-26 mA,
- bunch a portion of beam with induction module    head to tail velocity ramp, v/v ≈ 15% (≈0.2 s)
- Plasma column neutralizes space charge

- If TL limits compression,   bunch duration:

- L = drift length
- v = <velocity>

**Notes:**
Only compressing part of the beam.

These are space charge dominated beams...
High perveance...

## plasma sources for 1-2 meter drift compression.

**Filter cathode arc plasma source**
**Injection from end into weak solenoid**
**n****e**** ≈ 5 x 10****11  ****cm****-3 ****measured**

**Both approaches not yet optimized, higher density possible.  **

- BEAM

## Neutralized drift compression experiment (NDCX)

**Induction bunching****module:**
- -80 < V < +70 kV
- applied to the beam
- Building one more module with greater V

- BUNCHING
- MODULE

## Experiment for neutralized drift compression

**injector**

**quadrupoles --> solenoids**

**Support rails**

**Drift compression / plasma**

**Induction bunching **

**D**

**D, **
**Plasma**
**source**

**Energy Analyzer**

## 60x beam compression observed in a 1-meter neutralized drift experiment agreed with EM-PIC model results.

**60x beam compression observed in a 1-meter neutralized drift experiment agreed with EM-PIC model results.**

**LSP*****: EM - PIC code including plasma modeling and beam plasma interaction.**
***Voss Scientific, www.vosssci.com**

**4.5 ns FWHM**

**n****p**** ≈ 5 x 10****10  ****cm****-3**

***Average of 4 pulses, with detailed waveform input to modeling...***

***Toward target heating experiments: Will characterize shot-to-shot reproducibility / reliablility.***

**Notes:**
Voss Scientific, Say formerly MRC.

## Next: For beam compression suitable for near term WDM experiments, simultaneous longitudinal compression and transverse focusing must be demonstrated...

**This would demonstrate simultaneous longitudinal compression and transverse focusing on NDCX for the first time.**
**Later we will optimize the induction voltage waveform that would yield higher compression.**

***Angle at entrance to bunching module***** ********o**** = 7.5 milliradian****	****	**** ********o**** = 13.5 milliradian**

- Larger R near max. bunching

- Smaller R near max. bunching

**Beam radius at 210 cm**

**Compression I/I****o**** **

- 7.5 mr 13.5 mr

- E (keV)

***Compensation of transverse focusing effect in induction gap ****(test this summer****.****)*

- Net defocusing in gap due to energy change, Er

**Notes:**
Space charge couples transverse & longitudinal  dimensions

## A new bunching module will increase the voltage amplitude and voltage ramp duration

**14 --> 20 induction cores ****--> higher voltage amplitude & ********V********t**

**Gap geometry is flexible; opportunity to optimize.**

- V (kV)

- 250x compression (model)

- 60x compression measured, modeled

- 125x compression (model)

- Beam experiments in 2007.

**Notes:**
core is driven by a thyratron-switched modulator.  Because the modulator for each core can be designed to produce different waveforms and can be triggered independently, a variety of waveforms can be produced at the acceleration gap using the 14 discrete building blocks. 

## Gap geometry - design allows for straightforward modifications

**Removeable plates **

**insulator**

**Plasma & drift compression **

**Induction cores**

**acceleration gap**

## A final focus solenoid is needed to achieve Ttgt~1 eV. Modeling for NDCX and HCX input beams

**HCX, 1.6 MeV, I****o**** = 0.36 A**

**NDCX, 0.4 MeV, I****o**** = 0.07 A**

- B(Gauss)

- 100

- 1.5x104

- Log(ni)

- 7.00

- 13.00

- 25 cm solenoid

***Compressed         ******beam***

- Simulations: D. Welch (Voss Sci.) & A. Sefkow (PPPL)

**Notes:**
These simulations look at what is possible.

## 1st NDCX longitudinal energy spread measurement  Tl ≈ 1.5 eV (new electrostatic energy analyzer)

**T****//**** ≤ ********E)****2****/(2E) ≤ 1.5 eV.  **
**Upper limit due to coarse measurement intervals, uncertainty of instrumental resolution. **
**broadening due to finite entrance slit = 1 mm --> 8%. **
**New analyzer can measure up to 1 MeV ions, with resolution few x 10****-4****.**
**It was used to verify ion acceleration and beam dynamics of the prototype Pulse Line Ion Accelerator module.**
**valuable for disentangling contribution to focusing limits from initial beam conditions and bunching waveform fidelity.**

**Photo of analyzer**

- R = 50 cm

- entrance

- detector

## 2 . Initial experiments with a Pulse Line Ion Accelerator (PLIA) accelerator section.  Potential for inexpensive, high-gradient ion acceleration

- transformer coupling (5:1 step-up)

