如何更改力布局中节点之间的距离？

graph = {
  nodes: [{
      id: 'a',
      group: 1
    },
    {
      id: 'b',
      group: 1
    },
    {
      id: 'c',
      group: 1
    },
    {
      id: 'd',
      group: 2
    },
    {
      id: 'e',
      group: 2
    },
    {
      id: 'f',
      group: 2
    },
    {
      id: 'g',
      group: 3
    },
    {
      id: 'h',
      group: 3
    },
    {
      id: 'i',
      group: 3
    },
    {
      id: 'j',
      group: 4
    }
  ],
  links: [{
      source: 'a',
      target: 'b',
      value: 2
    },
    {
      source: 'a',
      target: 'c',
      value: 2
    },
    {
      source: 'b',
      target: 'c',
      value: 1
    },
    {
      source: 'd',
      target: 'e',
      value: 2
    },
    {
      source: 'd',
      target: 'f',
      value: 2
    },
    {
      source: 'e',
      target: 'f',
      value: 1
    },
    {
      source: 'g',
      target: 'h',
      value: 1
    },
    {
      source: 'g',
      target: 'i',
      value: 2
    },
    {
      source: 'i',
      target: 'h',
      value: 2
    },
    {
      source: 'a',
      target: 'j',
      value: 2
    },
    {
      source: 'b',
      target: 'j',
      value: 1
    },
    {
      source: 'c',
      target: 'j',
      value: 1
    },
    {
      source: 'd',
      target: 'j',
      value: 1
    },
    {
      source: 'e',
      target: 'j',
      value: 1
    },
    {
      source: 'f',
      target: 'j',
      value: 1
    },
    {
      source: 'g',
      target: 'j',
      value: 1
    },
    {
      source: 'h',
      target: 'j',
      value: 2
    },
    {
      source: 'i',
      target: 'j',
      value: 2
    },
  ]
}
var graph = this.graph

var svg = d3.select('svg')
var width = +svg.attr('width')
var height = +svg.attr('height')

var color = d3.scaleOrdinal(d3.schemeCategory20)

var simulation = d3.forceSimulation()
  .force('link', d3.forceLink().id(function(d) {
    return d.id
  }))
  .force('charge', d3.forceManyBody())
  .force('collide', d3.forceCollide(function(d) {
    return d.id === "j" ? 100 : 50
  }))
  .force('center', d3.forceCenter(width / 2, height / 2))

var link = svg.append('g')
  .attr('class', 'links')
  .selectAll('line')
  .data(graph.links)
  .enter().append('line')
  .attr('stroke-width', function(d) {
    return Math.sqrt(d.value)
  })

var node = svg.append('g')
  .attr('class', 'nodes')
  .selectAll('circle')
  .data(graph.nodes)
  .enter().append('circle')
  .attr('r', 5)
  .attr('fill', function(d) {
    return color(d.group)
  })
  .call(d3.drag()
    .on('start', dragstarted)
    .on('drag', dragged)
    .on('end', dragended))

node.append('title')
  .text(function(d) {
    return d.id
  })

simulation
  .nodes(graph.nodes)
  .on('tick', ticked)

simulation.force('link')
  .links(graph.links)

function ticked() {
  link
    .attr('x1', function(d) {
      return d.source.x
    })
    .attr('y1', function(d) {
      return d.source.y
    })
    .attr('x2', function(d) {
      return d.target.x
    })
    .attr('y2', function(d) {
      return d.target.y
    })

  node
    .attr('cx', function(d) {
      return d.x
    })
    .attr('cy', function(d) {
      return d.y
    })
}

function dragstarted(d) {
  if (!d3.event.active) simulation.alphaTarget(0.3).restart()
  d.fx = d.x
  d.fy = d.y
}

function dragged(d) {
  d.fx = d3.event.x
  d.fy = d3.event.y
}

function dragended(d) {
  if (!d3.event.active) simulation.alphaTarget(0)
  d.fx = null
  d.fy = null
}

.links line {
  stroke: #999;
  stroke-opacity: 0.6;
}

.nodes circle {
  stroke: #fff;
  stroke-width: 1.5px;
}

<script src="https://d3js.org/d3.v4.min.js"></script>
<div class="container">
  <svg width="960" height="600"></svg>
</div>