- 1.2 m

*Moving bucket*

* **helical coil structure, submerged in a di-electric medium (oil) powered by a pulsed, HV waveform --> energy gains many times higher than the input voltage to the helix.*
* **Beam energy modulation of -80 keV to +150 keV was measured using a PLIA input voltage waveform of -21 kV to +12 kV. *
* **Comparison to simulations, dynamics understood.*
* **Flashover presently limits <E> ≈ 150 kV /m -- see Coleman et al. poster*

## The predicted energy amplification and beam bunching were experimentally observed. 

**Experiment****	****WARP-3D**

_*Energy distribution:*_* Measured with new energy analyzer*
*Ringing waveform **** modulation*

*I/Io ****Ratio**** (PLIA / no PLIA)*

_*Bunching:*_* qualitative similarities between experiment and PIC*

**Flashover problem: Wave B****z**** & B****r**** accelerating electrons suspected, and being modeled.**
_**Plan:**_** **
**Bench tests with grading rings on the insulator vacuum interface (both open and closed rings).**
**If successful, then higher gradient tests with ion beam.**

## 3. Studying solenoid transport of high perveance beams for future WDM experiments

**Solenoids can transport high current, space charge (in one beam).**_*  *__*Objectives:*_
*match and transport a space-charge-dominated ion beam, maintain low emittance (Brillouin flow).*
*study associated electron-cloud and gas effects that may limit the beam quality or beam control in a longer transport system.** *
*Beam halo scraping  **** e**-** emission*
*Ionization of background gas**	**	*
*Expelled ions hitting vacuum wall**	*
*Ionization of desorbed gas**	***	**

- Compare / contrast with experience in magnetic quadrupoles. See A. Molvik talk.

## Begun experiment by diagnosing beam before and after 2 solenoids.  Later will extend to 4 solenoids.

**Using pulsed solenoids, B ≤ 3 Tesla, t = 4 ms.  Measured & modeled eddy current effects at beginning and end of beamline *****(e)*****. **

*(e)*

*(e)*

- S1

- S2

*Same injector as NTX/NDCX*

- E = 300 keV
- I = 25-45 mA
- BS1 =  BS2 = 2.7 T
- Diagnostics for transverse phase space:
- Slits, slit-collectors, scintillator / gated CCD camera

## Beam transverse phase space characterized  immediately after injector and after two solenoids.

- Transport of 45 mA beam with little or no beam loss

- Ion emission uniformity, diode dynamics modeling
- (I. Haber)

- Slit - scintillator data --> input for modeling

## Negatively biased e- or “traps” control electrons from aperture and intercepting diagnostics

**Negatively biased e- or “traps” control electrons from aperture and intercepting diagnostics**

**electrons are liberated wherever beam ions hit surfaces**
**aperture plates and diagnostics, such as slit-plates, are major sources**
**rings at -3 kV on both sides of an aperture keep electrons out of diode**
**third ring is placed after the solenoids to block electrons from diagnostics**

## At first, x-x’ phase space showed spurious time dependence due to e-trap, diagnostics in Bsol

**At first, x-x’ phase space showed spurious time dependence due to e-trap, diagnostics in B****sol**

**Injected: ******** = 23 mm•mrad**

**After two solenoids ******** = 35 mm•mrad ~ 2x ********i**

**WARP PIC envelope *****agrees***** near beam head. **
**But, large  envelope parameter variation within ~2 ********s. *****Electron/gas effects?****** ******Diagnostics in ~1 kG  fringe field?***

- WARP e-cloud modeling (W.Sharp)

**Notes:**
14.7 mm, -33 mrad

## Moved the diagnostics downstream 30 cm, Bz = 3 kG --> 0.1 kG.

- K+
- source

## 4-Solenoid Transport eXperiment, e- cloud diagnostics, and neutralized drift compression tune

**Faraday cup**
**4” square Scintillator**
**Horizontally & Vertically Driven Slits and Slit-Cups**

**254.31 cm**

**End diagnostic box**

**2.6 T****	****    1.4 T****	****         1.4 T            2.3 T**

**I****b**** = 26 mA (aperture)**

## First measurements through four solenoids, 26 mA beam (aperture)

**Issues**: centroid / alignment
- ni =  0.06 mm mrad   nf = 0.08 mm-mrad

## Addition of two more solenoids e-cloud studies, matching into NDC

## We have installed e--cloud diagnostic rings in four solenoids

**Biased rings (****≤1 kV) ****in bore of solenoid magnets to trap or collect e****-**** from surface emission, ionization,..**

**e****-****, neutral sources: aperture, desorption from wall (halo particle loss).**

## Rings in solenoids short enough that Ez > 0 to expel (or trap) electrons

**Negative electrode in solenoid will expel e****-****.  **
**Can expel electrons in outer 0.1-0.5 of beam radius.**
**Positive electrode between will suppress emission.**
**Reverse bias to emit and trap e****-****.**** **

## STX e-cloud diagnostics

## Preliminary results

**Preliminary results**

**Short electrodes (-500 V) in solenoids expel or attract electrons**

**long electrodes (+500 V) collect electrons along solenoidal field lines between magnets.**

## New diagnostic to measure Larmor rotation of beam within the solenoid

- E. H.

**New diagnostic to measure Larmor rotation of beam within the solenoid**

**Pinhole array followed by scintillator (********z ≈ 10 cm).  **
**Viewing geometry (for CCD camera).**
**For ********Larmor**** = 1.7 x 10****6**** rad / sec, r****beam**** ≈ 2.5 cm, ******** = 142 mrad.**

- Image on scintillator
- 10 cm downstream.

- Pinhole array

- 3-4 mm

**rev/sec**

## Implementation in the experiment

- 10 cm

- Viewing port for image intensified CCD camera

## conclusion and summary

- Neutralized drift compression:
- Will attempt simultaneous longitudinal beam compression with transverse compression this summer.
- Tl ≈ 1 eV: inferred from compressed pulse width & also consistent with uncompressed beam through new energy analyzer. More measurements planned, valuable for disentangling contribution to focusing limits from initial beam conditions and bunching waveform fidelity.
- A new induction bunching module may provide compression up to 200x.
- 5-15 Tesla final focus solenoid is planned to increase transverse compression to < 1 mm.
- Pulse Line Ion Accelerator:
- Experimental verification of the predicted PLIA beam dynamics: beam energy gains many times higher than the input voltage
- Flashover problem being pursued with design modifications, modeling...
- Solenoid transport
- Injected and matching high-perveance beam into solenoid channel.  Beam dynamics studies, gas and electron effects...

## Extra slides...

## 60x beam compression observed in a 1-meter neutralized drift experiment agreed with EM-PIC model results.

**60x beam compression observed in a 1-meter neutralized drift experiment agreed with EM-PIC model results.**

**LSP*****: EM - PIC code including plasma modeling and beam plasma interaction.**
***Voss Scientific, www.vosssci.com**

**4.5 ns FWHM**

**Exp. plasma on**

**Exp. plasma off**

**LSP **
**plasma off**

**LSP **
**plasma on**

**n****p**** ≈ 5 x 10****10  ****cm****-3**

***Early comparisons,***
***Effect of plasma...***

***Average of 4 pulses, with detailed waveform input to modeling...***

**Notes:**
Voss Scientific, Say formerly MRC.

## Next: For beam compression suitable for near term WDM experiments, simultaneous longitudinal compression and transverse focusing must be demonstrated...

**Next: For beam compression suitable for near term WDM experiments, **_**simultaneous longitudinal compression and transverse focusing**_** must be demonstrated...**

- 0                    50                    100                  150                  200
- Time (sec)

- 0                      50                  100                 150                 200
- Time (sec)

- Experiment

- Theory (LSP)

- Beam compression I/Io

*Snapshot at fixed location. Induction module voltage, entrance envelope held constant *

**Notes:**
Differences due to tilt core waveform accuracy.
Tilt waveform was very sometimes unstable -- tiny differences could account for differences in compression.
Most scans were done with stable shots, but still, fluctuations could play a role.
DW asked about Marx voltage droop and stability, and PR said Marx was stable.

DW used a generic waveform for the tilt waveform.  But didn’t use specific measured waveform for each compression shot.

## E-Cloud diagnostic objectives in solenoids

**• ****	**_**Minimize**_** electrons – suppressors (traps) biased negatively (≤1 kV) to expel electrons ****(≥23 ns ******** n****e****/n****b**** ~ 0.01%)**** and collectors (clearing rings) biased positively ****(≤1 kV)**** to collect them and suppress emission.**
**•****	****Suppressor electrodes short enough that potential peaks at center with axial E-field throughout to repel e- (length ~ diameter) **
**•****	****Bias potential of ≤1 kV is 2-4x beam potential – adequate?**
**•****	****Can expel electrons in outer 0.1-0.5 of beam radius.**
**•****	****Tilting bias (50 V/solenoid) expels some (to most) e- at all radii in ~1 μs.**
**	****	**** ****Or**** ****	**
**•****	**_**Maximize**_** electrons in Penning Trap geometry – “suppressors” (+) to confine electrons within solenoid and “collectors” (-) to emit e-.**
**•****	****Ionization of gas: ****n****e****/n****b**** ~ 10% for 20 μs**** pulse at 10****-5**** torr, assume 10****-16**** cm****2****. ****Desorbed gas moves a few cm, some will reach axis. May be able to photograph from diag. tank (at end).**
**•****	****Wall emission from negative “collectors”: n****e****/n****b**** ~ 14% for 10 μA loss (high?), 900 μA emission. Collect on positive “suppressors” when B-field turns off?**